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# CIGALE: An innovative gas neutralizer based high efficiency neutral beam injector concept for future fusion reactors

Alain Simonin, Hugo Bouvard, Basile Pouradier Duteil

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Alain Simonin, Hugo Bouvard, Basile Pouradier Duteil. CIGALE: An innovative gas neutralizer based high efficiency neutral beam injector concept for future fusion reactors. Nuclear Fusion, 2024, 64, pp.096029. 10.1088/1741-4326/ad645f. hal-04653002v2

HAL Id: hal-04653002

https://amu.hal.science/hal-04653002v2

Submitted on 27 Feb 2025

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Distributed under a Creative Commons Attribution 4.0 International License
# CIGALE: an innovative gas neutralizer based high efficiency neutral beam injector concept for future fusion reactors

A. Simonin¹*, H. Bourvard² and B. Pouradier Duteil¹

¹ CEA, IRFM, F-13108 St Paul lez Durance, France

² ARKADIA Energy, Aix-en-Provence, F-13290, France

E-mail: alain.simonin@cea.fr

Received 25 March 2024, revised 24 June 2024

Accepted for publication 17 July 2024

Published 5 August 2024

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CrossMark

## Abstract

This paper outlines the main features of a new high efficiency ($\eta > 62\%$) high power (~18 MW D⁰) neutral beam (NB) concept based on pragmatic solutions suitable with the reactor requirements. The injector is modular (several beamlines in parallel) with independent ion sources referenced to the ground potential and gas neutralizers held at +1 MV. This topology leads to numerous simplifications; it overcomes the main issues of conventional NB systems, such as the complex 1 MV electrical setup, the difficult ion source remote maintenance, the high caesium consumption. The other key parameter is the gas neutralization concept which minimizes the amount of gas by operating at a low gas target and low neutralizer duct conductance. The implementation of an energy recovery system for the residual 1 MeV D⁻ is essential to attain a high wall-plug efficiency. These specific features require thin laminar D⁻ beams provided by a pre-acceleration up to 100 keV in slotted grid apertures to form thin blade-like beamlets, followed by the post-acceleration to 1 MeV by merging the beamlets in a single beam in five gaps (+200 kV per gap). All these specific aspects minimize the beams losses and thermal loads along the beamline and enhance the injector reliability and availability.

Keywords: reactors, neutral beam injector, high efficiency, gas neutralizer, recovery energy

## 1. Introduction

The achievement of future D–T fusion plasmas represents outstanding challenge for all types of heating systems to meet essential reactor requirements, such as nuclear safety, a regular robotic maintenance of the system components without

any impact on the operation, a high power coupled to the plasma core with high efficiency, high reliability and stable conditions over long operations. The specific asset of neutral beam (NB) systems is its ability to couple high-power energetic atom (D⁰) beams to the plasma core independently of the plasma physical conditions; this is particularly true regarding the role played by NB systems in achieving worldwide advanced plasma scenarios despite significant plasma instabilities. AD–T fusion plasma will likely have to deal with unprecedented and powerful plasma instabilities [1, 2]. It is clear that the insensitivity of the heating systems to the physical conditions should be a prerequisite for reactors that cannot be set aside.

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Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote>* Author to whom any correspondence should be addressed.</footnote>
On the other hand, present conventional NB system concepts [3–5] are not designed and optimised to cope with the technological requirements cited above: the 1 MV electrical setup is complex and invasive, the beamline is faced to heavy heat loads and beam losses resulting to a low efficiency and reliability, the remote maintenance would require long injector shutdowns and a low availability.

It is apparent that a dedicated R&D on NB systems has to be addressed to come up with reliable and pragmatic solutions specifically suitable for the harsh environment of future fusion reactors.

An injector consists essentially of four subsystems with very different functions and constraints: (1) the ion source is the active element of the injector; its operation involves multiple connections to external auxiliary power supplies as well as a regular maintenance especially due to the use of Caesium (Cs) essential to the production of negative ions D⁻. The electrical power requested to supply the plasma source is much less than 1 MW; it has a low impact on the overall injector wall-plug efficiency.

Conversely, the other downstream components of the source (accelerator, neutralizer, Residual Ion Dump (RID)) are passive thermomechanical elements at different DC potentials that do not really require regular maintenance.

They are subjected to significant thermal loads resulting mainly from the different interactions of the powerful beam (~40 MW of D⁻) both with the gas present along the beam line, i.e. the generation of energetic charged particles [6, 7] (by ionization and stripping), and, the interception of the D⁻ beam by the metallic surfaces of the injector (accelerating grids, neutralization and RID panels). These two different aspects are decisive to achieve high injector performances in terms of availability, reliability and efficiency. The D⁻ source must be easily accessible to regular robotic maintenance operations without interrupting the injector operation, and the gas load must be minimized along the beamline for lower beam losses and thermal powers.

On present NBI systems, the source is referenced to the high potential (−1 MV [4, 5]), this excludes any possibility of maintenance in operation, and involves a long injector shutdown due to difficult access. It is apparent that a high level of injector availability is only possible if the source is referenced to the ground potential, easily accessible and subdivided into independent source modules so that the maintenance (or failure) of a source module component (e.g. Cs oven) is compensated by another one in operation. We point out that multiple ion sources at high voltage (−1 MV) [8] would significantly increase the complexity of the 1 MV electrical setup (more power supplies), the bushing dimension (more feed-throughs and connectors) and the stored energy.

On the other hand, photoneutralization [9], which would eliminate the gas injection in the neutralizer, or plasma neutralizer [10] are not mature for short or mid-term reactors. In the short term, only conventional beam neutralization on a gas target can be considered with the guideline of drastically minimizing gas injection.

The main objective of this paper is to present an injector concept that gathers these two different aspects, i.e. an ion source allowing an easy remote maintenance and a drastic reduction of the gas injection (source + neutralizer) and beam power losses, essential to cope with the reactor requirements. This new injector concept is based on (1) a low pressure modular ion source referenced to the ground potential, (2) an accelerator generating high energy thin laminar ion beams, (3) a gas neutralizer held at high voltage (+1 MV) operating at low gas injection, and (4), an energy recovery system of the residual 1 MeV D⁻ at the neutralizer exit.

The injector acronym 'CIGALE' stands for 'Concept of a NB Injector based on GAs neutraLization and Energy recovery'.

Chapter 2 describes the overall injector concept aiming to minimize gas injection in the neutralizer; the specific details of the injectors components are presented in the other chapters. Chapter 3 presents the source concept resulting from the return experience of the NBI based negative ion sources in operation over the past three decades. Chapters 4–7 are dedicated to the beamline simulations: the laminar beam optics (pre and post-accelerator), the gas distribution and D⁻ beam losses along the beamline, the stray electron suppression, the simulation of the gas neutralizer, and the energy recovery system. Chapter 8 presents a preliminary injector design, and an estimate of the wall-plug efficiency.

## 2. Injector concept

The injector concept (see figure 1) consists of adjacent beamlines spatially oriented in the vacuum vessel to focus each NB downstream on the Tokamak duct aperture. In the perpendicular direction, the beam focalization is carried out by the angle of inclination of the accelerating grids on their supports. Each beamline is supplied by a source module (length ~1.5 m, width ~10 cm) referenced to the ground potential and connected to its external power supplies via connectors (air-vacuum) located on the vacuum vessel rear flange.

The negative ions produced in one source module are extracted through slots in the plasma grid (see paragraphs 3 and 4), then accelerated and cleaned of parasitic particles (electrons) in a 100 keV pre-accelerator before their post-acceleration at 1 MeV to form a thin laminar beam (~4 cm width) at the neutralizer entrance. The pre-accelerator electrical setup is also referenced to the ground potential. When an over current (spark, HV breakdown) along the beamline is detected, the ion beam in the pre-acceleraror is interrupted in the micro-second range by fast switches (referenced to the ground potential), while the response time of the 1 MV power supply is much slower (millisecond range).

Two complementary routes are proposed to minimize gas injection into the neutralizer: the first is a low gas conductance of the neutralizer cell subdivided in thin ducts (fitting with the laminar beams). The second is a significant reduction of the gas target (nl): figure 2 shows the neutralization rate of 1 MeV
<figure><img src="image_5.png" /><figcaption>**Figure 1.** Scheme of the injector concept with six beamlines in parallel.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 2.** Comparison of ITER and CIGALE gas targets for 1 MeV D⁻ beams.</figcaption></figure>

D⁻ as a function of the gas target *nl* (*n*: D₂ density, *l*: neutralizer length). On ITER [3, 4], *nl* = 1.2 · 10²⁰ m⁻², the theoretical neutralization rate is 55% with 22.5% for each species of residual ions (D⁺ and D⁻ at 1 MeV). These residual ions are collected at full energy (1 MeV) on the RID, resulting in 2·8 MW of power thermal losses (supposing 40 A of D⁻ at the neutralizer entrance).

With the CIGALE gas target (*nl* = 0.55 · 10²⁰ m⁻²), i.e. a factor two lower than the ITER one, the D⁻ neutralization rate decreases to 40%, the residual D⁻ rate increases to 52%, while the D⁺ rate falls down to 7%. It is apparent that this operating point is only interesting if it is associated with the energy recovery of the residual 1 MeV D⁻, which consists in decelerating and collecting them at low energy (~50 keV).

Table 1 compares the power losses between the two injector concepts: assuming a 10 A D⁻ beam at 1 MeV at the entrance of one neutralizer duct, the CIGALE operating point combined with energy recovery of residual D⁻ leads to significant decreases of the D⁺ and D⁻ power losses (0.96 MW instead of 4.5 MW on ITER), yielding to a higher power balance (80.6% for CIGALE instead of 50% for ITER).

The lower neutralizer rate of CIGALE can be compensated by the number of source modules (beamlines) implemented in the vacuum vessel (to attain the required neutral power).

<table><caption>**Table 1.** Comparison of the power losses between ITER and CIGALE injector concepts. These values results from the neutralization fractions of figure 2 applied to a 10 A 1 MeV D⁻ beam at the neutralizer entrance. On ITER, the residual D⁻ are collected by the RID at full energy (1 MeV). On CIGALE, they are decelerated to down to 50 keV and collected on the recovery electrode (50 keV).</caption><thead><tr><th></th><th>n.l*10<sup>20</sup><br>(m<sup>-2</sup>)</th><th>P<sub>D0</sub><br>(MW)</th><th>P<sub>D-</sub><br>(MW)</th><th>P<sub>D+</sub><br>(MW)</th><th>η(%)</th></tr></thead><tbody><tr><td>ITER</td><td>1.2</td><td>5.5</td><td>2.25</td><td>2.25</td><td>50%</td></tr><tr><td>CIGALE<br>(with recovery)</td><td>0.55</td><td>4</td><td>0.26</td><td>0.7</td><td>80.60%</td></tr></tbody></table>

The efficiency ($\eta$) is defined by: $\eta = P_{D0}/(P_{D0} + P_{D-} + P_{D+})$.

We point out that the ion beam energy recovery has already been successfully demonstrated both for positive (H⁺, D⁺) and negative (D⁻) based NBI systems [11, 12] at 100 keV energy range. The common features of these NBI systems were:

(i) A grounded ion source,

(ii) A laminar ion beam aspect ratio,

(iii) A gas neutralizer cell held at high voltage with plasma confinement (electrostatic or magnetic) at the duct extremities to avoid plasma leakages,

(iv) At the neutralizer exit, an electrostatic deflection of the energetic residual ions towards the recovery electrode (at low voltage), while the other ion species (impossible to recover) were stopped and collected at full energy close to the neutralizer.

On CIGALE, the concept remains the same (it is presented in chapter 7) with noticeable differences such as the beam energy (1 MeV) and several adjacent beamlines in the vacuum vessel.

A potential issue of the CIGALE concept is the large surface held at high voltage (+1 MV). For this purpose, a specific attention must be paid to the high voltage holding aspect. Indeed, to minimize (and mitigate) as much as possible arcings between electrodes and the electron field emission by the cathodic surfaces [13–15] exposed to the intense electric fields, appropriate conditioning protocol is required. Such protocol
combines fine polishing (Ra < 0.1) (or electro-polishing), cleaning with high resistivity water, outgassing at high temperature of the metal surfaces and dust suppression.

### 3. Ion source concept

Each ion source module has to operate at very low pressure ($p \ll 0.3$ Pa) and produce an uniform and dense plasma column illuminating the accelerator extraction surface (see figure 3). The extraction surface (facing of one module) consists of five grids segments (called 'plasma grids') of 30 cm length each, drilled by three slots (width: 1 cm, length: 28 cm) in order to shape the D⁻ into a laminar beam of 5 cm width in the pre-accelerator stage. Each plasma grid (PG) segment is fixed along the support and spatially oriented (tilted) to focus the NB downstream towards the Tokamak first wall aperture. Assuming a uniform D⁻ current density of 250 A m⁻² along the surface, the extracted D⁻ beam current (per module) is around 10 A.

Additional stringent specifications are required for reactors, such as steady state operations, very low Cs consumption, the remote maintenance of the source active components, e.g. the Cs oven or the plasma drivers (cathodes or RF antenna), without interrupting the injector operation. Significant efforts and progresses were made in recent decades in the R&D of negative ion sources [16–22], these Return Of Experiences (ROE) are instructive to define the most appropriate source concept for future reactors.

#### 31. ROE of the different types NBI based negative ion sources

Up to now, three different plasma drivers have been considered and explored for large negative ions sources dedicated to NBI systems: the filamented cathodes [16–18], the RF driven inductive coupling [19–21], and the Helicon antenna [22].

31.1 **Helicon antenna.** A Helicon [22] antenna in a perfect axis-cylindrical magnetic confinement generated by magnetic coils has the ability to generate a long, dense and stable magnetized plasma column at low operating pressure with substantial production of Cs free H⁺ ions along the plasma edge [22]. However, the implementation of a long extraction surface in contact with the plasma column significantly attenuates the density and disturbs the wave propagation [23].

31.2 **RF driven ICP source.** The choice of an inductively coupled plasma RF source [19–21] for ITER [3, 4] results from the infinite lifetime of the external RF antenna where the microwaves are coupled to the plasma through a dielectric window and a metallic Faraday screen. The reverse side of the medal of ICP is its incompatibility with any magnetic field within the RF driver, which would trap the electrons and prevent the power transfer from the RF waves to the electrons. That results in significant issues on the plasma parameters:

<figure><img src="image_7.png" /><figcaption>**Figure 3.** Extraction surface of one source module and Cs oven implantation.</figcaption></figure>

(i) The plasma potential is high in the source chamber namely within the RF driver ($V_p \sim 50–70$ V) [24].

(ii) At low operating pressure (below 0.3 Pa), plasma instabilities appears [24], the plasma potential ($V_p$) and electron temperature ($T_e$) increase significantly [25].

(iii) The high plasma potential ($V_p \sim 50–70$ V) leads to acceleration in the sheath of the different positive ion species ($D^+$, $D_2^+$, $D_3^+$ and $Cs^+$) towards the walls, resulting in surface erosion. This issue is enhanced by the combination of the high plasma potential with the heavy Cs nuclei; indeed, the evaporated Cs atoms in the plasma are quickly stripped by the hot electrons over several degrees of ionization (Cs ionization energies: 1st level: 3.9 eV, 2nd level: 23.1 eV, 3rd level: 35 eV, etc) and then accelerated. The sputtering yield of these energetic Cs ions bombarding surfaces is several orders of magnitude higher than that of other plasma ions ($D^+$, $D_2^+$, $D_3^+$) [26]. The surfaces within the RF driver are particularly exposed, due to the combination a high $V_p$ (~70 V) with the hot primary electrons; for this reason, molybdenum coating is required to minimize sputtering.

(iv) The ion acceleration by the sheath and their recombination on the metal walls induces energetic atoms $D^0$ and gas heating [25] in the plasma. Hot atoms $D^0$ bombarding the caesiated surface in the extraction region will produce D⁻ with transverse energy resulting in D⁻ beam optics with high divergence (>10 mrad) [27].

31.3 **Filamented cathodes sources.** Conversely, filamented cathode source types [16–18] operate very well with a high magnetic confinement (Cusp type magnetic topology). The plasma potential remains close to the potential of the walls ($V_p < 5$ V), the D⁻ beams have low divergence (~5 mrad) at operating pressure of 0.3 Pa [16, 17] and, the electrical power transfer efficiency from filamented cathodes to the primary electrons is high (>90%).

The major limitation of hot cathodes is their limited lifetime due to tungsten (W) evaporation at high temperature (at 2500 K, thermal evaporation rate ~2 $10^{-9}$ g cm⁻²) [28]. It does not exist yet an experimental database on the W filament lifetime in a pure hydrogen (Cs free) plasma. The ROE on heavy ion plasmas [29, 30] (Ar, Kr, Yt, Sm, etc.) shows that the sputtering rate is dominant over the evaporation rate, the
integrated W filament on-time (filament diameter of 2 mm) is estimated about 2–3 days [29, 30].

In a pure (Cs free) deuterium plasma uniformly distributed and with thick filaments (diameter >2 mm), we can expect much higher lifetime. So far, that remains an open question that must be tackled experimentally [31, 32], but it is clear that filamented cathodes are incompatible with Cs evaporation in the plasma bulk (due to a high sputtering rate). A new Cs seeding methodology has to be considered aiming to exclusively depose Cs on the PG surfaces where D⁻ are produced and extracted. This would significantly reduce the Cs rate in the plasma, the Cs consumption, and facilitate the control of the Cs coating (~0.6 mono-layer) essential for a homogeneous production of D⁻ on the surface.

In summary, it turns out that the filament-driven source exhibits better performances than the RF ICP sources if two essential conditions are met:

(i) The source is designed to ease regular and rapid remote maintenances to replace the used filaments by new ones without breaking the injector vacuum (and injector operation).

(ii) No caesium evaporation in the plasma volume in order to avoid enhanced W filament erosion.

### 32 Caesium management (see figure 3)

To depose (or refresh) the Cs layer on the grid surface (where D⁻ are produced), the method consists in moving the heated Cs dispenser tube along the grid chamfers. The oven containing the liquid Cs is outside the vacuum vessel, it is connected to the dispenser tube via a bellows. The Cs vapour is locally routed through tubes heated to 300 °C and deposited on the surface through holes facing the chamfers. The bellows is compressed to move the dispenser along the grid chamfers (excursion length equal to the grid length ~28 cm). When not in use, the oven is at ambient temperature and isolated from the vacuum vessel by a shut vacuum valve, the bellows is relaxed and the dispensers are located between the grid segments (outside the beam region).

Assuming a Cs evaporation rate of 40 mg h⁻¹ (~oven reservoir temperature of ~165 °C in the case of the SPIDER ovens [21]), the PG is kept at ~20 °C (resp. 100 °C), the sticking coefficient of Cs on the surface is greater than 90% (resp. 50%), resulting in less than 0.5 s (resp. 1 s) of excursion time for deposition of 0.6 Cs mono-layer on the grid chamfers. The ion beam is switched off during this excursion time to avoid the ion bombardment (D⁺ back-streaming ions) on the tube dispenser.

The oven maintenance (replacement) by remote handling does not affect the source operation: with the two vacuum valves closed, the oven can be disconnected from the source and replaced by another one ready for operation. Each source module is equipped with one oven at each extremity to supply the five grid segments.

This methodology should allow a fine control of the Cs coating on the PG and drastically minimizes the Cs consumption. The Cs coating lifetime, i.e. the frequency of Cs coating

<figure><img src="image_8.png" /><figcaption>**Figure 4.** Cross section of the filamented driven source with the multi-cusp magnetic confinement.</figcaption></figure>

refreshing, is so far another open question. With a low W filament erosion rate in the plasma, frequent Cs evaporation will likely not be required.

### 33 Principle of a source module (see figure 4)

The production of primary (hot) plasma at the rear of the module is generated by heated W filaments located on the two lateral edges of the plasma, away from the back-streaming energetic D⁺ beam. The magnetic confinement is a multicusp arrangement. To avoid an overall plasma drift on the source front, the conventional ‘large magnetic filter field parallel to the PG’ is replaced by a cusp layout generated by permanent magnets inserted along and within the water cooled grid chamfers. The intensity of this cusped magnetic filter is adjusted to avoid the penetration of hot electrons into the extraction region while maintaining sufficient transparency for D⁺ ions. In this extraction region (between chamfers), the D⁻ are produced on the caesiated chamfer surfaces by the D⁰ and D⁺ bombardment, the goal being to have locally an ion-ion plasma (D⁺ and D⁻) between the chamfers to minimize the co-extracted electron current and heat load downstream on the accelerating grids.

The filament feedthroughs are fixed on a flange at the back of the vacuum vessel. The used filaments remote maintenance consists to replace the flange by another one equipped with new filaments. To do so, a container (equipped with vacuum valves) is implemented behind the flange on the vessel wall and pumped. The container with the old flange is evacuated and replaced by the new one. A specific robotic tool relevant with the nuclear safety requirements is required for this operation, the challenge being to replace (in remote) the flange while maintaining the injector in operation.

A 3D PIC code derived from the ‘LePIC 2D/3D’ code [33] is being developed to simulate plasma transport in the extraction region. Preliminary code results show that hot electrons are trapped by the cusp field lines and diverted to the leading edges of the chamfers (see figure 5-left). We note the magnetic filter transparency for the D⁺ ion flux on figure 5-right essential to balance the negative space charge of the D⁻ produced on the PG caesiated surface. The dimensions of the simulation domain are respectively 6.4 cm along the beam axis (oz),
<figure><img src="image_9.png" /><figcaption>**Figure 5.** Simulation of the electron and H⁺ ion transport across the magnetic cusp of the chamber.</figcaption></figure>

and 3.2 cm along the transverse (ox) and vertical (oy) directions. The grid resolution is 1096 × 512 × 512 nodes with one particles per cell, the initial plasma density is $n_e = 5 \cdot 10^{16}$ m⁻³ and $n_{H2} = 1 \cdot 10^{17}$ m⁻³. The code will also be used to study the origins of the aberrations of the ion beam in the acceleration channel that result mainly from the transport of D⁻ in the magnetized plasma, their transverse energy in the meniscus and the geometry of the aperture.

## 4. Laminar beam accelerator concept

### 4.1. The 100 keV pre-accelerator beam optics

The first three grids that make up the pre-accelerator are shown on figure 6. The PG faces the plasma and is kept at the source potential (0 V). It features 3 slots of 1 cm width (see figure 3) from which the 3 blade-like beams are extracted, thanks to the extraction grid (EG) which is polarized at values of the order of 3 kV. The first acceleration grid (AG1) completes the pre-acceleration system and is polarized at values of the order of 100 kV.

The geometry of the grids and the choice of acceleration voltages were optimized using the IBSimu simulation package [34] and OPERA 3D. figure 6(a) features the pre-accelerated beam, in optimal conditions. When plotting a map of the electric potential around the PG slots one can clearly observe the concave shape of the virtual surface from which the negative ions are extracted, the so-called meniscus (figure 6(b)). The divergence of the beam (along the transverse y-axis) at the exit of the pre-accelerator ($x = 100$ mm) and before reaching the 200 kV grid ($x = 155$ mm) is calculated from the trajectories simulated in OPERA 3D and reaches values of the order of 4 mrad (the maximum beam divergence of the emittance diagram).

### 4.2. Electron suppression in the pre-accelerator

The extraction and pre-acceleration grids serve a dual purpose. They must on one hand extract the D⁻ beam and optimize the beam optics, and on the other hand intercept the unavoidable co-extracted electrons before they reach the post-accelerator. The power density of the co-extracted electrons intercepted on the EG surface with slot apertures is reduced for two reasons:

(i) The electrostatic lens being stronger, the EG voltage is only ~3 kV against ~8 kV for cylindrical apertures.

<figure><img src="image_10.png" /><figcaption>**Figure 6.** 100 keV Pre-accelerator beam optics modelling with OPERA 3D ($V_{EG} = 3.2$ kV, $V_{AG1} = 100$ kV, $j_D^{−} = 250$ A m²).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 7.** Pre-accelerator magnetic configuration.</figcaption></figure>

(ii) With the slot geometry, electrons are uniformly distributed along the slot while they are focalized on hot spots (with the cylindrical apertures).

The co-extracted electron suppression is done by inserting permanent magnets in the PG and EG, as shown in figure 7, where a colour map of the transverse magnetic field By and its profile along the $y = z = 0$ axis are shown. In this simulation, $Sm_2Co_{17}$ permanent magnets are used to produce a local transverse magnetic field up to the order of $5 \cdot 10^{-2}$ T on axis at the EG. This is sufficient to fully magnetize the co-extracted electrons, trapping them between the PG and the EG, as confirmed by the simulation shown on figure 8.

### 4.3. The 1 MeV post-accelerator beam optics

Figures 9 and 10 show a 3D simulation of the post acceleration stage to 1 MeV for one grid segment (see figure 10). The D⁻ beam is composed by three individual laminar beamlets (beamlet dimensions: 1 cm width, 28 cm height) emitted at the pre-accelerator exit with a primary energy of 100 keV. The emittance profile of each 100 keV D⁻ beamlet is shown on figure 11.
<figure><img src="image_12.png" /><figcaption>**Figure 8.** Suppression of co-extracted electrons in the pre-accelerator.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 9.** 3D simulation (Top view) of the 2 A D⁻ beam optics in the 1 MeV post-acceleration stage. The post-accelerator merges three 100 keV D⁻ beam sheets (see figure 6) and accelerates them up to 1 MeV.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 10.** Side view of figure 9 with beam axis inclined, showing the simulation domain (grid segments). The D⁻ beam height is 28 cm per grid segment.</figcaption></figure>

At the pre-accelerator exit, the total D⁻ beam width is 5 cm (see figure 9), the accelerated current (per grid segment) is 2A. The three beamlets are merged downstream into a single beam via large slotted apertures (~6 cm width) in the post-accelerating grids resulting in a D⁻ beam width of 4 cm at 1 MeV (at the neutralizer entrance).

The horizontal and vertical emittance profiles at 1 MeV for the total beam and without magnetic field in the accelerating channel are depicted on figure 12. The beam dimensions at

<figure><img src="image_15.png" /><figcaption>**Figure 11.** Emittance profile of one 100 keV beamlet emitted at the pre-accelerator exit.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Figure 12.** Horizontal and vertical emittance profiles of the total merged 1 MeV beam at the neutralizer entrance (see figure 9).</figcaption></figure>

1 MeV are 4 cm width and 28 cm height and the maximum divergence in both directions is 6 mrad.

This post-accelerator with large apertures is advantageous in the sense that the beam optics remains low divergent (<6 mrad), and most importantly, it minimize the interactions of the energetic D⁻ beam with the grid metal surfaces which is the main cause of HV breakdowns. That design releases the alignment constraints between the grids and favours the gas pumping resulting in lower gas density (and lower D⁻ stripping losses) in the accelerating channel.

The stray electrons emitted by the D⁻ stripping with the background gas in the post-accelerating channel have to be stopped at each stage to avoid heavy thermal loads (and power losses) on the grids. For this purpose, permanent Sm-Co magnets are inserted in each post-accelerating grid to generate vertical magnetic fields.

Figure 13-left shows the electrons beam trajectories within the different accelerating gaps; they are swept out from the D⁻ beam (the D⁻ beam is masked) and immediately stopped on the front face of the following grid.

To avoid a transverse deflection of the D⁻ beam, the magnets have alternating polarities at each stage. Figure 13-right
<figure><img src="image_17.png" /><figcaption>**Figure 13.** Left: stripped electrons trajectories in the magnetized accelerating channel. Right: transverse 1 MeV D⁻ beam emittance profile.</figcaption></figure>

shows the resulting transverse D⁻ beam emittance profile; it does not highlight a significant beam degradation.

We note a slight dispersion of each beamlet emittance profile with a maximum divergence of 8 mrad. This magnetic field topology does not affect the vertical emittance diagram.

Downstream, the NB is vertically and horizontally focused on the Tokamak first wall aperture by the tilt of the beamlines (see figure 1) and by the curvature radius of the accelerating grids supports. Taking into account of the beam divergence of figures 12 and 13, the NB cross section in the duct, at 20 m away from the source, ranges around 40·60 cm². A specific attention to the beam reionisation in the duct by the background gas diffusing from the Tokamak chamber has to be paid to assess the thermal loads (by 1 MeV D⁺ ions).

**5. D⁻ beam stripping losses in the accelerating channel**

Stripping losses (the detachment of the extra electron of the D⁻ ion when colliding with D₂ gas in the pre-accelerator and accelerator) must be minimized in order to retain the highest D⁻ beam intensity and reduce the power losses. The D⁻ stripping cross-sections by D₂ are maximal in the 1–10 keV region [35], i.e. in the pre-accelerator. Furthermore, the gas pressure in this region is expected to be rather high due to the leaking of the gas from the source through the PG and EG. We therefore expect most of the stripping to occur in the pre-acceleration region, and the stray electrons should be intercepted before the post-acceleration stages.

Figure 14 displays the pressure profiles along the beam axis, assuming that the pressure in the source and in the vessel are kept constant ($p_{vessel} = 0.01$ Pa) and that the conductances of the grid apertures and of the space between the grids can be modelled as simple rectangular ducts. We show the pressure profiles in the cases $p_{source} = 0.3$ Pa and 0.1 Pa. The pressure profile depends on the lateral position of the source module in the injector: the modules on the edges benefit from higher gas conductance.

The following results are given for the worst-case, i.e. the innermost module (where the pressure and stripping are the highest). Two significant results can be highlighted from these estimations displayed on figure 15: most of the stripping

<figure><img src="image_18.png" /><figcaption>**Figure 14.** Pressure profiles along the accelerator.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>**Figure 15.** Stripping yield and beam intensity along the accelerator.</figcaption></figure>

occurs in the pre-acceleration stage, and the D⁻ beam losses is only 10% with $p_{source} = 0.1$ Pa, and attains 20% at 0.3 Pa.

At 0.1 Pa, the D₂ gas injected in the source region (composed with five modules) is estimated to 1.05 Pa·m³ s⁻¹ (resp. 1.6 Pa·m³ s⁻¹ in H₂), while it is 3.15 Pa·m³ s⁻¹ (resp. 4.8 Pa·m³ s⁻¹ in H₂) at 0.3 Pa.

**6. The gas neutralizer**

As explained in chapter 1, the CIGALE injector relies a low gas target (nl). Monte Carlo simulations with the ‘MOLFLOW’ code [36] have been performed to evaluate the gas distribution (H₂ or D₂) injected in a single neutralizer duct.

Figure 16 compares the pressure profiles between one ITER and CIGALE neutralizer duct with the same external conditions, i.e. same pumping speed at the duct extremities (see figure 17) and same gas injection topology in the duct centre.

The D₂ gas injection estimated by MOLFLOW (see table 2) in a single duct is 4.77 Pa·m³ s⁻¹ for ITER and 1.01 Pa·m³ s⁻¹ for CIGALE with smooth duct internal surfaces. The gas density at the neutralizer exit is six time lower on CIGALE than on ITER.

Figure 17 shows the design of gas neutralizer with V-shaped internal surfaces which tends to reflect the gas towards the duct centre. MOLFLOW simulations confirms a slight gas reduction of 0.98 Pa·m³ s⁻¹ for the same gas target (nl = 0.55·10²⁰ m⁻²).
<figure><img src="image_20.png" /><figcaption>**Figure 16.** Comparison of the gas density profiles for one ITER neutralizer duct with CIGALE duct (with V-shaped surfaces).</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>**Figure 17.** MOLFLOW simulation of the gas density in a duct with V-shaped surfaces, with $D_2$ injection of 0.98 Pa·m³ s⁻¹. The sticking coefficients of the pumping walls equal to 1.</figcaption></figure>

<table><caption>**Table 2.** Comparison of the gas injection for different neutralizer duct shapes.</caption><thead><tr><th>One Neutralizer duct</th><th>nl*10<sup>20</sup> (m<sup>-2</sup>)</th><th>H<sub>2</sub> (Pa·m<sup>3</sup> s<sup>-1</sup>)</th><th>D<sub>2</sub> (Pa·m<sup>3</sup> s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>ITER-like</td><td>1.2</td><td>7.29</td><td>4.77</td></tr><tr><td>CIGALE smooth</td><td>0.55</td><td>1.63</td><td>1.01</td></tr><tr><td>CIGALE V-shaped</td><td>0.55</td><td>1.35</td><td>0.98</td></tr></tbody></table>

ITER-like: 3 m long, 10 cm width, 1.5 m height.

CIGALE smooth: 2 m long, 4 cm width, 1.5 m height.

CIGALE V-shaped: 2 m long, 4 cm width, 1.5 m height.

Under these conditions, the ITER gas neutralizer with four ducts would necessitate 19.08 Pa·m³ s⁻¹, while the V-shaped CIGALE neutralizer with five ducts would only necessitate 5·0.98 = 4.9 Pa·m³ s⁻¹, i.e. 4 times less amount of gas.

Consequently, the total gas injected in a CIGALE injector operating with five source modules (source pressure at 0.1 Pa) and a gas neutralizer with V-shaped ducts, would amount to

about 1.05 + 4.9 = 5.95 Pa·m³ s⁻¹. Under these operating conditions, a pumping speed of 600 m³ s⁻¹ is required for 10 mPa tank background pressure in $D_2$ operations.

## 7. The energy recovery system

Figure 1 shows the overall energy recovery concept for six beamlines in parallel, and figure 18 represents the beam recovery simulation for one beamline, i.e. a 10 A D⁻ beam at 1 MeV at the neutralizer duct entrance (the beam dimensions are 4 cm width, 1.5 m height). Taking into account of the CIGALE neutralization fractions of figure 2, the residual ion beams at the neutralizer exit are: 5.2 A of D⁻ and 0.7 A of D⁺. These two ion beams are separated by a transverse electric field of ~30 kV cm⁻¹ generated by two electrodes sheltered from the beams: one electrode at 800 kV (connected to the 800 kV bushing stage) and the opposite electrode at 1 MV.

The deflected 1 MeV D⁺ are collected on an actively cooled target at +1 MV, while the D⁻ are decelerated down to 50 keV and collected on the recovery electrode (at 50 kV). It is important to point out that this recovery electrode has to be designed to efficiently trap the secondary electrons resulting from the 50 keV D⁻ ion bombardment in order to avoid their retro-acceleration towards the 1 MV neutralizer. We note that the secondary electrons emitted on the D⁺ target are trapped on the surface by the potential well generated by the 800 kV deflection electrode.

The efficiency of the recovery system requires high optical quality of the 1 MeV D⁻ beam at the neutralizer exit. For this purpose, the thin neutralizer ducts are actively cooled to scrap the beam halo (beam optic aberrations).

At the neutralizer exit (in the deflecting region), the low gas density ($n \sim 1.1 \cdot 10^{18}$ m⁻³, see figure 16) and the high transverse electric field avoid the formation of plasma sheath, and, the beam reionisation rate in this region is low: supposing a 20 A 1 MeV D⁰ beam at the neutralizer exit, the D⁺ (1 MeV) reionized beam current is evaluated to 8 mA.

## 8. Injector wall-plug efficiency and design

### Wall-plug efficiency assessment

The wall-plug efficiency is evaluated assuming five beamlines in operation in the vacuum vessel, corresponding to 50 A of extracted D⁻ current from the five source modules. That leads to 20 MW of NB at the injector exit. The NB loss in the tokamak duct is estimated to 10% (~5% of D⁰ reionization and 5% interception by the walls) resulting in 18 MW neutral power coupled within the plasma core.

The electrical and thermal powers involved to operate the injector are:

(i) Electrical power to supply the ion source: ~0.5 MW for five modules
<figure><img src="image_22.png" /><figcaption>**Figure 18.** 3D simulation of the energy recovery system for a 1.5 m height D⁻ beam resulting from the neutralization of a 10 A D⁻ beam at the neutralizer entrance (see table 1).</figcaption></figure>

(ii) Power losses in the accelerator: the pressure distribution along the accelerating channel (see figure 14) allows to evaluate the D⁻ stripping losses and thermal loads on the grids by the stripped electrons. Assuming an ion source operating at 0.1 Pa, the power loss in the pre-accelerator is evaluated to 0.4 MW (resp. 0.8 MW at 0.3 Pa). In the post-accelerator, the stripping rate and gas load are much lower, the emitted electrons being stopped at each stage (see figure 13), the thermal power on each post-accelerating stage ranges around 100 kW leading to 0.5 MW power loss for the five accelerating grids.

(iii) Power losses in the neutralizer-recovery: (see table 1) 1.3 MW of D⁻ on the recovery electrode and 3.5 MW on the D⁺ beam target.

(iv) Electrical power for cryo-pumps and water-refrigeration: ~1 MW.

(v) Electrical power in the 1 MV power supply supposing 95% efficiency (same than the ITER one):

When coupled with the energy recovery system, the drain current flowing in the 1 MV power supply is the subtraction of the recovered current (26 A) to the accelerated current (50 A) (see figure 15 [9]).

Consequently, the electrical power requested to accelerate the D⁻ beam is 24 MW, and the electrical power loss (within the power supply) is $(1-0.95)^*24 = 1.2$ MW

Taking into account of all these power losses, the global injector wall-plug efficiency is around 63%.

## Injector conceptual design

Figure 19 presents the beamline components under vacuum using the Solidworks software. It consists of three independent units aligned on a rail lying on the vessel floor:

(i) The ‘source + 100 keV pre-accelerator’,

(ii) The UHV components ‘Bushing + post-accelerator + neutralizer + deflector + collector’,

(iii) The ‘energy recovery system’.

<figure><img src="image_23.png" /><figcaption>**Figure 19.** Overview of the injector design showing the different components in the vacuum vessel.</figcaption></figure>

The UHV components are connected on the top to the 1 MV bushing inner part (under vacuum), and on the bottom, they lie on 1 MV insulators fixed on the rail for support and alignment.

The upper part of the bushing is pressured (~0.5 MPa) under SF₆ gas (or dry air) and connected to the external DC 1 MV Power Supply (1 MV-PS) located outside of the reactor building. The 1 MV bushing is crossed by water cooling lines and a tube with D₂ gas to supply the UHV components and neutralizer.

On CIGALE, there is no need of intermediate high voltage platform (with power supplies held at +1 MV) between the 1 MV-PS and the bushing.

Instead of a SF₆ pressured invasive and complex transmission line, the 1 MV electrical setup could benefit by the emergence of commercial solutions like (UHV) cables (from 200 kV up to 1 MV) used for the electricity transport (setup not represented on figure 20).

They would be specifically manufactured to the needs of the injector, i.e. DC current below of 50 A and HV holding from 200 kV to 1 MV. Each cable termination in the bushing SF₆ tank would be equipped with snubbers to absorb their stored energy and mitigate the HV breakdowns. Such UHV cables would greatly simplified the whole 1 MV electrical setup, and favours the accessibility and maintainability of the 1 MV bushing.
<figure><img src="image_24.png" /><figcaption>**Figure 20.** Vertical cross section of the beamline.</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>**Figure 21.** Horizontal cross section of the beamline.</figcaption></figure>

The six sources with their 100 keV pre-accelerators are supported in the vacuum vessel by a support fixed on the rail (see figure 19), which ensures the alignment with the beamline components downstream.

The source modules are powered via feedthroughs (and insulators) implanted on the grounded back flange of the vessel (see figure 20–21).

The low gas injection in CIGALE releases the constraint and size of the pumping system. Cryo-condensation pumps are required to achieve a pumping speed of 1000 m³ s⁻¹ for deuterium (or hydrogen) operation. Figure 21 shows a top view of the injector with ten individual cryopumps (~100 m³ s⁻¹ per cryopump) implemented along the vessel walls.

During the long shot durations in D₂, the 400 m³ s⁻¹ margin on the pumping speed allows the regeneration of the cryopumps while maintaining the injector in operation. To do this, each cryopump is located behind a shutter that is closed during the pump regeneration (outgassing) while the others remain functional. Such a feature requires a dedicated study by experts.

Figure 21 also shows the location of the Cs ovens at each end of the source. The ovens are in the air, fixed on bellows (see figure 3) allowing the excursion of the heated tubes (with Cs vapour) along the grid segments.

## 9. Conclusion

This paper outlines the main features of a new NB concept (called CIGALE) based on pragmatic and simple technical solutions. It overcomes several issues of conventional NB systems while being suitable with the main reactor requirements. Its modular concept with independent oriented beamlines eases the adaptation to different reactor needs (beam power and beam angles).

A spare beamline in the vacuum vessel allows maintaining the injector in operation during a source module maintenance (shutdown of one beamline). The ion source modules are referenced to the potential of the earth; that configuration leads to noticeable advantages, namely for the source (and Cs oven) remote maintenance.

The comparison of the different source plasma drivers experienced over the last decades (ICP RF driver, filamented cathodes or helicon) shows that the filament driven source has the best performances but it necessitates high filaments lifetime on reactors for long shots operation (or steady state). To fulfill such stringent constraint, two requirements are essential:

(i) The remote replacement of the used filaments by new ones without breaking the tank vacuum while maintaining the injector in operation,

(ii) No evaporation of caesium in the plasma volume to avoid rapid filament aging (W erosion).

A new caesium management is proposed: Cs is deposited only on the PG chamfers near the extraction aperture by a mobile dispenser. This method allows a fine control of the Cs layer and significant reductions of Cs consumption and Cs density in the plasma.

In addition, the Cs oven accessibility and remote maintenance are easy and have no impact on the operation of the injector.
A new plasma source magnetic confinement based on multi-cusp topology is proposed to operate the source at low pressure and to favour the plasma homogeneity. The standard ‘magnetic filter field’ is replaced by cusped magnetic fields on the leading edges of the PG chamfers.

The 1 MV electrical setup is greatly simplified with the source at the ground potential and the 1 MV power supply could be directly connected to the bushing by commercial UHV cables.

The other key parameter of this injector concept is the low gas injection resulting from a gas neutralizer with a reduced gas target (~a factor two of the ITER one).

An energy recovery system is essential on CIGALE to attain a high wall plug efficiency (η ~ 63%). With five beamlines in operation in the vacuum vessel, the estimated injected neutral power in the plasma core is similar to the ITER one (~18 MW).

A thin laminar D⁻ beam is required to suit with the thin gas neutralizer duct (4 cm width). The accelerator is composed with a 100 keV pre-accelerator based on slotted apertures. In the post-accelerator, the blade-like beamlets are merged through large apertures in the grids (from 200 kV to 1 MV). These large apertures releases the grid alignment constraints and minimizes the interaction of the D⁻ beam with the metal grid surfaces.

The vacuum vessel is equipped with 10 independent cryopumps implanted along the vessel walls to provide a total pumping speed of ~1000 m³ s⁻¹. In D₂ operation, the margin of 400 m³ s⁻¹ allows cryopump regeneration during long shot (or steady state) while maintaining the injector in operation.

To conclude, this new NB concept highlights significant advantages as it overcomes the main issues of conventional NB systems that are incompatible with reactor requirements.

It is new route for NB systems that should be explored targeting at mid-term, the development and optimization of a single beamline (one module) at scale one.

The ion source is the key component of the injector; its performances (operating pressure, D⁻ current density, extracted electron current density, Cs consumption, Cs coating lifetime, and filament lifetime) will define the final injector parameters and performances. To this aim, the development of a source module prototype is ongoing on the DEIS testbed at IRFM.

## Acknowledgments

This work received support from the french government under the France 2030 investment plan, as part of the Initiative d'Excellence d'Aix-Marseille Université—A*MIDEX "AMX-22-RE-AB-034. The authors would like to thank Pr. Gilles Cartry (PIIM laboratory, CNRS, Aix-Marseille University) for helpful discussions on the source concept, Dr Gwenael Fubiani (Laplace Laboratory, CNRS, Toulouse University) for learning and advices on the LePIC 2D/3D" code and Christian Dechelle (IRFM, CEA Cadarache) for his helpful contribution to the injector drawings.

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