Hammerhead: a compact 300 kV vacuum bushing

Domain: physics
Published: 2025-12-19
Editorial outcome: minor_revision

Abstract
High-voltage vacuum bushings are vital for particle accelerators, X-ray tubes, electron microscopes, fusion devices, and electron sources. Commercial bushings, however, are limited to ~100 kV, and exceeding this limit has proven to be difficult. This work reports the physics-informed design, simulation, and experimental validation of Hammerhead, a compact vacuum bushing that has been tested up to 330 kV and used reliably at 300 kV. The design utilizes a coaxial electrode configuration and a ceramic insulator that bridges the gap using a flashover-resistant geometry. Compared with the state-of-the-art, Hammerhead nearly doubles the voltage holdoff per unit of volume, does not require ultra-high vacuum conditions or sub-micron polishing, and can be manufactured in a moderate clean room environment. It also integrates directly with a high-voltage coaxial cable, without the need for insulating fluids or pressurized gas. Here, it is shown-over 144 h of experimental testing-that Hammerhead operates stably at 300 kV with dark current below 10 μA.

Introduction
High-voltage systems in vacuum are essential to a diverse array of technologies ranging from particle accelerators and fusion devices to X-ray tubes and electron microscopes. In many of these technologies, within vacuum, up to hundreds of kilovolts of potential are used to accelerate charged particle beams or to generate high-energy photons. The reliable access to such extreme voltages in a vacuum environment is known to be challenging, plagued by a lack of universal theory for failure mechanisms such as vacuum breakdown and surface charge flashover1,2. Unlike solids and fluids, the density of neutral particles within a vacuum environment is not high enough to enable an avalanche of ionization events. Therefore, traces of surface defects or contamination play a critical role in providing the means for the breakdown3.

A high-voltage vacuum bushing (or feedthrough) is a component that allows electrical power to pass from ambient (or gas-insulated) environments into vacuum chambers. High-voltage vacuum systems have traditionally shown large dimensions to reduce the electric field intensity. Metal surfaces exposed to high electric fields emit electrons through quantum tunneling (field emission) which can lead to localized heating, X-ray emission, as well as electrical breakdown4. When an insulator is introduced, the problem becomes more complex as the insulator surface often has a lower breakdown strength relative to the breakdown strength of the vacuum gap and the bulk of the insulator5. It is commonly believed that flashover often initiates at the cathode triple junction, the junction between cathode, insulator and vacuum, where there is a significant field emission current6. When a flashover or vacuum breakdown is triggered, the resulting discharge quickly crosses the high-voltage gap. Design solutions for these problems place emphasis on the need to minimize field intensities and manage surface charge at crucial interfaces, which has commonly led to the use of wider, longer or multi-stage designs as well as intensive conditioning procedures.

Vacuum bushings rated above 100 kV are exceptionally difficult to develop due to the aforementioned failure mechanisms. Market offering of high-voltage bushings tends to cap at ~100 kV which mandates a custom design at higher voltage needs. Figure 1 shows the cross-section of a commercially available 100 kV bushing. This design is not fully enclosed by a grounded structure. On the air side it employs a sawtooth pattern to increase creepage distance and mitigate flashover; on the vacuum side it relies on a long, slightly angled flashover path, which accounts for its substantial height of approximately 32 cm. The center conductor (cathode) has a diameter of about 4 mm, and the alumina ceramic insulator is brazed to a 15 cm-diameter ConFlat® flange. Both the anode and the cathode are typically fabricated from 304 stainless steel. Because the high-voltage portion includes external threaded connections, additional clearances and field-shaping electrodes are required, and a fully shielded cable cannot extend to the termination; the grounded shield must be cut back sufficiently far from the high-voltage connection point.

Several examples of bushing designs for voltages exceeding 100 kV can be found in the literature7–11. Conventional methods have resorted to arrangements, including long ceramic standoffs or pressurizing the bushing with insulating gases such as sulfur hexafluoride (SF6), thus increasing size, complexity, and environmental impacts. A well-known example is the high-voltage vacuum bushing developed for the International Thermonuclear Experimental Reactor (ITER) neutral-beam injector which must provide 1 MV of insulation in vacuum. It is formed by five stages, using layered ceramic insulators and SF6 gas barriers to meet the voltage requirement; together with mechanical, cooling-channel and tritium-safety provisions, these features result in a meter-scale assembly with a large footprint7,9. After more than two decades since the initial work on the referenced design12, full 1 MV operation was reported in 201913. Although the ITER bushing is a specialized component, the theoretical and experimental advances made during its development offer valuable insights for next-generation vacuum bushing designs.

Several development efforts have targeted the 300 kV regime. Notable examples include bushing development efforts for Jefferson Lab’s photogun and the University of Wisconsin-Madison’s inertial electrostatic confinement devices. Jefferson Lab demonstrated a compact photoemission gun using an inverted-insulator geometry capable of achieving a stable 300 kV11. This design features centrifugal barrel-polished electrodes, auxiliary electrodes for screening the triple junction, and a 10−10 Pa-scale vacuum. To reach the nominal design voltage, krypton gas conditioning was employed14.

Since the construction of the first inertial electrostatic confinement (IEC) devices at the University of Wisconsin-Madison in 1994, the research group has pursued operation at voltages approaching 300 kV. The initial bushing concept was based on modifying a commercial 100 kV bushing by replacing the stock conductor and insulator with a custom assembly15. Subsequently, to overcome the limitations of that arrangement, a new insulator-conductor stalk design was developed that could reliably exceed 100 kV; this design demonstrated stable operation up to 150 kV. In 2009, the group pursued a further bushing redesign aimed at eliminating the pinhole failures that had plagued the earlier device, hoping to enable 300 kV operation16. Although the redesigned bushing achieved voltages above 200 kV, the 300 kV goal remained out of reach.

This work presents a compact 300 kV vacuum bushing design, Hammerhead, originally developed to address the voltage requirements of the Orbitron, a compact crossed-field fusion device17. However, this technology is applicable to a wide range of applications requiring high-energy electrons, such as X-ray sources, particle accelerators, and electron microscopes. The design concept—previously proposed by the authors18—is validated through simulations, theoretical modeling, and 144 h of experimental data, demonstrating stable operation at the target voltage in a significantly more compact form factor. Hammerhead features a coaxial geometry and utilizes a ceramic insulator to bridge the interface between vacuum and atmospheric pressure. The center electrode (cathode) is at negative potential relative to the grounded outer electrode. The cathode geometry inherently shields the triple junction without the need for auxiliary screening electrodes. By directly interfacing the high-voltage cable, the bushing is able to operate without SF6 or oil insulation. Compared with Jefferson Lab’s 300 kV bushing, which is the most compact design in its voltage class reported in the literature—Hammerhead reduces the radial dimension by roughly 20% and the axial dimension by about 25%, while avoiding the need for pressurized gas, sub-micron polishing, ultra-high-vacuum conditions or high-grade cleanroom environments.

Results
Hammerhead design
The overall design of the Hammerhead bushing is illustrated in Fig. 2. On the air side, the high-voltage connection to the cathode is made via a custom flat connector of 20 cm outer diameter. The connector houses a coaxial X-ray cable insulated with ethylene-propylene rubber (commercially marketed as C2236). The cable is potted into the connector using a two-part H1369 epoxy resin obtained from Parker Medical Inc., which also fabricated the connector. The curing process is carried out in atmospheric pressure; the temperature is slowly ramped up to ~125 ∘C followed by a slow cooling to room temperature. A compressed silicone-rubber gasket between the flat faces of the connector and the ceramic insulator eliminates any air gap and ensures a reliable dielectric seal. The cable’s grounded shield is terminated at the entry to the connector assembly, while the semi-conductive layer extends about 1 cm into the potting to reduce the electric stress at the interface.

On the vacuum side, the bushing comprises a central molybdenum cathode (99.9% purity), a concentric insulator machined from MACOR® machinable glass-ceramic, and a concentric 304L stainless-steel anode with a 20 cm ConFlat flange. MACOR—which consists of approximately 55% fluorophlogopite mica and 45% borosilicate glass—is selected for its excellent vacuum compatibility, high dielectric strength, and ease of machining. Its machinability allows the fins, 10–19 mm high with radii and thicknesses of 1.3 mm and 2.5 mm, respectively, to be produced with conventional tooling. A knife-edge or O-ring groove is machined into the ceramic to seal against the stainless-steel flange. Additional design details are provided in ref. 19.

To limit permittivity mismatches at interfaces, the constituent materials are chosen to have comparable relative permittivities: MACOR has ϵr ≈ 6, the epoxy ϵr ≈ 4, the silicone gasket ϵr ≈ 3, and the cable dielectric ϵr ≈ 2.8. Finite-element simulations confirm that the resulting electric-field stresses remain below the dielectric strength of each material. The remainder of this section discusses key failure and loss mechanisms and their impact on the design approach.

Field electron emission
Through a quantum-mechanical mechanism, electrons can escape from solid surface into vacuum when exposed to strong electric fields. When the field strength reaches a critical threshold, the electrons closest to the Fermi level penetrate the surface barrier toward vacuum space. Quantitative understanding of this phenomenon stems from formulation first presented by Fowler and Nordheim20, later improved by Murphy and Good21. It incorporates image-charge effects that reduce the surface potential barrier due to the Schottky-Nordheim effect22, resulting in a formula that approximates tunneling current density as below:

J = ϕ A t2 F (2 y) e−Bϕ3= F 2v(y) (1)

where A and B represent two universal constants which depend on factors that address image-force effects and barrier rounding23. F is the intensity of the local electric field at the emitter, and ϕ is the work function of the material. Finally, t(y) and v(y) are slowly varying functions of ϕ and F. t(y) is close to unity, but v(y) takes values slightly less than unity. The current density, J, needs to be multiplied by the emitting surface area to obtain the field emission current from an emission site.

Emission from broad-area electrodes originates at discrete microscopic sites associated with surface irregularities or inclusions that locally enhance the electric field. Local field enhancement factor β describes the relationship between the local electric field at the microscopic feature, F, and the applied (macroscopic) electric, Fmacro, in the form of F = βFmacro. A small number of high-β emitters provide most of the overall emission current24. It has also been demonstrated that field-enhancement features can transform over time due to Maxwell stresses and heat-activated processes25.

Material properties such as work function and surface topography, together with mechanical quality, determine both the onset and intensity of electron emission. Previous studies indicate that refractory metals such as molybdenum exhibit high work functions and successfully reduce dark current at macroscopic electric fields near 100 MV/m26. Moreover, the high thermal stability of refractory metals confers enhanced resistance to the formation of new emission sites during testing. For these reasons, molybdenum is selected as the cathode material, with a geometry designed to keep the macroscopic electric field below 40 MV/m, which is about 60% lower than the switch-on field for molybdenum reported in ref. 26.

Vacuum breakdown
Although an ideal vacuum gap would have infinite dielectric strength, decades of research have shown that it can rapidly transition from an insulator to a conductive plasma through vacuum breakdown. In vacuum, which lacks an ionizable medium, breakdown is triggered when neutral and charged particles are released from the electrode surfaces as a result of intense electric fields27. Despite extensive study, the physics of vacuum breakdown have been debated28. The two major mechanisms have been used to describe the initiation of the vacuum breakdown: (1) vaporization of field emission sites on the cathode, and (2) breakdown through microparticle impact (clump theory).

In field-emission-based breakdown, current densities exceeding 108 A/cm2 can vaporize an asperity within nanoseconds; the vapor is then ionized and initiates Townsend avalanches, ultimately producing a conductive plasma29. Beyond electric field strength, mechanical and crystallographic properties also influence breakdown thresholds; experiments demonstrate that hexagonal close-packed metals have the greatest breakdown strength, followed by body-centered cubic and then face-centered cubic structures30. When the local electric field near an emission site reaches the GV/m level required for field emission, mechanical stresses can exceed tens of MPa, which may approach the threshold for dislocation movement. The interplay between surface defects, field-emission currents (causing Joule heating and Nottingham-effect-related thermal effects), and dislocation activity may further enhance the local field, establishing a feedback loop among mechanical, thermal, and electrostatic processes31.

In clump theory, microparticles detach from an electrode and are accelerated across the gap by the electric field; their impact melts or vaporizes material on the opposing electrode, triggering plasma formation32. A contaminated vacuum environment along with system deterioration promotes this breakdown method. High-speed imaging and particle detectors have observed such clumps prior to breakdown33. Anode-initiated breakdown via microparticles tends to be more prevalent in unconditioned or contaminated gaps, where many loose particles or flakes are present34.

In the Hammerhead design, molybdenum is chosen in part because its high melting point reduces the likelihood that metallic asperities will grow or vaporize. To tackle the breakdown mechanism due to microparticles and contaminants, this study employs a moderate clean room followed by glow-discharge cleaning and conditioning to prepare the surfaces for operation in vacuum (more details in the Theory section).

Surface charging and flashover
Surface flashover in vacuum is a complex discharge phenomenon. The mechanism occurs along an insulator’s surface and typically happens at a voltage below the breakdown of the solid dielectric and the vacuum gap. Two widespread theories suggest initiation at the cathode triple junction where the cathode, insulator and vacuum meet and result in a high localized electric field strength and emission current35. The leading theory is that secondary electron emission avalanche (SEEA), posits that initial electrons emitted from the cathode triple junction strike the insulator surface, releasing secondary electrons, which in turn knock out more electrons36. The cascading process continues until reaching the anode and in the final stage, it transitions into a gas phase breakdown in the desorbed vapor or ablated material37. The other prevalent theory, electron-triggered polarization relaxation (ETPR), suggests that injected electrons are trapped in the surface or bulk trap sites of the insulator, and once a critical density is reached, rapid detrapping (or a polarization relaxation wave) initiates the discharge38.

No single mechanism comprehensively explains all observations—contemporary literature recognizes multiple, often concurrent, mechanisms depending on material and conditions. Nevertheless, the fundamental stages (field electron emission, surface electron multiplication, and eventual discharge) are consistently seen across a range of vacuum dielectrics.

Triple junction field enhancement and surface micro-features play a dominant role in initiating vacuum flashover. The triple junctions often undergo a strong local electric field intensification due to the discontinuity in material permittivity and electrode geometry. In addition to mismatched relative permittivities, the reduced potential barrier due to the presence of the dielectric also contributes to elevated electron emission at the cathode triple junction. With this in mind, the Hammerhead’s cathode is designed to trap the triple junction into an equipotential well where the electric field is lower than the threshold needed for electron field emission. Additionally, the sawtooth pattern of the insulator assists with surface flashover prevention and mitigation in three ways: (1) it increases the flashover path between the two electrodes to over 400 mm, (2) forces any electron propagating along surface to move against or perpendicular to the direction of applied electric field thus reducing the flashover likelihood, and (3) as the next section shows, it can capture electrons and distort the local field in such a way that the electric field on the cathode is reduced.

Lastly, similar to how field emission conditions the surface, flashover events can also condition the surface. When an insulator is subjected to repetitive flashover events at low to moderate energies, residual surface charge accumulates and weak emission sites (microscopic protrusions or contaminants) are burned off or removed, which ultimately improves flashover resistance39.

Electro-quasistatic Simulation
As a preliminary study of the projected electric stress on the bushing and the feasibility of the design, the Poisson’s equation are resolved in COMSOL Multiphysics®. The 2-D axisymmetric model is discretized using approximately 7.1 × 105 triangular elements (~3.6 × 105 vertices) together with 15,768 edge elements, yielding an average element quality of 0.90 and a minimum quality of 0.018. The boundary mesh on the ceramic-metal interface contained 15,744 vertices and exhibited an average growth-rate quality of 0.97. A mesh-refinement study is carried out in the cathode triple junction region by successively doubling the local mesh density until the computed peak field changes by less than 1%. The electric potential and electric field distributions are shown in Fig. 3 for the cathode at the nominal voltage (−300 kV).

Figure 3a shows the center conductor (the cathode) held at -300 kV with respect to the grounded outer boundary. As confirmed by Fig. 3b, the electric field is highest near the cathode surface, which is characteristic of the coaxial electrode configuration. The maximum electric field on the cathode occurs at its rounded top edges and reaches 36.7 MV/m, while the average electric field on the cathode is approximately half of that value. At 300 kV, the maximum electric field is close to the empirical breakdown limits32,40; however, the choice of molybdenum and the insulator charging behavior (discussed in the next subsection) help mitigate the risk for a well-conditioned cathode but as shown later in this section, it assists with providing a qualitative picture of how charging develops.

The density of electrons born from the cathode surface follows the Murphy-Good formula (see Eq. (1)). No field enhancement factor is included given the lack of information on the magnitude or spatial distribution. Relativistic corrections are applied in the simulation, since at 300 keV the electron speed approaches a significant fraction of the speed of light.

According to the simulation data, about 31.9% of these electrons emitted from the cathode strike the insulator’s surface. Since MACOR has high resistivity (>1016 Ω cm), each incident electron on the insulator surface contributes to the surface charge accumulation. To better understand the charging extent and polarity, a histogram on the count of electrons as a function of impact energy is illustrated in Fig. 4b. The minimum energy of striking electrons is 11.3 keV and the maximum 278.0 keV. The broad peak with the large width around 130 keV shows the large population of the electrons that impact the first fin’s surface facing the cathode. The next four narrow peaks at approximately 180 keV, 210 keV, 240 keV, and 270 keV correspond to the bombardment of the next four insulator fins farther away from the cathode, with higher electron energies.

To understand the insulator charging extent due to electron bombardment, an illustrative secondary electron yield curve as a function of incident electron energy is displayed in Fig. 5. The secondary electron yield of MACOR for the aforementioned energy range (11–290 keV) has not been characterized experimentally yet. However, there are a significant number of reports showing that ceramic insulators have E1 < 100 eV, Emax ranging from 250 to 700 eV and E2 ≈ 1–4 keV41. This suggests that electrons with an striking energy greater than 11 keV cause the impacted areas of the insulator to become negatively charged, since the quantity of electrons reaching the surface is greater than that of the secondary electrons leaving it. This negative charge slows down incident electrons until their energy is similar to that of E2, resulting in a state of equilibrium. Whether the electron flux is sufficient to reduce the energy to E2 cannot be predicted without knowledge of the emitter distribution on the cathode. However, post-experiment observations show that the insulator remains nearly intact, which suggests that the negative charging is not extreme enough to cause local field concentrations exceeding the dielectric strength. Furthermore, if the insulator near the cathode (i.e., the first fin) accumulates negative charge due to secondary electron emission, this negative potential will locally reduce the electric field strength at the cathode surface, thereby weakening field-emission current. Thus, it can be hypothesized that a state of equilibrium can be reached in which a slight degree of charging is maintained by a decreased level of field-emission current, resulting in enhanced flashover resistance.

Experimental results
The I–V behavior of the Hammerhead bushing under both gas and current conditioning is depicted in Fig. 7a, revealing important information about their effectiveness and the progress of the test as the applied voltage increases. The two solid lines show the mean cathode current as a function of voltage for gas-conditioned (blue) and current-conditioned (orange) operation. The shaded band around each line indicates the standard deviation of the measurements from the mean.

In these experiments, gas conditioning is applied first to bring the system to a target voltage, followed by current conditioning at the same voltage setpoint. At voltages below 150 kV, both methods result in negligible dark current, indicating minimal field emission. However, at higher voltages, gas-conditioned samples tend to exhibit elevated dark-current levels. This increase is attributed to enhanced ion-induced secondary electron emission as described in the previous section. Furthermore, gas conditioning is intentionally conducted first to improve the cathode surface through sputtering and ion implantation, effectively removing high-β emission sites. As a result, by the time current conditioning is initiated, many of the most prominent emitters have already been passivated, leading to the generally lower emission activity during the latter phase.

The conditioning procedure continues relatively smoothly until 165 kV, after which, longer wait times are needed to remove stubborn field emission sites. Figure 7b displays the time spent for each conditioning method over 15 kV voltage ranges between 30 kV and 330 kV. The gas conditioning period for 165–180 kV is about 8 h which is longer than entire period leading to the 165 kV operating point.

Throughout the conditioning process, discharge events occur; they appear as fluctuations in voltage along with spikes in cathode dark current and X-ray emission. By triggering diagnostics such as fast-frame imaging on these events, insight into the underlying physics can be gained. Figure 8 shows a characteristic vacuum breakdown between the cathode and anode. There is a 200-350 ns delay between the external trigger and the moment the sensor begins exposure. The camera operates in circular-buffer mode and is configured to save footage whenever a rapid transient in the cathode voltage is detected. Since each frame integrates light over 200 μs at 5000 frames per second, the image captures not only the discharge initiation but also its subsequent evolution.

Occasional de-conditioning events are observed during the process. They appear as spikes in current and X-ray emission followed by a more stable increase in dark current and X-ray level. While most of these events are relatively benign, extreme cases can require tens of hours of conditioning to revert the damage. An example of this can be seen on Day 3 in Fig. 9. This figure displays the voltage and current ranges of experiments for each of the 23 days of testing. The depth of color for each point denotes the cumulative hours spent conditioning at that voltage; darker points indicate longer conditioning durations.

On Day 3, a local maximum in voltage is observed; it takes nearly six days of conditioning to return to that voltage again on Day 9. It is worth noting that the current on Day 9 is less than that on Day 3, indicating that the cathode is more thoroughly conditioned after recovering from the de-conditioning event than it was prior. These de-conditioning events are hypothesized to stem from two main mechanisms: (1) explosive vaporization of field emitters on the cathode surface, and (2) cathodic contamination via sputtered particulates-metallic or insulating-that are dislodged during breakdown and transported across the vacuum gap. Upon redeposition, these particulates can generate new high-field enhancement sites (high β) or exacerbate existing ones, leading to renewed field emission. In either case, the result is a temporary degradation in conditioning efficacy and a setback in voltage holdoff, necessitating further emitter suppression and surface reconditioning to restore stability.

Between Days 9 and 13, conditioning proceeds nominally with no major de-conditioning events, and an average dark current of approximately 80 μA until 300 kV. While no major setback is observed in this period, the dark current increases from 40 μA at 250 kV on Day 11 to 120 μA at 270 kV on the next day.

Upon reaching the target voltage of 300 kV, transitioning from gas conditioning to current conditioning proves challenging. On Day 14, the system reaches 300 kV under krypton backflow, but the same voltage cannot be maintained when the system is returned to high vacuum without krypton. This observation indicates that the effect of gas conditioning is more complex than simply being a tool for removing tough emitters. A plausible explanation is that the increase in background pressure due to krypton backflow enhances scattering of electrons and ions as a result of shorter mean free path, thereby reducing the probability that they traverse the gap with energies near the full potential. Reversible removal of implanted krypton upon returning to high vacuum is unlikely, as such desorption typically requires bake-out temperatures that are not reached during these tests.

Next, the krypton backflow is reintroduced and the voltage is raised to 330 kV, 10% above the nominal operating voltage. The cathode is held at 330 kV under gas conditioning for 1 h (Day 19), before the voltage is then reduced to ~250 kV. From there, current conditioning resumes and proceeds for the subsequent two days until the cathode is stable at 300 kV. Hammerhead’s cathode is then held stably at 300 kV for over 2 h under high vacuum, with dark current remaining below the ±10 μA resolution of the measurement system. The average dark current during this period is approximately 8 μA, indicating a successful recovery and stabilization. The success of krypton conditioning at higher voltage can be attributed to two effects: (1) the longer conditioning duration allows sputtering of stubborn emission sites until the emitter flux becomes too low to initiate breakdown, and (2) the higher energy of Kr+ ions increases the likelihood of implantation, leading to greater emitter passivation through an increased work function.

Post-experiment microscopy and surface analysis
Following conditioning completion, the cathode is inspected with Optical Imaging, Optical and Laser Profilometry, Scanning Electron Microscopy (SEM), as well as Energy Dispersive Spectroscopy (EDS). Fig. 10 displays a characteristic clump of a copper contaminant that has impacted the cathode. It is observed that copper (Cu) and silver (Ag) particulates are embedded in the cathode surface. It is hypothesized that during the disassembly and reassembly procedures, contaminants enter the chamber, originating from the bushing’s Cu seals and Ag-plated fasteners. These contaminants are likely then clumped onto the cathode as dictated by Clump Theory32, in which microscopic particles or clumps become dislodged from one electrode and traverse the vacuum gap, leading to a disruptive discharge upon impact on the opposite electrode32,42.

Post-experiment surface analysis shows that clumps of different materials (e.g., Au and Cu) are clustered at discrete sites, suggesting either pre-aggregation before acceleration or electrostatic focusing to specific impact points by local field enhancements (see refs. 43–46 for discussions of field-enhancement features). As shown in ref. 7, the trajectory of a charged particle released from rest in a static electrostatic field is governed by the field geometry (with the direction set by the sign of the charge), and thus particles follow field lines. In contrast, if some clumps are ejected with nonzero initial velocities or possess different charge-to-mass ratios (q/m), inertial effects can modify their paths; combined with electrostatic focusing, this can produce the observed clustering. Further analysis of these hypotheses will be presented in a future publication.

Methods
Cleaning procedure
The cathode and insulator are manufactured from bulk materials using computer numerical control (CNC) machining. The cathode is polished to a surface roughness equal to or less than 1 μm through manual sanding using 2000 grit sand paper. The cathode material is molybdenum (Type 361) with a purity of 99.9%, the insulator is manufactured from MACOR® Machinable Glass Ceramic, and the anode is a commercially available vacuum chamber constructed of 304L Stainless Steel. Following fabrication, the samples are subjected to a cleaning process to remove hydrocarbons and other contaminants that remain from the manufacturing and handling processes. Initially, the parts are hand-washed with soap and water to remove bulk contaminants. Next, each sample is subjected to a series of ultrasonic cleaning baths. The first cycle is 15-min in a mixture of detergent and heated deionized water. Following this cycle, each sample is rinsed with deionized water before being transferred to the second cycle, 2-min in a deionized water bath for a second ultrasonic cleansing. Next, the sample is rinsed again before entering a third cycle, this time 15-min in isopropyl alcohol. Lastly, each sample is brought to a class-10,000 clean room for hand drying with a lint-free cloth.

After installing the Hammerhead bushing on the vacuum chamber and achieving a 5 × 10−6 Pa pressure, the entire vacuum chamber is the heated gradually to 80 ∘C over the course of 8 h, held at temperature to be baked out for 24 h, and then gradually cooled down over an additional 8 h. The vacuum pressure after bake-out is settled at 4 × 10−6 Pa.

Prior to testing, glow discharge conditioning is implemented as one final cleaning measure for the bushing’s electrodes. Ar/O2 gas mixture with 9/1 ratio flows into the chamber according to the procedure described in ref. 47. By reversing the polarity of the power supply, it is possible for both electrodes to be exposed to ions. This is done with a target current density of 25 μA/cm2. The gas mixture cleans the surface employing several fundamental mechanisms. Ar leverages sputtering to remove impurities from the surfaces, while the addition of O2 facilitates surface reactions producing CO and CO2, which accelerates the removal of surface carbon from the chamber48. With a total pressure within the 13–133 Pa range and applied voltage magnitude between 300 and 600 V, the targeted current density was achieved for the outer and center electrodes with 20 mA and 1.5 mA, respectively.

Gas and current conditionings serve complementary roles. Introducing a low-pressure krypton backfill allows field-emitted electrons to ionize krypton atoms; the resulting ions sputter sharp asperities and implant into the cathode, raising its work function and passivating high-β emission sites. Once these prominent emitters have been suppressed, current conditioning in high vacuum can more effectively blunt the remaining sites through Joule heating and provide a smooth transition to steady high-vacuum operation at the nominal voltage.

Current conditioning
Despite the careful design of Hammerhead, the compact geometry of the bushing leads to an electric field strength on the cathode surface that is sufficiently strong for field emission. These electrons can bombard the insulator and the anode eventually trigger breakdown phenomena discussed, under certain conditions. In the high voltage testing, two phases of controlled conditioning discharges are used to remove surface irregularities and field emitters, current conditioning and gas conditioning. In total, the two methods are implemented over 144 h to achieve stable operation at 300 kV in high vacuum.

The leading method of conditioning, current conditioning (also known as spot-knocking conditioning), involves raising the applied voltage in small increments and holding it until no discharge or current/X-ray spike is observed. The mechanism by which current conditioning improves the voltage hold-off is due to the burn-off of the emitter through Joule heating of the field emission sites with high current densities49. While effective, current conditioning can often be time-consuming and sometimes unreliable. One alternative, gas conditioning can more efficiently burn off the emitters, and is heavily employed in this work.

Gas conditioning
In the gas conditioning method, an inert gas is introduced to the chamber at relatively low voltages; the gas is then maintained as the voltage is ramped up. The voltage is raised in 2 kV increments and held until the cathode current and X-ray emission stabilize before the voltage is increased again. During this process, some of the gas molecules are ionized by the field-emitted electrons from the cathode. These newly formed ions improve voltage hold-off through two mechanisms: sputtering and ion implantation. In the case of sputtering, the newly formed ions are attracted to the field-emission sites (often metallic protrusions or insulating particulates) because of the high local electric field (high-β sites). As the ions impact these sites, they blunt the protrusions; this process is known as sputtering. In the other mode, ions are implanted into the cathode surface, ultimately increasing the work function of the metal and thereby reducing the likelihood of electron tunneling from the cathode, as shown in Eq. (1).

In addition to sputtering and implantation, the ionization of krypton molecules by electron impact and their subsequent collision with the cathode can liberate more than one electron per incident ion, depending on the impact energy. This secondary emission can increase the dark current. The interplay between the raised work function and the enhanced secondary emission creates a situation in which, depending on the operating parameters, the krypton backflow may either increase or decrease the total dark current.

In this work, krypton (Kr) is used while various other gases, including He, Ar, N2, O2, and H2, have also been used for gas conditioning in the literature50. The flow is introduced at a rate ranging between 8.33 × 10−9 m3/s and 1.67 × 10−7 m3/s, which at the highest, raises the background pressure to ~0.15 Pa. The benefits of using Kr for gas conditioning have been discussed in the works done at Jefferson Lab11,51. In the case of Hammerhead, heavy Kr+ ions bombarding the cathode have a similar atomic mass to molybdenum (84 vs. 96 amu), so at energies above ~1 keV a single ion can eject multiple Mo atoms. This efficient sputtering rounds off micro-tips and smooths nanoscale asperities-features that concentrate field emission. Lighter ions such as He+ have smaller sputtering effect. Krypton ions that do not sputter away are implanted within the upper tens of nanometers of the cathode (e.g., a 300 keV Kr+ ion has a projected range of ~63 nm), and these implanted noble gases can raise the local work function on the order of 1 eV51. The combination of controlled sputtering and shallow implantation reduces tunneling currents and suppresses field emission without significant bulk modification, although excessive ion bombardment can roughen the surface.

Gas and current conditionings serve complementary roles. Introducing a low-pressure krypton backfill allows field-emitted electrons to ionize krypton atoms; the resulting ions sputter sharp asperities and implant into the cathode, raising its work function and passivating high-β emission sites. Once these prominent emitters have been suppressed, current conditioning in high vacuum can more effectively blunt the remaining sites through Joule heating and provide a smooth transition to steady high-vacuum operation at the nominal voltage.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper presents the design and development of an ultra-compact 300 kV vacuum bushing, which operates without requiring ultra-high vacuum conditions or sub-micron polishing and can be manufactured in a moderate cleanroom environment. The test results demonstrate that a bushing with 20 cm radial and 30 cm axial dimensions sustained a DC 300 kV voltage for over 144 hours, which is undoubtedly impressive. Below are some suggestions to help more readers appreciate the ingenuity of this work:\n1. The paper repeatedly mentions that 100 kV is the limit for existing commercial bushings. It would be helpful to briefly introduce the structure of conventional 100 kV bushings and compare them with the proposed design to highlight the structural innovation of the Hammerhead. Additionally, the description of the Hammerhead’s structure is overly simplified—key details such as specific dimensions and material selections should be provided in the paper.\n2. In Figure 1, the cable inserted into the potting—how far does its surface grounding layer extend? How is the electric field in the red-circled areas (see the attachment) of Figure 3 controlled at a low level? Is the potting also under vacuum during operation? If so, how is vacuum achieved in this section?\n3. Under normal operating conditions, what is the current flowing through the bushing? Is the cavity temperature uniformly distributed, or does it exhibit a gradient?\n4. The paper highlights Current Conditioning and Gas Conditioning as key factors enabling high-voltage endurance in a compact space. How do these two mechanisms work synergistically? In Figure 8, what do the outer contours of the two solid lines represent? In Figure 11, what do the varying color depths of the data points indicate?\n5. Figures are presented out of order in the text: Figure 10 appears before Figure 9.", "reviewer_2": "Overall, the paper is well written and reports interesting data that is beneficial to the community. I recommend that the paper be published. However, I also recommend that the authors consider the below points and potential revisions to clarify some points and provide some additional details.\n1. Why was MACOR selected? Was it strictly for machinability? Would an Al2O3 insulator, or something else, provide superior performance?\n2. Some details of the potting process would be helpful. What epoxy was used? Was the feedthrough potted in a vacuum environment? A pressure chamber?\n3. Regarding both of the above points, what considerations were given to selecting materials with respect to their dielectric constants, noting that dielectric mismatches lead to some field enhancement?\n4. It is mentioned that anomalies in the voltage and current are used to trigger the camera. What is the insertion delay on the camera? Even for fast ICCD cameras, the insertion delay is on the order of 100 ns and I would suspect the insertion delay on the 5000 fps camera is even higher. This raises the question of, what does the imaging capture? Presumably, this is some late-stage phenomena, but exactly what is not clear.\n5. The deconditioning data and discussion is interesting. Of course, such behavior has been anecdotally reported by other authors as well. Generally, low energy systems condition upward, and high energy systems condition downward. The former does not have sufficient energy to sputter considerable material into the vacuum assembly, and does not have enough energy to substantially damage things. At higher energy, sputtering and damage are possible. Thus, it would be interesting to know how much energy is in your system and available to drive the discharge. This would include any available energy in your power supply as well as the energy capacitively stored in your HV coaxial cable.\nAgain, I believe reader would benefit from the author's input on the above points. Overall, the paper is interesting and should be published.\nNote as few minor typos that may need reconsideration.\n- typo \"as is indicate\" on page 5\n- Final sentence of pg. 6 (that goes to page 7) may need revision (it mentions that the length is increased three-fold).\n- Space missing in the first paragraph of pg. 7.\n- Krypton and Molybdenum and other elements are sporadically capitalized.\n- Missing space in page 10 (Geiger-Muller detector sentence).\n- The sentence \"The voltage is raised in 2-kV steps and it is waited until cathode current and X-ray emission are stabilized before stepping up the voltage again. \" may need to be revised.", "reviewer_3": "The paper describes the prototype of a remarkably compact 300 kVdc cable-to-vacuum bushing, which represents a noteworthy step ahead in the optimization of EHV vacuum insulated devices for both research and industrial use.\nThe comment are reported in the attached file", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": ""}, {"round": 2, "reviewer_comments": {"reviewer_1": "Thank you for the authors' revision. I think this paper can be accepted in this form.", "reviewer_2": "Consistent with my previous review, it is my opinion that the manuscript presents new and novel materials that merit publication. The authors have carefully considered all of my feedback in questions from the previous review, and have revised the manuscript accordingly. It is my opinion that the paper is suitable for publication.", "reviewer_3": "Review -#2\nThe authors have addressed most of the issues raised in Review #1, and we consider the paper suitable for publication. The points below contain requests for clarification, text revisions, and additional suggestions, which are not mandatory unless explicitly indicated.\nSuggestion - Introduction\nConcerning the 100 kV commercial feedthrough, it’s suggested to make a reference to one or more specific products.\nMoreover, in order to compare correctly the Hammerhead feedthrough with the commercial ones, it is suggested to give at least an estimation of the operating voltage (or rated voltage) of the Hammerhead. 300 kV are in fact to be considered as test voltage.\nChange text request - Introduction - mandatory\nChange: “It employs five acceleration grids, layered ceramic insulators and SF6-gas barriers to meet the voltage requirement” into “It is formed by five stages, made by layered ceramic insulators and SF6-gas barriers to meet the voltage requirement”.\nClarification request - Field emitted electron trajectory\nClarify if the current emission is calculated for an average field of 18.6 MV/m or it is used the Murphy-Good formula (eq. 1). It seems that such electric field is the lower limit for FN emission.\nClarification request - Experimental result\nIt is stated that “A plausible explanation is that the increase in background pressure due to krypton backflow enhances scattering of electrons and ions, thereby reducing the probability that they traverse the gap with energies near the full potential”. At 0.15 Pa (your max operating Kr pressure) the mean free path of gas molecules is of the order of centimetres (lower distance between cathode and anode). Do you mean that the scatter can occur because of the fulfilling of this condition? If yes, it would be better adding few words about to support your explanation. On the contrary, if the Kr is effective also at pressures order of magnitudes lower than 0.15Pa other physical processes may play a role.\nChange text request - Post-Experiment Microscopy and Surface Analysis - Mandatory\nThe words “and the applied voltage” shall be removed because, if the initial velocity V0 is zero, the particle trajectory is not affected neither by the q/m value, nor by the voltage (for non-relativistic motion)\nSuggestion - Vacuum Breakdown\nWe go back again on the breakdown induced by dislocation. You consider that the stress that triggers the dislocation movement is in the MPa range because in the calculation of the stress caused by the electric field, you include the enhancement factor of the tip. But such a tip appears because of the dislocation movement. The suggestion is to reconsider the statement.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Comments from Reviewer 1:\nWe sincerely thank the reviewer for their insightful comments and recommendations, which have been carefully considered and implemented in the revised manuscript.\nIn what follows, the reviewer’s comments are in blue and our responses are in black. The original and revised parts of the manuscript are highlighted in red and green, respectively.\n1. The paper repeatedly mentions that 100 kV is the limit for existing commercial\nbushings. Itwouldbehelpfultobrieflyintroducethestructureofconventional100\nkVbushingsandcomparethemwiththeproposeddesigntohighlightthestructural\ninnovationoftheHammerhead. Additionally,thedescriptionoftheHammerhead’s\nstructure is overly simplified—key details such as specific dimensions and material\nselections should be provided in the paper.\nResponse: Thank you for your suggestion. We have added the figure and description of the\ncommerciallyavailable100kVbushinginadditiontoamoredetaileddescriptionofHammerhead’s\nstructure.\nOriginal: TheoveralldesignofHammerheadbushingisillustratedinFigure2. Ontheairside,\nthe high-voltage connection to the cathode is achieved via an interconnect system comprised of a\ncoaxial cable and a custom flat connector, manufactured by Parker Medical Inc., which interfaces\nwith the air-side of an electrically insulating ceramic (MACOR). A compressed silicone rubber\ngasket placed between the two flat surfaces eliminates any air gap, ensuring a reliable dielectric\nseal.\nOn the vacuum side, the bushing features a central high-voltage cathode composed of 99.9%\nmolybdenum,aconcentricinsulatingstructuremadeofMACORwithsawtoothsurfacefeaturesto\nmitigate field enhancement and increase flashover path length, and a concentric anode fabricated\nfrom 304L stainless steel...\nRevised: Figure 1 shows the cross-section of a commercially available 100 kV bushing. This\ndesign is not fully enclosed by a grounded structure. On the air side it employs a saw-tooth pat-\ntern to increase creepage distance and mitigate flashover; on the vacuum side it relies on a long,\nslightly angled flashover path, which accounts for its substantial height of approximately 32 cm.\nThe center conductor (cathode) has a diameter of about 4 mm, and the alumina ceramic insulator\nis brazed to a 15 cm-diameter ConFlat® flange. Both the anode and the cathode are typically\nfabricated from 304 stainless steel. Because the high-voltage portion includes external threaded\nconnections, additional clearances and field-shaping electrodes are required, and a fully shielded\ncablecannotextendtothetermination; thegroundedshieldmustbecutbacksufficientlyfarfrom\nthe high-voltage connection point.\n...\nOn the air side, the high-voltage connection to the cathode is made via a custom flat con-\nnector of 20 cm outer diameter. The connector houses a coaxial X-ray cable insulated with\nethylene-propylene rubber (commercially marketed as C2236). The cable is potted into the con-\nnector using a two-part H1369 epoxy resin1. The curing process is carried out in atmospheric\npressure; the temperature is slowly ramped up to ∼ 125◦C followed by a slow cooling to room\ntemperature. A compressed silicone-rubber gasket between the flat faces of the connector and\nthe ceramic insulator eliminates any air gap and ensures a reliable dielectric seal. The cable’s\ngrounded shield is terminated at the entry to the connector assembly, while the semi-conductive\nlayer extends about 1 cm into the potting to reduce the electric stress at the interface.\nOn the vacuum side, the bushing comprises a central molybdenum cathode (99.9 % purity), a\nconcentric insulator machined from MACOR® machinable glass-ceramic, and a concentric 304L\nstainless-steelanodewitha20cmConFlatflange. MACOR—whichconsistsofapproximately55%\nfluorophlogopite mica and 45% borosilicate glass—was selected for its excellent vacuum compat-\nibility, high dielectric strength, and ease of machining. Its machinability allows the fins, 10–19\nmm high with radii and thicknesses of 1.3 mm and 2.5 mm, respectively, to be produced with\nconventional tooling. A knife-edge or O-ring groove is machined into the ceramic to seal against\nthe stainless-steel flange. Additional design details are provided in [19].\n2. InFigure1,thecableinsertedintothepotting—howfardoesitssurfaceground-\ning layer extend? How is the electric field in the red-circled areas (see the attach-\nment) of Figure 3 controlled at a low level? Is the potting also under vacuum\nduring operation? If so, how is vacuum achieved in this section?\nResponse: In our design the outer shield is stripped back so that it ends at the point where\nthe grounded housing of the flat connector begins; the semiconductive layer, on the other hand,\nextends into the potting by about 1 cm to provide a smooth transition between the cable and the\npotting portion. We have clarified this detail in the text.\nRevised: The curing process is carried out in atmospheric pressure; the temperature is\nslowly ramped up to ∼ 125◦C followed by a slow cooling to room temperature. A compressed\nsilicone-rubber gasket between the flat faces of the connector and the ceramic insulator eliminates\nany air gap and ensures a reliable dielectric seal. The cable’s grounded shield is terminated at\nthe entry to the connector assembly, while the semi-conductive layer extends about 1 cm into the\npotting to reduce the electric stress at the interface.\n3. Under normal operating conditions, what is the current flowing through the\nbushing? Is the cavity temperature uniformly distributed, or does it exhibit a\ngradient?\nwhich results in a total power loss of less than 3 W. At this power dissipation level, the temper-\nature throughout the cavity remains essentially uniform. This quantitative information has been\nadded to the Results section.\nRevised: ... Moreover, with the stable dark current below 10 µA at the nominal voltage, the\ntotalpowerdissipationislessthan3W;therefore,thetemperaturethroughouttheanode–cathode\ngap should remain essentially uniform during the standard operation.\n4. ThepaperhighlightsCurrentConditioningandGasConditioningaskeyfactors\nenablinghigh-voltageenduranceinacompactspace. Howdothesetwomechanisms\nworksynergistically? InFigure8,whatdotheoutercontoursofthetwosolidlines\nrepresent? In Figure 11, what do the varying color depths of the data points\nindicate?\nResponse: Gas conditioning and current conditioning serve complementary purposes. Intro-\nducing a low-pressure krypton backfill allows field-emitted electrons to ionize krypton atoms; the\nresulting ions sputter sharp asperities and implant into the cathode, increasing its work function\nand passivating high- β emission sites. Once the prominent emitters have been removed, current\nconditioning in high vacuum can more effectively blunt the remaining sites via Joule heating. We\nhave clarified this synergy in the Theory Section. In Figure 8, the two solid lines show cathode\ncurrent versus voltage for gas-conditioned (blue) and current-conditioned (orange) operation. The\nshadedbandaroundeachlineindicatesthestandarddeviationofthemeasurementsfromthemean;\nthe caption is be updated accordingly. In Figure 11 the depth of color for each point denotes the\ncumulative hours spent conditioning at that voltage; darker points indicate longer conditioning\ndurations. Both captions and text are revised to make these points explicit.\nRevised: Gasandcurrentconditioningservecomplementaryroles. Introducingalow-pressure\nkrypton backfill allows field-emitted electrons to ionize krypton atoms; the resulting ions sputter\nsharp asperities and implant into the cathode, raising its work function and passivating high-β\nemission sites. Once these prominent emitters have been suppressed, current conditioning in high\nvacuumcanmoreeffectivelyblunttheremainingsitesthroughJouleheatingandprovideasmooth\ntransition to steady high-vacuum operation at the nominal voltage.\n...\n(explaining Figure 8) The two solid lines show the mean cathode current as a function of voltage\nfor gas-conditioned (blue) and current-conditioned (orange) operation. The shaded band around\neach line indicates the standard deviation of the measurements from the mean.\n...\n(explaining Figure 11, see next page) Each data point is plotted with low opacity; overlapping\npoints appear progressively darker, indicating more time spent conditioning at that voltage.\nplotted with low opacity; overlapping points appear progressively darker, indicating more time\nspent conditioning at that voltage.\n5. Figures are presented out of order in the text: Figure 10 appears before Figure\n9.\nResponse: Thankyouforpointingthisout. Theorderiscorrectedintherevisedversion(Fig.\n8 → Fig. 9 → Fig. 10 → Fig. 11).\nWe sincerely thank the reviewer for their insightful comments and recommendations, which\nhave been carefully considered and implemented in the revised manuscript.\nIn what follows, the reviewer’s comments are in blue and our responses are in black. The original\nand revised parts of the manuscript are highlighted in red and green, respectively.\n1. WhywasMACORselected? Wasitstrictlyformachinability? WouldanAl2O3\ninsulator, or something else, provide superior performance?\nResponse: MACOR was chosen because it offers a combination of high dielectric strength\n(∼ 129 MV/m), low outgassing, and excellent machinability. Its machinability allows the com-\nplex fin features to be produced with conventional tooling, avoiding costly diamond machining.\nAlthough Al O is also an excellent insulator, its greater hardness makes fabrication of fins more\n2 3\nexpensive. Moreover, Al O has a higher relative permittivity (∼ 9.8 versus ∼ 6 for MACOR),\n2 3\nwhich slightly increases the triple-junction field. Fabrication of an alumina version of the bushing\nis currently underway and will be reported in a future work.\nRevised: On the vacuum side, the bushing comprises a central molybdenum cathode (99.9\n% purity), a concentric insulator machined from MACOR® machinable glass-ceramic, and a con-\ncentric 304L stainless-steel anode with a 20 cm ConFlat flange. MACOR—which consists of ap-\nproximately 55% fluorophlogopite mica and 45% borosilicate glass—was selected for its excellent\nvacuum compatibility, high dielectric strength, and ease of machining. Its machinability allows\nthe fins, 10–19 mm high with radii and thicknesses of 1.3 mm and 2.5 mm, respectively, to be\nproduced with conventional tooling. A knife-edge or O-ring groove is machined into the ceramic\nto seal against the stainless-steel flange. Additional design details are provided in [19].\n...\n...(in Conclusions section, discussing the future works) and (4) evaluating Hammerhead with\nalternative ceramics such as Al O which is currently under fabrication.\n2 3\n2. Some details of the potting process would be helpful. What epoxy was used?\nWas the feedthrough potted in a vacuum environment? A pressure chamber?\nResponse: Thank you for your suggestion. Details of the potting process are incorporated in\nthe revised version.\nRevised: TheconnectorhousesacoaxialX-raycableinsulatedwithethylene-propylenerubber\n(commerciallymarketedasC2236). Thecableispottedintotheconnectorusingatwo-partH1369\nepoxy resin1. The curing process is carried out in atmospheric pressure; the temperature is slowly\nramped up to ∼125◦C followed by a slow cooling to room temperature.\n3. Regarding both of the above points, what considerations were given to se-\nlecting materials with respect to their dielectric constants, noting that dielectric\nmismatches lead to some field enhancement?\ncanintensifyelectricfields. Inourassembly,MACORhasϵ ≈6,theepoxyhasϵ ≈4,thesilicone\nr r\ngasket has ϵ ≈ 3, and the EPR cable has ϵ ≈ 2.8. We have added a paragraph explaining that\nr r\nthese values were used in the finite-element simulations to ensure that the maximum field stress\nremained below the dielectric strength of each material.\nRevised: To limit permittivity mismatches at interfaces, the constituent materials are chosen\ntohavecomparablerelativepermittivity: MACORhasϵ ≈6,theepoxyϵ ≈4,thesiliconegasket\nr r\nϵ ≈ 3, and the cable dielectric ϵ ≈ 2.8. Finite-element simulations confirm that the resulting\nr r\nelectric-field stresses remain below the dielectric strength of each material.\n4. Itismentionedthatanomaliesinthevoltageandcurrentareusedtotriggerthe\ncamera. What is the insertion delay on the camera? Even for fast ICCD cameras,\nthe insertion delay is on the order of 100 ns and I would suspect the insertion\ndelayonthe5000fpscameraisevenhigher. Thisraisesthequestionof,whatdoes\nthe imaging capture? Presumably, this is some late-stage phenomena, but exactly\nwhat is not clear.\nResponse: We appreciate the insightful comment regarding the insertion delay of the high-\nspeed camera and its implications. According to the manufacturer, the delay between an external\ntrigger and the moment the sensor begins exposure (measured as the time between the trigger\ninput and the frame-sync output) is typically on the order of 200–350ns. The camera operated in\ncircular-buffer mode and was configured to save footage whenever a rapid transient in the cathode\nvoltage was detected. This ensured that each recorded video segment captured the onset of any\nvacuum arc or flashover event on the insulator. Because each frame integrates light over 200µs\nat 5kfps, the recordings do not resolve the nanosecond-scale formation of the plasma channel, but\ninsteadcaptureboththeinitiationandthesubsequentevolutionofthedischargeovermicrosecond-\nto millisecond-timescales. We have incorporated this information into the manuscript.\nRevised: Figure 9 shows a characteristic vacuum breakdown between the cathode and anode.\nThere is a 200–350 ns delay between the external trigger and the moment the sensor begins expo-\nsure. The camera operates in circular-buffer mode and is configured to save footage whenever a\nrapid transient in the cathode voltage is detected. Since each frame integrates light over 200 µs at\n5 kfps, the image captures not only the discharge initiation but also its subsequent evolution.\n5. The deconditioning data and discussion is interesting. Of course, such behavior\nhas been anecdotally reported by other authors as well. Generally, low energy\nsystems condition upward, and high energy systems condition downward. The\nformer does not have sufficient energy to sputter considerable material into the\nvacuumassembly,anddoesnothaveenoughenergytosubstantiallydamagethings.\nAthigherenergy,sputteringanddamagearepossible. Thus,itwouldbeinteresting\nto know how much energy is in your system and available to drive the discharge.\nThiswouldincludeanyavailableenergyinyourpowersupplyaswellastheenergy\ncapacitively stored in your HV coaxial cable.\nResponse: Thank you for pointing this out; we fully agree on the impact of stored energy on\nthe conditioning trend. As suggested, we have added both the total stored energy and its detailed\nbreakdown.\nRevised: The stored energy in the setup comprises the contributions from the power supply,\nthe coaxial cable, the high-voltage probe, and the bushing assembly. At 300 kV the power supply\n7\nstores approximately 167 J. The 6-m-long coaxial cable, with a capacitance of about 102 pF/m,\nyields a total capacitance of ∼ 612 pF. The high-voltage probe and the bushing assembly add\n20 pF and 28 pF, respectively, giving a combined capacitance of ∼ 660 pF, which corresponds to\n∼29.7Jat300kV. Asaresult, thetotalstoredenergywithinthesetupat300kV isabout197J.\n6. Note as few minor typos that may need reconsideration.\n- typo ”as is indicate” on page 5\n- Final sentence of pg. 6 (that goes to page 7) may need revision (it mentions that\nthe length is increased three-fold).\n- Space missing in the first paragraph of pg. 7.\n- Krypton and Molybdenum and other elements are sporadically capitalized.\n- Missing space in page 10 (Geiger-Muller detector sentence).\n- The sentence ”The voltage is raised in 2-kV steps and it is waited until cathode\ncurrent and X-ray emission are stabilized before stepping up the voltage again. ”\nmay need to be revised.\nResponse: We thank the reviewer for pointing out these errors. The revisions are detailed\nbelow.\nOriginal Revised\nas is indicate as is indicated\nthe sawtooth pattern of the insulator fins in- the sawtooth pattern of the insulator assists\ncreases surface flashover path length three- withsurfaceflashoverpreventioninthreeways\nfolds\nKrypton, Molybdenum krypton, molybdenum\nOccasionally, Further, Furthermore,\nThe voltage is raised in 2-kV steps and it is The voltage is raised in 2 kV increments and\nwaited until cathode current and X-ray emis- helduntilthecathodecurrentandX-rayemis-\nsionarestabilizedbeforesteppingupthevolt- sion stabilize before the voltage is increased\nage again again.\nWe sincerely thank the reviewers for their insightful comments and recommendations, which\nhave been carefully considered and implemented in the revised manuscript.\nIn what follows, the reviewers’ comments are in blue and our responses are in black. The original\nand revised parts of the manuscript are highlighted in red and green, respectively.\n1. -To use SI units\n- To use different symbols for Electric Field (e.g. F) and Energy (E)\n- To replace triple point with triple junction\n- p.2 cross → crosses\nResponse: We thank the reviewer for pointing out these errors.\nOriginal Revised\nTorr Pa\nsccm m3/s\nE (electric field) F (electric field)\ntriple point (everywhere) triple junction (everywhere)\ncross crosses\n2. On p.2 Actually the ITER bushing has been already successfully operated at\nfull voltage, i.e. up to 1 MV. [A. Kojima et al 2019 Nucl. Fusion 59 086042 DOI\n10.1088/1741-4326/ab1ab0]. The test configuration exploited additional shields\nto fully decouple the interaction among voltage stages and the vacuum chamber.\nSince, in the previous tests the breakdowns were due to direct BD between MV-\nconductorandground(notarealbushingissueduetoflashoveralongtheceramic)\nrather than due to flashover on the ceramic rings.\nResponse: Thank you for pointing out this oversight. The sentence in the introduction that\nimpliestheITER1MV bushinghasnotreachedfullvoltageiscorrectedcitingKojimaetal.,Nucl.\nFusion 59, 086042 (2019).\nOriginal: ...Anassemblyofsuchgrandscalereachesmetersinlengthandstillhasnotoperated\nat full voltage - highlighting the complexity of these sorts of problems.\nRevised: ...Aftermorethantwodecadessincetheinitialworkonthereferencedesign[11],\nfull\n1MVoperationwasreportedin2019[12]. AlthoughtheITERbushingisaspecializedcomponent,\nthe theoretical and experimental advances made during its development offer valuable insights for\nnext-generation vacuum bushing designs.\n[11] Watanabe, K., et al: Development of a multiaperture, multistage electrostatic accelerator for\nhydrogen negative ion beams. Review of Scientific Instruments 69(2), 986–988 (1998).\n[12] Kojima, A., et al: Demonstration of 1 MV insulation for the vacuum insulated beam source\nin the ITER neutral beam system. Nuclear Fusion 59(8), 086042 (2019).\n9\n3. Page3. withreferencetothesentence...achievingastable300kVinamarkedly\nsmaller voltage to volume ratio compared to ITER’s 1-MV bushing [8], it is not\ncorrect comparing the ITER’s 1MV bushing to other bushings, in particular with\nlower operating voltages, since the complexity/volume is strongly non-linear with\nthe voltage. Furthermore in the ITER’s bushing there are additional requirements\nsuchasmechanical,needsofcoolingchannelsfortheaccelerator,doubleinsulating\nbarrier for tritium safety, which enlarges the bushing size.\nResponse: We appreciate the reviewer’s insightful comment. We agree. We have revised\nthe Introduction section to remove direct comparison to the ITER bushing and instead com-\npare Hammerhead to compact 300 kV designs for the Jefferson Lab’s photogun and University of\nWisconsin-Madison’s inertial electrostatic fusion reactor.\nRevised: ...A well-known example is the high-voltage vacuum bushing developed for the In-\nternational Thermonuclear Experimental Reactor (ITER) neutral-beam injector which must pro-\nvide approximately 1 MV of insulation in vacuum. It employs five acceleration grids, layered\nceramic insulators and SF -gas barriers to meet the voltage requirement; together with mechani-\n6\ncal, cooling-channel and tritium-safety provisions, these features result in a meter-scale assembly\nwith a large footprint.\nSeveral development efforts have targeted the 300 kV regime. Notable examples include bush-\ning development efforts for Jefferson Lab’s photogun and the University of Wisconsin–Madison’s\ninertial electrostatic confinement devices...\n4. Onpage3,withreferencetothesentence...theseimprovementsresultinapprox-\nimately20%radialand25%axialsizereductioncomparedtothestate-ofthe-art,...\nthere is not a clear reference indicating the ”state of the art” where typical length\nrequired for a 300kV bushing is explicitly indicated. It is recommended providing\nthe main features of one or more existing bushing in the same voltage range and\nwithsimilarfeaturesandcomparingtheHammerheadtoitratherthantheITER’s\nbushing.\nResponse: Wehaveclarifiedthatthecomparisonismadetothemostcompact300kVbushing\nreportedintheliterature,namelyJeffersonLab’sphotogun,whichisthemostrelevantbenchmark\nat the 300 kV level targeted in this work.\nOriginal: Theseimprovementsresultinapproximately20%radialand25%axialsizereduction\ncompared to the state-of-the-art.\nRevised: Compared to Jefferson Lab’s 300 kV bushing—which, to the authors’ knowledge, is\nthe most compact design in its voltage class reported in the literature—Hammerhead reduces the\nradial dimension by roughly 20% and the axial dimension by about 25%, while avoiding the need\nfor pressurized gas, sub-micron polishing, ultra-high-vacuum conditions or high-grade cleanroom\nenvironments.\n5. On page 4, Referring to fig. 1, the description of the Hammerhead bushing\nshould be accompanied by more details . e.g. :\n- potting material and relevant electrical characteristics.\n- cable type (special or commercial; material and relevant electric characteristics).\n- rubber gasket material and relevant characteristics.\n-detaileddesignoftheMACORinsulator(e.g. heightofthefins,finstipradii)and\nmaterial relevant characteristics. To indicate also what MACOR is (machinable\nglass ceramic). MACOR is a TM.\nResponse: Thank you for pointing this out. We expanded the section to describe the potting\nprocess,thecabletype, therubbergasket, andtheceramic,includingitscompositionandgeomet-\nric parameters.\nOriginal: On the air side, the high-voltage connection to the cathode is achieved via an inter-\nconnect system comprised of a coaxial cable and a custom flat connector, manufactured by Parker\nMedical Inc., which interfaces with the air-side of an electrically insulating ceramic (MACOR).\nA compressed silicone rubber gasket placed between the two flat surfaces eliminates any air gap,\nensuring a reliable dielectric seal.\nOn the vacuum side, the bushing features a central high-voltage cathode composed of 99.9%\nmolybdenum,aconcentricinsulatingstructuremadeofMACORwithsawtoothsurfacefeaturesto\nmitigate field enhancement and increase flashover path length, and a concentric anode fabricated\nfrom 304L stainless steel. More design details can be found in [15].\nRevised: On the air side, the high-voltage connection to the cathode is made via a custom\nflat connector of 20 cm outer diameter. The connector houses a coaxial X-ray cable insulated\nwith ethylene-propylene rubber (commercially marketed as C2236). The cable is potted into the\nconnector using a two-part H1369 epoxy resin1. The curing process is carried out in atmospheric\npressure; the temperature is slowly ramped up to ∼ 125◦C followed by a slow cooling to room\ntemperature. A compressed silicone-rubber gasket between the flat faces of the connector and\nthe ceramic insulator eliminates any air gap and ensures a reliable dielectric seal. The cable’s\ngrounded shield is terminated at the entry to the connector assembly, while the semi-conductive\nlayer extends about 1 cm into the potting to reduce the electric stress at the interface.\nOn the vacuum side, the bushing comprises a central molybdenum cathode (99.9 % purity), a\nconcentric insulator machined from MACOR® machinable glass-ceramic, and a concentric 304L\nstainless-steelanodewitha20cmConFlatflange. MACOR—whichconsistsofapproximately55%\nfluorophlogopite mica and 45% borosilicate glass—was selected for its excellent vacuum compat-\nibility, high dielectric strength, and ease of machining. Its machinability allows the fins, 10–19\nmm high with radii and thicknesses of 1.3 mm and 2.5 mm, respectively, to be produced with\nconventional tooling. A knife-edge or O-ring groove is machined into the ceramic to seal against\nthe stainless-steel flange. Additional design details are provided in [19].\n6. Onpage5,TheAuthorshouldexplainbetterthemeaningof“liberalthreshold”\nand the value of 32 MV/m should be justified, adding references or rationale. At\np. 7isreportedinsteadthemaxelectricfieldonthecathodesurface,\nequalto36.7\nMV/m. Why it is larger than the threshold?\nResponse: Thank you for pointing this out. We have revised the design threshold to the\ncorrectvalueof40MV/m. Theterm“liberal”referstochoosingavalueapproximately60%lower\nthan the switch-on field previously reported for molybdenum∗. The correction and accompanying\nclarification have been added.\n∗ Furuta, F., et al: Reduction of field emission dark current for high-field gradient electron\ngun by using a molybdenum cathode and titanium anode. Nuclear Instruments and Methods in\n538(1), 33–44 (2005)\nOriginal: The particular design of the cathode structure was developed to keep surface elec-\ntric fields under 32 MV/m which represents a liberal threshold for preventing the activation of\nadditional emission sites.\nRevised: ...For these reasons, molybdenum is selected as the cathode material, with a geom-\netry designed to keep the surface electric field below 40 MV/m, which is about 60 % lower than\nthe switch-on field for molybdenum reported in [26].\n7. On page 5, Referring to the breakdown nucleation process induced by the\nmechanicalstimulateddislocationmovement,itisdeclaredthatatextremeelectric\nfield (w/o β enhancement) the mechanical stress (electrostatic pressure) is in the\norder of tens MPa. Actually, the electrostatic pressure @100 MV/m is 44.2 kPa.\nAsamatteroffact,suchastresscaninducedislocationmovementincopper,butit\nis not proved that it is sufficient to cause dislocation movement in other materials\n(e.g. molybdenum). The Authors should address better this point.\nResponse: Thank you for raising this point. In the present context, we strictly focus on the\nMaxwell stress acting on field-enhancement features, where the electrostatic pressure is defined\nas 1ε (βE)2. Near these features, the local electric field can reach the GV/m range, leading to\n2 0\npressures exceeding several MPa and potentially reaching tens of MPa. We have clarified this in\nthe text.\nOriginal: When electromagnetic fields reach extreme levels, mechanical stresses can exceed\ntens of MPa, crossing a critical threshold and resulting in dislocation movement.\nRevised: When the local electric field near an emission site reaches the GV/m level required\nforfieldemission,mechanicalstressescanexceedtensofMPa,potentiallycrossingacriticalthresh-\nold and resulting in dislocation movement.\n8. The increased electron emission from triple junction is due by the field en-\nhancement at the triple junction, but it is also accompanied by a reduction of the\npotential barrier due to the presence of the insulator.\nResponse: Thank you for raising this point. We have included this nuance in the revised\nmanuscript.\nRevised: In addition to mismatched relative permittivities, the reduced potential barrier due\ntothepresenceofthedielectricalsocontributestoelevatedelectronemissionatthecathodetriple\njunction.\n9. On page 6, Maybe the Authors mean “the sawtooth pattern of the insulator\nfins reduces the breakdown probability three-folds:”\nOriginal: the sawtooth pattern of the insulator fins increases surface flashover path length\nthree-folds:\nRevised: the sawtooth pattern of the insulator assists with surface flashover prevention and\nmitigation in three ways:\n10. Page 7, To avoid trivial sentences like “The equipotential lines are the densest\nimmediately adjacent the cathode, indicating that the electric field intensity is the\nhighest at the cathode surface”\nResponse: We have rephrased this statement.\nOriginal: Theequipotentiallinesarethedensestimmediatelyadjacentthecathode,indicating\nthat the electric field intensity is the highest at the cathode surface. Figure 3b indicates that the\nmaximum electric field at its rounded edges can reach 36.7 MV/m,...\nRevised: As confirmed by Figure 3b, the electric field is highest near the cathode surface,\nwhich is characteristic of the coaxial electrode configuration. The maximum electric field on the\ncathode occurs at its rounded top edges and reaches 36.7 MV/m, ...\n11. On pages 7/8 . Referring to the computational analysis:\n- the electric field calculation seems to be based on pure electrostatic analysis (i.e.\ndepending only by geometry and permittivity). This analysis is valid if time scale\nof the applied voltage (e.g. with ac 50 Hz voltage application) is much lower than\nthetransitiontimeconstantfromelectrostaticdistributiontocurrentdistribution,\nwhichisevaluatedasτ =ρ.ϵ .ϵ =1014×6×8.85×1012 =5310s(1h:28′)(using\n0 r\nMACOR as main insulating material for the resistivity and dielectric constant).\nIn this case, the dc voltage applied time results much longer than such a time\nconstant, so the electric field distribution calculation must be performed using the\ninsulator bulk and surface conductivity. Often, the difference between the two\ndistributions appears at the triple junction.\n-TheexactpositionoftheTripleJunctionisnotvisibleinfig3. Indicatetheexact\ncontact point between conical part of insulator and cathode hollow screen. Also, a\nzoomviewofthefieldonTJwouldbeusefultounderstandthesimulatedgeometry\nat the interface. How can you be sure that this result is not mesh-size dependent?\nThe exact field at TJ is usually strongly dependent on shape of interface and in\nsome cases diverges to infinite. The authors should address the above mentioned\npoints.\nResponse: Thankyouforyourinsightfulcomment. Peryoursuggestion,intherevisedanalysis\nanelectro-quasistaticsimulationisperformedinwhichtheconductivityofeachmaterialisincluded.\nAsmentioned,thisincreasesthefieldatthetriplejunctionfrom0.34MV/mtoabout1.04MV/m,\nwhich is still well below the switch-on field for electron emission. The revised manuscript presents\nthe updated results, include a zoomed-in inset of the triple junction in Fig. 3, and discuss mesh\nindependence; the mesh is refined until the maximum field changed by less than 1%.\n13\n(a) Electric potential (b) Electric field (c) Electric field near\ndistribution. distribution. cathode triple junction.\nFigure 3: Two-dimensional, axisymmetric, steady-state electro-quasistatic simulation of Hammer-\nhead at −300 kV (r–z cross-section).\nRevised: The 2-D axisymmetric model is discretized using approximately 7.1×105 triangular\nelements (∼ 3.6×105 vertices) together with 15,768 edge elements, yielding an average element\nqualityof0.90andaminimumqualityof0.018. Theboundarymeshontheceramic–metalinterface\ncontained15,744verticesandexhibitedanaveragegrowth-ratequalityof0.97. Amesh-refinement\nstudy is carried out in the cathode triple junction region by successively doubling the local mesh\ndensity until the computed peak field changed by less than 1%. The electric potential and electric\nfield distributions are shown in Figure 3) for the cathode at the nominal voltage (−300 kV).\n...\nFigure 3c shows the electric field distribution near the vicinity of the most critical spot, the\ntriple junction, which undergoes an electric field of about 1.04 MV/m. ...\n12. On pages 8/9/10. These observations are related to the Field emitted Electro\nTrajectories, in connection with the Surface Charging paragraph.\n1. Fig. 4 should indicate the trajectories of the electrons, differentiated (e.g. with\ncolours) by energy. What is reported is simply the potential map. Moreover, in\nthe figure it seems that no electron passes beyond the convolution surface of the\ninsulator, and then no charging due to impinging electrons is present. This is\nunlikely.\n2. Itisunclearifthefluxoftheelectronsemittedbythecathodeiscalculatedusing\ntheFNemissionformula,andwhichhypothesisisdoneconcerningthe (enhancing\nelectric field factor) distribution along the cathode surface. From this calculation\ncan be derived at least a rough estimation of the insulator surface charge.\n3. Are the electron trajectories calculated in classical or relativistic motion? For\ntheelectronstherewouldbeachangeofthetrajectorycomparablewiththeradial\ndimension of the fins.\n4. At page 9, it is stated that the negative charge deposited by electrons from\n14\nthe cathode “slows down” the following electron, until the impact energy decrease\nfrom 130 keV (energy at the first fin) to 1-4 keV (SEY =1) energy indicated in\nthe fig. 5 plot. This conjecture implies that the surface charge deposited on the\ninsulator are sufficient to create a very strong modification of the potential lines.\nThis fact means that:\no Such negative charge on the insulator surface could produce intensification of\nlocal electric field in the bulk of the insulator and along its surface, potentially\nabove its dielectric strength.\noSuchadramaticchangeoftheelectricpotentiallineswouldhaveagreatinfluence\non the trajectory of the electrons.\no We guess that the SEY curve in presence of surface charge density could be no\nlonger applicable.\noDotheauthorsbelievethatthegoodperformanceofthebushingaredetermined\nby the electrical charging of the MACOR insulator?\nThe Authors should address the a.m. points and to make some rough calculation\nif the conjecture at point 4 is valid.\nResponse: Thank you for the detailed comment. We emphasize that the simulation of field-\nemitted electron trajectories is only a preliminary tool; it is not a fully self-consistent model of\nsurface charging. Even so, it provides a qualitative picture of charging trends that informs the\ndesign. Our responses to the specific points are given below.\n12.1Atthereviewer’ssuggestion,therevisedfigureusesalowerparticledensitysothatindividual\ntrajectories are clearly distinguishable. The simulation does not include insulator charging and is\nintended solely as an initial step toward understanding charging patterns; additional explanation\nhas been added to the text.\n12.2 The density of emitted electrons is calculated with the Fowler–Nordheim formula and no\nfield-enhancement factor, because the distribution of β values is unknown. This simplification is\nnow stated explicitly in the manuscript.\n12.3Relativisticcorrectionsareincorporatedinthesimulation; thispointisclarifiedintherevised\nversion.\n12.4 The insulator surface acquires negative charge, which slows subsequent electrons. Whether\ntheelectronfluxissufficienttoreducetheirenergytoE cannotbedeterminedwithoutknowledge\n2\nof the emitter distribution on the cathode. Post-experiment inspection shows that the insulator\nremains nearly intact, suggesting that the negative charging is not high enough to create local\nfields exceeding the dielectric strength. The secondary-electron-yield curve in Fig. 7 pertains only\nto the energy distribution in Fig. 5. Although we cannot attribute the good performance solely\nto charging, we hypothesize that negative surface charge locally reduces the electric stress on the\nmetal. Because MACOR has a very high dielectric strength (∼ 129 MV/m), it can tolerate the\nresulting local fields.\nRevised:\nIn this section a preliminary assessment of electron trajectories is performed in COMSOL Multi-\nphysics. At an average electric field of 18.6 MV/m the cathode emits electrons toward the anode,\nand some of these electrons bombard the insulator surface. Figure 4 shows the electrons’ kinetic\nenergy throughout the domain, originating from the cathode surface and eventually impacting ei-\nthertheanodeortheinsulator. Notethatthissimulationdoesnotincludesurfacecharging,butas\nshownlaterinthissection,itassistswithprovidingaqualitativepictureofhowchargingdevelops.\nThe density of electrons born from the cathode surface follows the Murphy–Good formula\n(see Equation 1). No field enhancement factor is included given the lack of information on the\nemission.\nFigure 4: The trajectory and kinetic energy of field-emitted electrons prior to insulator charging.\nmagnitude or spatial distribution. Relativistic corrections are applied in the simulation, since at\n300 keV the electron speed approaches a significant fraction of the speed of light.\n...\nWhether the electron flux is sufficient to reduce the energy to E cannot be predicted without\n2\nknowledgeoftheemitterdistributiononthecathode. However,post-experimentobservationsshow\nthat the insulator remains nearly intact, which suggests that the negative charging is not extreme\nenough to cause local field concentrations exceeding the dielectric strength. Furthermore, if the\ninsulatornearthecathode(i.e.,thefirstfin)accumulatesnegativechargeduetosecondaryelectron\nemission,thisnegativepotentialwilllocallyreducetheelectricfieldstrengthatthecathodesurface,\nthereby weakening field-emission current.\n13. On page 12, More detail should be given describing the Kr conditioning (e.g.\nKr pressure, current, estimated ion implantation...)\nkrypton conditioning.\nOriginal: Various gases, including He, Ar, N , O , and H , have been used for gas condition-\n2 2 2\ning in the literature [42]. In this work krypton is chosen due to its effectiveness at higher voltages\nbecause of its greater mass, and thus higher sputtering yields. Moreover, krypton ions implant at\nshallower depths compared to lighter gases such as helium, which has been shown to enhance the\nsurfaceworkfunction[43]. Theflowisintroducedataraterangingbetween0.5sccmand10sccm\nas is often seen in the literature [8]. Similar to as in the current conditioning method, voltage is\nincreased in small increments and held until dark current and X-ray emission are stabilized.\nRevised: In this work, krypton (Kr) is used while various other gases, including He, Ar, N ,\n2 2\nat a rate ranging between 8.33×10−9 m3/s and 1.67×10−7 m3/s, which at the highest, raises\nthe background pressure to ∼ 0.15 Pa. The benefits of using Kr for gas conditioning have been\ndiscussed in the works done at Jefferson Lab [11, 46]. In the case of Hammerhead, heavy Kr+ ions\nbombarding the cathode have a similar atomic mass to molybdenum (84 vs. 96 amu), so at ener-\ngies above ∼ 1 keV a single ion can eject multiple Mo atoms. This efficient sputtering rounds off\nmicro-tipsandsmoothsnanoscaleasperities—featuresthatconcentratefieldemission. Lighterions\nsuch as He+ have smaller sputtering effect. Krypton ions that do not sputter away are implanted\nwithin the upper tens of nanometers of the cathode (e.g. a 300 keVKr+ ion has a projected range\nof ∼ 63 nm), and these implanted noble gases can raise the local work function on the order of\n1 eV [46]. The combination of controlled sputtering and shallow implantation reduces tunneling\ncurrentsandsuppressesfieldemissionwithoutsignificantbulkmodification,althoughexcessiveion\nbombardment can roughen the surface.\n14. On page 13, The difference in dark current between current and gas condi-\ntioning is defined at higher voltage as “slight”, when they differ of a factor 3-4.\nResponse: Thank you for pointing this out. We have corrected the wording in the revised\nversion.\nOriginal: However, at higher voltages, gas-conditioned samples tend to exhibit slightly ele-\nvated dark-current levels.\nRevised: However,athighervoltages,gas-conditionedsamplestendtoexhibitelevateddark-current\nlevels.\n15. On page 13: It seems there is a contradiction between the sentences:\n“ionsareimplantedinthecathodesurface,ultimatelyincreasingtheworkfunction\nof the metal, which reduces the likelihood of electrons tunnelling from the cathode\nsurface, as shown in Equation 1” and the observation\n“gas-conditionedsamplestendtoexhibitslightlyelevateddarkcurrentlevels. This\nincrease is attributed to enhanced ion-induced secondary electron emission, which\ncan elevate the overall dark current”.\nThe Author should address this point.\nResponse: These two are not mutually exclusive. Ion implantation increases the cathode\nworkfunctionandreducesfield-emittedcurrent. However,thesamekryptonionscanalsoenhance\n17\nsecondaryelectronemissionwhentheystriketheinsulatororcathode, resultinginadditionaldark\ncurrent. Duringgasconditioning,thenetcurrentisthesumofreducedfieldemissionandincreased\nion-induced secondary emission. We have added a paragraph to reconcile these competing effects:\nunder krypton backfill, the dominant mechanism at high voltage becomes ion-induced secondary\nemission, which is why the total current appears higher even though emitters are being removed.\nRevised: In addition to sputtering and implantation, the ionization of krypton molecules by\nelectronimpactandtheirsubsequentcollisionwiththecathodecanliberatemorethanoneelectron\nper incident ion, depending on the impact energy. This secondary emission can increase the dark\ncurrent. The interplay between the raised work function and the enhanced secondary emission\ncreates a situation in which, depending on the operating parameters, the krypton backflow may\neither increase or decrease the total dark current.\n16. On page 14/16: The Authors should improve clarity in their observation. For\nexample,\n- it is stated that between days 9 and 13 the current is approx. 80 µA, when from\nfig. 11 the current jumps from 40 µA to 120 µA from day 11 to 12, with a voltage\nincrease from 250 kV to 270 kV.\n- how one can infer from figure 11 that transitioning from gas conditioning to\ncurrent conditioning proves challenging ? What’s the meaning ? Is it related to\ntheobservationthatthecurrentfromgasconditioningtohighvacuumconditioning\nincreased from 50 µA to 80 µA ?\n- what the Authors do mean withThis observation indicates that the effect of gas\nconditioningismorecomplexthansimplybeingatoolforremovingtoughemitters?\nResponse: Your detailed insights are much appreciated. The dark current fluctuates between\n40 µA and 120 µA as the voltage is increased from 250 kV to 270 kV, averaging around 80 µA;\nwe\nhave specified this range instead of giving a single value. Regarding the transition, we observed\nthat when krypton backfill was removed at 300 kV the dark current rose abruptly, indicating that\nthe beneficial effects of sputtering and ion implantation diminish without the gas present. The\nstatement that gas conditioning is “more complex” means that krypton does more than simply\nburn off emitters; it temporarily increases the cathode work function and modifies surface charge,\nbut these benefits can disappear once the gas is removed. We have expanded this explanation and\ninterpret the data accordingly.\nBetween days 9 and 13 conditioning proceeds Between days 9 and 13 conditioning proceeds\nnominally with no major de-conditioning nominally with no major de-conditioning\nevents, and a relatively low dark current of events,andanaveragedarkcurrentofapprox-\napproximately 80 µA until 300 kV. imately 80 µA until 300 kV. While no major\nsetback is observed in this period, the dark\ncurrent increases from 40 µA at 250 kV on\nday 11 to 120 µA at 270 kV on the next day.\nHowever, upon reaching the design voltage, Upon reaching the target voltage of 300 kV,\ntransitioningfromgasconditioningtocurrent transitioning from gas conditioning to cur-\nconditioning proves challenging. As is ob- rent conditioning proves challenging. On day\nserved on day 14, removing the added Kryp- 14 the system reaches 300 kV under kryp-\ntonbackfillandbringthesystembacktohigh- ton backflow, but the same voltage cannot be\nvacuum operation is not immediately achiev- maintained when the system is returned to\nable at 300 kV. This observation indicates high vacuum without krypton. This observa-\nthattheeffectofgasconditioningismorecom- tion indicates that the effect of gas condition-\nplex than simply being a tool for removing ing is more complex than simply being a tool\ntough emitters. This is likely due to implan- for removing tough emitters. A plausible ex-\ntationofKryptonionswhichelevatethecath- planation is that the increase in background\node’sworkfunction. Thisaddedbenefitseems pressure due to krypton backflow enhances\nto only last as long as the backfill is opera- scattering of electrons and ions, thereby re-\ntional, as soon as Krypton backfill is stopped ducing the probability that they traverse the\nthe benefit goes away. gap with energies near the full potential. Re-\nversible removal of implanted krypton upon\nreturning to high vacuum is unlikely, as such\ndesorptiontypicallyrequiresbake-outtemper-\naturesthatarenotreachedduringthesetests.\nTo compensate, the conditioning is pushed Next, the krypton backflow is reintro-\ngradually to 330 kV (10% safety factor) en- duced and the voltage is raised to 330 kV,\nabling ion implantation to occur at higher en- 10 % above the nominal operating voltage.\nergies and potentially deeper within the sur- ... The\nface. This additional exposure is intended to success of krypton conditioning at higher\nenhanceemitterpassivationandsurfacestabi- voltage can be attributed to two effects:\nlization, thereby improving post-conditioning (1) the longer conditioning duration allows\nperformance under vacuum. sputtering of stubborn emission sites until\nthe emitter flux becomes too low to initiate\nbreakdown, and (2) the higher energy of Kr+\nions increases the likelihood of implantation,\nleadingtogreateremitterpassivationthrough\nan increased work function.\n19\n17. On page 16, p.16 observation - mandatory It is stated that the concentra-\ntion of the clumps can be due to local field concentration. We remind that the\ntrajectory of a charged particle in a electrostatic field is determined only by the\ngeometry and by the voltage, not by its charge and mass, provided the initial ve-\nlocity is to zero. A local concentration of the electric field exerts a force acting\nonly along the length in which the electric field line deviates from the ideal one.\n(see N. Pilan, P. Veltri and A. De Lorenzi, ”Voltage Holding Prediction in Multi\nElectrode-multi Voltage Systems Insulated in Vacuum,” in IEEE Transactions on\nDielectrics and Electrical Insulation, vol. 18, no. 2, pp. 553-560, April 2011, doi:\n10.1109/TDEI.2011.5739461).\nResponse: We appreciate the reviewer’s comment and the reference provided. We agree that,\nfor particles released from rest, their trajectories in an electrostatic field depend only on the field\ngeometry and are independent of mass or charge. However, we cannot rule out that some clumps\nare detached with non-zero initial velocities. In our experiments, clusters of copper and silver par-\nticulatesarefoundatthesamelocationsonthecathode,suggestingthatcertaindetachmentevents\nmay impart initial momentum to the particulates; in such cases the mass and charge distribution\ncould influence their trajectories. We have acknowledged this possibility and cite the suggested\nreference in the revised manuscript.\nOriginal: These clumps often appear clustered together indicating that they are either clus-\ntered prior to acceleration to the cathode, or that they are accelerated to a given point due to a\nlocalfieldconcentration. Suchfieldenhancements arecoveredthoroughlyin areviewby Noerand\nas such will not be covered in depth here [45-48].\nRevised: Post-experiment surface analysis shows that clumps of different materials (e.g., Au\nand Cu) are clustered at discrete sites, suggesting either pre-aggregation before accel- eration or\nelectrostatic focusing to specific impact points by local field enhancements (see [43–46] for discus-\nsionsoffield-enhancementfeatures). Asshownin[7],thetrajec-toryofachargedparticlereleased\nfrom rest in a static electrostatic field is governed by the field geometry and applied voltage (with\nthedirectionsetbythesignofthecharge),andthusparticlesfollowfieldlines. Incontrast,ifsome\nclumps are ejected with nonzero initial velocities or possess different charge-to-mass ratios (q/m),\ninertial effects can modify their paths; combined with electrostatic focusing, this can produce the\nobservedclustering. Furtheranalysisofthesehypotheseswillbepresentedinafuturepublication.\n18. (General Comment) The paper is divided into two clearly distinct parts. The\nfirstpart(Par2-3)isrelatedtothemechanismunderlyingthebreakdowninitiation\nand other processes like surface charging of the insulator. The second part (Par.\n4) is related to the experimental results. The effort deployed by the Authors to\nput in relationship the first with the second part is appreciable, but there are not\nevidencesthatexperimentalfindingsarealwaysclearlyassociatedtothemechanism\nand processes elucidated in the first part. For this reason, it is suggested to skip\npartially the content of the first part, focusing on the clearer correlation between\nmodelandresultsandgiveinsteadmoredetailsontherealizationandtestingofthe\nbushingprototype,whichisactuallythemostinterestingpartofthiscontribution.\nIn particular, the observations done regarding the different effects of the gas and\nhigh vacuum conditioning should be better formulated.\nResponse: We appreciate this perspective. In the revision we have condensed the overview of\nbreakdownmechanismswhilebetterhighlightinghoweachoneofthesemechanismshasinfluenced\n20\nour design decisions. Also, in the revised version, the correlation between the processes and the\nexperimental results are expanded in the results section.\n19. (General Comment) Instead making comparison with the ITER bushing\n(feedthrough) for the Neutral Beam Injector - which is an extremely specific\ncomponent - it would be much more effective making detailed comparison with\nfeedthroughs of the same insulation range (200 kV-400 kV)\nResponse: We fully agree and have made this change throughout the manuscript. The com-\nparison to the ITER 1 MV bushing has been removed, except for a brief introduction noting that\ntheadvancesachievedduringitsdevelopmentprovidevaluableinsightsfornext-generationvacuum\nbushings. Instead, we now compare Hammerhead with two designs operating at the same 300 kV\nlevel: the Jefferson Lab photogun bushing and the University of Wisconsin–Madison inertial elec-\ntrostatic confinement (IEC) bushing.\nWe sincerely thank the reviewer for their insightful comments and recommendations, which\nhave been carefully considered and implemented in the revised manuscript.\nIn what follows, the reviewers’ comments are in blue and our responses are in black. The original\nand revised parts of the manuscript are highlighted in red and green, respectively.\n1. Suggestion - Introduction\nConcerningthe100kVcommercialfeedthrough,it’ssuggestedtomakeareference\nto one or more specific products. Moreover, in order to compare correctly the\nHammerhead feedthrough with the commercial ones, it is suggested to give at\nleastanestimationoftheoperatingvoltage(orratedvoltage)oftheHammerhead.\n300 kV are in fact to be considered as test voltage.\nResponse: Wethankthereviewerforthisconstructivesuggestion. Intherevisedmanuscript,\nwe have clarified the comparison by citing representative examples of three commercially available\n100 kV feedthroughs.\nAdditionally, we have revised the description of the Hammerhead feedthrough in the abstract\ntodistinguishbetweenitstestvoltagethathasbeenupto330kVanditsoperatingvoltage,300kV.\nOriginal:...Hammerhead,anultra-compactvacuumbushingthatcanbeusedreliablyat300kV.\nRevised: ...Hammerhead, a compact vacuum bushing that has been tested up to 330 kV and\nused reliably at 300 kV.\n...\n(footnote with regard to commerical 100kV feedthrough):\n[1] Examples of commercially available 100 kV feedthroughs include, but are not limited to, Kurt\nJ. Lesker Company (model EFT1C12156A), MPF Products Inc. (model A20322-1-CF), and Cer-\namTec North America LLC (model 6722-01-CF).\n2. Change text request - Introduction - mandatory\nChange: “Itemploysfiveaccelerationgrids,layeredceramicinsulatorsandSF6-gas\nbarrierstomeetthevoltagerequirement”into“Itisformedbyfivestages,madeby\nlayered ceramic insulators and SF6-gas barriers to meet the voltage requirement”.\nResponse: Thank you for the proposed revision. The sentence in the introduction has been\nmodified as suggested.\nOriginal: ...It employs five acceleration grids, layered ceramic insulators and SF -gas barriers\n6\nto meet the voltage requirement.\nRevised: ...It is formed by five stages, using layered ceramic insulators and SF gas barriers\n6\nto meet the voltage requirement.\n3. Clarification request - Field emitted electron trajectory\nClarifyifthecurrentemissioniscalculatedforanaveragefieldof18.6MV/morit\n2\nis used the Murphy-Good formula (eq. 1). It seems that such electric field is the\nlower limit for FN emission.\nResponse: We thank the reviewer for raising this point. As clarified in the second paragraph\nof the Field Emitted Electron Trajectories section, the density of emitted electrons is obtained\nusing the Murphy–Good formula (Eq. 1) without a field-enhancement factor, as the distribution\nof β values is unknown. Thus, the average field of 18.6 MV/m was not employed.\nOriginal: The density of electrons born from the cathode surface follows the Murphy–Good\nformula (see Equation 1). No field enhancement factor is included given the lack of information\non the magnitude or spatial distribution.\n4. Clarification request - Experimental result\nIt is stated that “A plausible explanation is that the increase in background\npressure due to krypton backflow enhances scattering of electrons and ions, thereby\nreducing the probability that they traverse the gap with energies near the full\npotential”. At 0.15 Pa (your max operating Kr pressure) the mean free path of\ngas molecules is of the order of centimetres (lower distance between cathode and\nanode). Do you mean that the scatter can occur because of the fulfilling of this\ncondition? Ifyes,itwouldbebetteraddingfewwordsabouttosupportyourexpla-\nnation. Onthecontrary,iftheKriseffectivealsoatpressuresorderofmagnitudes\nlower than 0.15Pa other physical processes may play a role.\nResponse: Wethankthereviewerforthisvaluableobservation. Ourinterpretationisbasedon\nthe fact that, at a krypton pressure of approximately 0.15 Pa, the mean free path of gas molecules\nbecomes comparable to the cathode–anode distance, leading to a non-negligible probability of\nelectron and ion scattering. This has been clarified in the revised manuscript.\nAtpressuressignificantlylowerthan0.15Pa,weagreethatthiseffectisunlikelytoexplainthe\nobserved improvement, and other physical processes, such as secondary electron emission or field-\ninduced desorption, may be involved. This has been discussed further in the following paragraph.\nOriginal: A plausible explanation is that the increase in background pressure due to krypton\nbackflow enhances scattering of electrons and ions, thereby reducing the probability that they\ntraverse the gap with energies near the full potential.\nRevised: A plausible explanation is that the increase in background pressure due to krypton\nbackflowenhancesscatteringofelectronsandions,sincethemeanfreepathbecomesshorter,\nthereby reducing the probability that they traverse the gap with energies near the full potential.\n5. Change text request - Post-Experiment Microscopy and Surface Analysis -\nMandatory\nThewords“andtheappliedvoltage”shallberemovedbecause,iftheinitialvelocity\nV0 is zero, the particle trajectory is not affected neither by the q/m value, nor by\nthe voltage (for non-relativistic motion)\nResponse: Thank you for pointing this mistake out. The sentence has been corrected as\nsuggested.\nelectrostatic field is governed by the field geometry and applied voltage (with the direction set by\nthe sign of the charge), and thus particles follow field lines.\nRevised: Asshownin[7],thetrajectoryofachargedparticlereleasedfromrestinastaticelec-\ntrostatic field is governed by the field geometry (with the direction set by the sign of the charge),\nand thus particles follow field lines.\n6. Suggestion - Vacuum Breakdown\nWegobackagainonthebreakdowninducedbydislocation. Youconsiderthatthe\nstress that triggers the dislocation movement is in the MPa range because in the\ncalculation of the stress caused by the electric field, you include the enhancement\nfactorofthetip. Butsuchatipappearsbecauseofthedislocationmovement. The\nsuggestion is to reconsider the statement.\nResponse: Thank you for the insightful comment. We agree that an initial field-enhancing\nfeature or local geometry is required to raise the stress from the kPa to the MPa or GPa range.\nOur revised text clarifies that field-induced stresses are plausible with localized enhancement but\nmay not be sufficient to initiate dislocation motion. Instead, dislocation activity may precede and\ngenerate surface asperities that subsequently amplify local electric fields and stresses, creating a\nfeedback loop between mechanical, thermal and electrostatic effects.\nOriginal: WhenthelocalelectricfieldnearanemissionsitereachestheGV/mlevelrequiredfor\nfieldemission,mechanicalstressescanexceedtensofMPa,potentiallycrossingacriticalthreshold\nand resulting in dislocation movement. These dislocations lead to surface extrusions that increase\nthe emission current and local field concentration [31].\nRevised: When the local electric field near an emission site reaches the GV/m level required\nfor field emission, mechanical stresses can exceed tens of MPa, which may approach the threshold\nfor dislocation movement. The interplay between surface defects, field-emission currents (causing\nJoule heating and Nottingham-effect-related thermal effects), and dislocation activity may further\nenhance the local field, establishing a feedback loop among mechanical, thermal, and electrostatic\nprocesses [31].\n4"}]

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