Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 TUP40

DEVELOPMENT OF A HIGH CURRENT INJECTOR WITH THERMIONICCATHODE ELECTRON GUN FOR A SUPERCONDUCTING RF LINAC∗
A. B. Kavar† , F. Hinode, I. Nagasawa, K. Nanbu, K. Kanomata, K. Shibata, K. Takahashi,
K. Kumagai, K. Kudo, S. Kashiwagi, T. Muto, H. Hama , Tohoku University, Sendai, JapanK. Masuda, National Institutes for Quantum and Radiological Sci. and Tech. (QST), Aomori, Japan
Abstract
We are conducting research on a thermionic gridded electron gun with high average current for a superconducting
electron linac which will be used for wastewater purification. The electron gun will generate the pulsed beam byusing a DC high voltage and driving the grid at high speed.
The current dependence of emittance is investigated numerically based on the existing electron gun setup. We will now
proceed to optimize and improve the geometry and configuration of the injector components such as focusing magnets
and bunching cavities to inject a high current beam into thesuperconducting RF cavities. In order to evaluate the actual
beam produced by the electron gun, a test stand for the electron gun is currently being constructed. In this paper, we
will discuss the results of numerical studies and the current
status of the electron gun test stand.

INTRODUCTION
The growing global concern about water pollution andshortage in recent years has highlighted the need for effective
wastewater treatment solutions. Among the emerging technologies, Electron Beam (EB) accelerator-based wastewater
treatment has received a lot of interest because of its potential
to provide a robust and sustainable solution [1]. Studies havedemonstrated an extraordinary cleaning ability of EB based
treatments [2, 3]. However, such treatments can be commercially successful only if they are economically viable. To
establish an economically viable EB-based plant, several
key requirements such as high beam current for effective
treatment rate, scalability to handle varying wastewater flow
rates, compact design making it suitable for integration into
existing facility, low maintainence requirement etc. need
to be met. Linear accelerators based on conduction cooled,
Superconducting Radio Frequency (SRF) technology havebeen an ideal choice in regards to the necessity of meetingthese requirements. We are therefore focusing our efforts on
using SRF technology for wastewater treatment.
Our overall aim is to design and develop a compact, highcurrent electron linear accelerator using a 𝑁𝑏3 𝑆𝑛 SRF cavities with a conduction cooling system. The 𝑁𝑏3 𝑆𝑛 SRF
cavities in the main accelerator section is being developed incollaboration with KEK and NIMS. The injector part including an electron gun and buncher section will be designed
and experimentally tested at ELPH, Tohoku University. For
stable operation of the SRF accelerator, the electron injector∗ WORKSUPPORTED BY PIONEERING RESEARCH SUPPORTPROJECT FELLOWSHIP BY JAPAN GOVERNMENT.
† kavar.anjali.bhagwan.r4@dc.tohoku.ac.jp

Figure 1: Schematic of proposed high-intensity injector.
system should provide an accurately shaped short bunches
without tails and halo [4]. Because even a small amountof beam loss in the superconducting cavity will cause aquench. Most common approach to fulfil this requirement is
an rf photoinjector which can generate picosecond electron
bunches but requirement of drive laser makes the geometrycomplex, costly and maintenance extensive. On the other
hand, thermionic emission is a less complex method which
is able to provide high current density with additional benefits of structure simplicity, high emission capability, lowmaintenance, low cost and also, the ease in finding the spares.
However, to produce short electron bunches, the cathode gridmust be controlled at high speed. We have initiated study forthe development of an injector comprising a DC thermionicgridded electron gun operated by a fast grid pulser followed
by a bunching section with conceptual schematic as shownin Fig. 1. In this paper we will discuss about the prototype
of accelerator system, the targeted design parameters andthe numerical studies of electron source.

EB BASED WATER PURIFICATION
When water is subjected to EB, it produces highly reactive
species [5] such as OH. , H. , e− and other secondary reactive
species within 1 µs as shown in Fig. 2. These short-lived
radicals carry out a cascade of simultaneous oxidation andreduction reactions breaking down organic and inorganic
pollutants into simpler compounds. As a result 70-100 %of degradation, removal, and detoxification of pollutantspresent in water has been observed [1]. Furthermore, theradicals disrupt the cell membranes and genetic materialof microorganisms, leading to their inactivation with aneffective dose of 1-2 kGy [6].
No additional chemicals are required to start these reactions. Most notably, both highly oxidizing and reducing- 396 -

Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 TUP40

Figure 2: Various reactive ions and radicals produced byelectron beam irradiation of water.
reactive species are generated simultaneously with an yieldhigher than any other advanced oxidation process. EB-based
processing also offers an additional benefit of Process controllability and Compatibility with conventional methods.

DESIGN PARAMETERS AND
CHALLENGES
High-power accelerators are being designed and developed to build economically viable EB-based plants. We set
our first target such that the accelerator system will be capable of processing 5 million liters per day (MLD) of water at
a dose of 2 kGy. Assuming an irradiation efficiency of 60%,
the required beam power is 200 kW from Eq. (1).
Power(kW) =Dose(kGy) × Water amount(kg/s)
efficiency(1)

The transfer efficiency of power from an electron beam towater strongly depends on the irradiation system, such as
electron beam energy and scanning beam size at the irradiation point, and so on. The efficiency of 60 % is set withreference to the EB treatment plant using 1 MeV DC accelerator in South Korea [7] and the proposed treatment plantusing 10 MeV SRF accelerator by Fermi Laboratory [8]. In
our design, five L-band single cell SRF cavities are used
to accelerate the beam up to 5 MeV. Average beam current
is 40 mA with 200kW of beam power. Each cavity cell isequipped with a coupler of 40 kW to feed the RF power forthe accelerating gradient of about 10 MV/m.
The SRF linac injector to be developed with an average
beam current of 40 mA includes a DC-grid thermal electron gun and a sub-harmonic buncher system operating at
216 MHz, which corresponds to 1/6 of the linac’s RF frequency of 1.3 GHz. To achieve an average current of 40 mA,
the repeition rate required must be feasible. Presently, theavailable pulser, developed by Kentech Instruments Ltd., canachieve a maximum repetition rate of 27 MHz with 400 ps

Figure 3: Geometry of the electron gun (Up). Vacuum field
distribution along beam axis in the gun (Down).
in FWHM. Considering the space charge limit, if cathodeis operated for a Gaussian electron bunch with a charge of
1 nC and a bunch length of 𝜎𝑡 = 167 ps (6𝜎𝑡 = 1ns), a bunch
repetition rate of 40 MHz will be necessary. Our challenge
is thus to develop a high-speed grid pulser that can operate
at 40 MHz or higher. We will develop a fast grid pulser
using two transistors and step recovery diode that operate
at high speed. Another and most important aspect is theemittance of the electron beam produced by the electron
gun which must be small enough to avoid beam loss in the
superconducting cavities.
Adhering to these goals, our initial focus will be to build
an electron gun test stand at the ELPH facility and develop
the high repetition rate, high average current electron gun.
We will then investigate the current dependence of emittanceand bunch length.

NUMERICAL STUDIES OF GUN
Numerical study has been performed for an existing gun[9]. Geometry of the gun and the electric field distributionof the gun is as shown in Fig. 3. At a first, we wanted
to investigate current dependence of electron beam emittance using simulation code of CST [10]. To start with, thisstudy was performed with CW beam for the current rangeof 100 mA to the maximum achievable current of 2.5A withgun voltage of 72 kV. The results were demonstrating an- 397 -

Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 TUP40

Figure 4: Emittance evolution along the beam axis calculatedwith CST and KUCODE for 1A with 72 kV of gun potential.

Figure 5: Variation of the beam emittance along longitudinal
direction for different beam current.
uncommon trend of almost the same emittance value fordifferent currents. We therefore used KUCODE [11], which
has been used for electron gun simulations, and compared
our results with those of CST. The current dependence of theemittance calculated by the two codes was different. Focusing on the difference in the phase-space distribution of thebeam emitted from the cathode, a modification was made to
reduce the mesh size of the cathode surface. By modifying
mesh cells and mesh size for the gun geometry within CST,
we obtained very close results with CST and KUCODE as
shown in Fig. 4. From this modification, it was found thatemittance is sensitive to the mesh around the cathode surface
and that the mesh should be set carefully. The emittanceobtained after the mesh optimization was not constant anddecreased with increasing beam current as shown in Fig. 5.
Analysis of transverse fields (space charge and vacuum field)
acting on beam stated the reason of an inter-play between
beam spreading space charge force and the focusing vacuum
field. Space charge effect is low for lower current resulting
in a stronger transverse focusing due to vacuum field. As a

Figure 6: Phase space distribution for 0.1A, 1A and 2.5A
at 5 mm downstream the anode i.e. at 42 mm from cathodesurface (z=0).

Figure 7: Beam extraction from thermionic cathode with
72 kV for different bunch charge.
result, phase space distribution of the beam for higher current becomes linear whereas there comes non-linearity inthe beam for lower current as shown in Fig. 6, giving an
above result of decreasing emittance with current.
Simulations for bunched beam are then performed with
optimized mesh cells and mesh size in CST. First of all, we
wanted to investigate the pulse lengthening due to bunchcharge and gun voltage. In this study, simulations involving gun potentials of 50 kV, 72 kV and 100 kV and bunchcharge of 1nC and 2nC have been performed. An initialbunch length of 𝜎𝑡 = 167 ps (6𝜎𝑡 = 1ns) was assumed. Witha bunch charge of 1 nC, the peak current corresponds toabout 2 A. While studying the effect of bunch charge on
pulse lengthening, two peaks has been observed for higher
bunch charge, whereas no such peaks were observed forlower bunch charge as shown in Fig. 7. These peaks represent suppression of charge extraction due to the influence of
space charge. From the simulations, the space charge limits
for potentials of 50 kV, 72 kV, and 100 kV are obtained to be- 398 -

Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 TUP40

Figure 10: Principle of double slit emittance measurement.

Figure 8: Emittance variation of a 1 nC and 2 nC bunch fordifferent gun potential.

Figure 9: Bunch length variation along longitudinal position
with different potential (72 kV and 100 kV). Bunch charge
is 1 nC.
approximately 0.6 nC, 1.7 nC, and 2.6 nC, respectively. To
get a desired average current of 40 mA with a feasible repetition rate, bunch charge of at least 1 nC needed to consider.
The further study we therefore have performed for the gun
potential of 72 and 100 kV. With the same bunch charge of
1 nC, emittance for higher gun potential is less as comparedto that of lower gun potential as shown in Fig. 8. Also, no
significant difference in bunch length for 72 and 100 kV as
shown in Fig. 9. Emittance for higher bunch charge of 2nC
for 100 kV has also been checked for which the emittnace
is even larger than that for 1 nC, 72 kV. Further optimizations including solenoid and buncher system will therefore
be performed with bunch charge of 1nC and 100 kV.

EMITTANCE MEASUREMENT METHODS
Actual evaluation of the beam becomes necessary to validate, refine, and provide a ground truth that simulations
can be compared against. We therefore want to evaluate thebeam experimentally.

There exist various techniques for the experimental quantification of the emittance. Methods used for high energy
electron beam uses interactions of the beam with probes
used for measurement. However, no such directional interactions can be observed at low energy. Alongwith this,space-charge effects which are much prominent at low enegies leads to the beam spreading affecting the accuracy ofemittance measurements. However, there are few methods
that are most commonly used for effective measurement of
the emittance without getting influenced by space charge.
These are slit-and-grid systems and pepper-pot techniques.
Pepper pot is a most commonly used as it gives a single
shot measurement. In this method, a beam is stopped by amask containing holes. Only a small part of the beam i.e.
beamlets are allowed to pass through these holes. Since thecharge is significantly suppressed, the beamlets are emittance dominated and thus they traverse the drift with beam
divergence only. After a proper drift, a scintillator screen
produces an image of the beamlets. Analysis of the size
evolved and the intensity passed through the holes gives theemittance value. However, a whole beam can not be scannedwith this method as for the precise measurement of the beam
divergence, two beamlets should not overlap on the screen
constraining the holes to be placed apart [12]. To perform
pepper pot emittance measurement as per the beam properties, we therefore need to fix hole dimension, drift lengthand hole separation which are interdependent.
Comparatively, the principle of double slit emittancemethod in which two slits are used to measure emittanceas shown in Fig. 10, allows the whole beam scan. In thismethode, first slit of the device restricts the width of theparticle beam in one direction creating a narrow, emittancedominated beamlet. After traversing the drift length L fromfirst slit, the profile of the small beamlet is measured by
the second slit. For every position of first slit, there is corresponding profile scan of second slit. By scanning both
slits throughout the whole beam area, a beam distribution in
the transverse phase space obtained at the second slit position can be retraced back to get phase space data at first slit
position and hence the emittance.
Therefore to perform the double slit scan, parameters suchas slit width and drift length L need to be fixed. These parameters does not have any interdependence. We therefore
can find the suitable values for these parameters. In order toevaluate the actual beam produced by the electron gun, wetherefore have decided to use double slit emittance measure-- 399 -

Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 TUP40

electron gun and a sub-harmonic buncher system which will
generated an average current of 40 mA. Numerical studies performed till now have showed that there is no bunch
lengthening over the drift of 400 mm for the bunch with 1 nC
bunch charge and 𝜎𝑧 of 0.167 nS. Further studies concluded
that for bunch charges of 1 nC to 2 nC, a gun voltage of
100 kV is more suitable than 72 kV in terms of beam emittance. Additionally, to accurately evaluate the emittance ofthe produced electron beam, we are planing to implement a
double-slit emittance measurement method.

Figure 11: Electron beam trajectory after focusing.

REFERENCES[1] Trojanowicz et al., ”Gamma-ray, X-ray and electron beam
based processes”, Elsevier, 2018, pp. 257–331.
[2] Sampa, M. HO, et al., ”Electron beam wastewater treatment
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[3] Chmielewski, A. G., ”Radiation Processing: Environmental
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[4] Gonin, Ivan et al., Built-in Thermionic Electron Source for
SRF Linacs. No.FERMILAB-CONF-21-822-DI-SQMS-TD.
Fermi National Accelerator Lab.(FNAL), Batavia, IL (UnitedStates), 2021.
[5] Hanna, Samy M. ”Examples of radiation wastewater treatment
implemented in various countries”, Twelfth International Water Technology Conference, IWTC12, 2008.

Figure 12: Solenoid field strength at first slit location.
ment method. The double-slit apparatus at the ELPH facility
can accommodate a beam with a diameter of 30 mm. In response to this, we have employed a solenoid with which thebeam size is effectively reduced to a value significantly lessthan 10 mm, as depicted in Fig. 11. The initial position of
the first slit has been strategically chosen to be 300 mm awayfrom the cathode’s surface, as this is a region characterized
by a relatively weak magnetic field strength, as illustrated inFig. 12. The precise arrangement and configuration of the
double-slit setup including slit width and dirft length L will
now be determined through an analysis of the simulation by
the CST software.

SUMMARY
In the pursuit of an economically viable wastewater treatment solution, our research focuses on developing a highcurrent thermionic injector for a superconducting electron
linac. The system incorporates a DC gridded thermionic[6] Alexander V. Ponomarev and Boris G. Ershov, ”The Green
Method in Water Management: Electron Beam Treatment”,
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[7] Han, Bumsoo et al., ”Operation of industrial-scale electron
beam wastewater treatment plant”, Radiat. Phys. Chem 81.9
(2012): 1475-1478.
[8] Dhuley, R. C. et al., ”Design of a 10 MeV, 1000 kW average power electron-beam accelerator for wastewater treatment
applications”, Phys. Rev. Accel. Beams 25.4 (2022): 041601.
[9] S. Miura et al., ”Manufacturing of the electron gun of highintensity linac in Tohoku University”, Proc. of the 14th Annual
Meeting of Particle Accelerator Society of Japan, pp. 11781181, (2017).
[10] CST STUDIO SUITE, Computer Simulation Technology AG[11] K. Masuda : Ph. D Thesis, Dept. of Engineering, Kyoto Univ,(1998).
[12] Apsimon et al., ”A numerical approach to designing a versatile pepper-pot mask for emittance measurement”, Nuclear
Instruments and Methods A 943 (2019): 162485.

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