RF modulated thermionic electron gun
We present design and simulation results on the electron source for S-band technological electron linac. This device is a combination of an RF gun and a DC diode gun. The gun consists of a thermionic cathode, which is mounted on the wall of the TM₀₁₀ cavity that is under negative high voltage potent...
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irk-123456789-1957892023-12-07T12:15:36Z RF modulated thermionic electron gun Mytrochenko, V.V. Zhyglo, V.F. Kushnir, V.A. Linear charged-particle accelerators We present design and simulation results on the electron source for S-band technological electron linac. This device is a combination of an RF gun and a DC diode gun. The gun consists of a thermionic cathode, which is mounted on the wall of the TM₀₁₀ cavity that is under negative high voltage potential relatively to an anode. The particles dynamics has been studied by simulation. At an average particle energy of 95 keV, the beam pulse current is 0.44 A, the emittance does not exceed 4.8 mm·mrad, and 70% of all electrons are concentrated in the 72° phase range. Наведено результати розрахунку джерела електронів для лінійного технологічного прискорювача електронів десятисантиметрового діапазону. Пропонована нами гармата складається з термоелектронного катода, який встановлений на стінці TM₀₁₀-резонатора, що знаходиться під високим негативним потенціалом відносно анода. Методом чисельного моделювання досліджено динаміку частинок. При середній енергії частинок 95 кeВ імпульсний струм пучка дорівнює 0,44 А, еміттанс не перевищує 4,8 мм·мрад і 70% всіх електронів зосереджено в діапазоні фаз 72°. 2022 Article RF modulated thermionic electron gun / V.V. Mytrochenko, V.F. Zhyglo, V.A. Kushnir // Problems of Atomic Science and Technology. — 2022. — № 5. — С. 77-81. — Бібліогр.: 16 назв. — англ. 1562-6016 PACS: 29.20.Ej, 07.77.Ka DOI: https://doi.org/10.46813/2022-141-077 http://dspace.nbuv.gov.ua/handle/123456789/195789 en Problems of Atomic Science and Technology Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Linear charged-particle accelerators Linear charged-particle accelerators Mytrochenko, V.V. Zhyglo, V.F. Kushnir, V.A. RF modulated thermionic electron gun Problems of Atomic Science and Technology |
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We present design and simulation results on the electron source for S-band technological electron linac. This device is a combination of an RF gun and a DC diode gun. The gun consists of a thermionic cathode, which is mounted on the wall of the TM₀₁₀ cavity that is under negative high voltage potential relatively to an anode. The particles dynamics has been studied by simulation. At an average particle energy of 95 keV, the beam pulse current is 0.44 A, the emittance does not exceed 4.8 mm·mrad, and 70% of all electrons are concentrated in the 72° phase range. |
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Article |
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Mytrochenko, V.V. Zhyglo, V.F. Kushnir, V.A. |
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Mytrochenko, V.V. Zhyglo, V.F. Kushnir, V.A. |
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Mytrochenko, V.V. |
| title |
RF modulated thermionic electron gun |
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RF modulated thermionic electron gun |
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RF modulated thermionic electron gun |
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RF modulated thermionic electron gun |
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RF modulated thermionic electron gun |
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rf modulated thermionic electron gun |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Linear charged-particle accelerators |
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RF modulated thermionic electron gun / V.V. Mytrochenko, V.F. Zhyglo, V.A. Kushnir // Problems of Atomic Science and Technology. — 2022. — № 5. — С. 77-81. — Бібліогр.: 16 назв. — англ. |
| series |
Problems of Atomic Science and Technology |
| work_keys_str_mv |
AT mytrochenkovv rfmodulatedthermionicelectrongun AT zhyglovf rfmodulatedthermionicelectrongun AT kushnirva rfmodulatedthermionicelectrongun |
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2025-07-16T23:58:58Z |
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ISSN 1562-6016. Problems of Atomic Science and Technology. 2022. №5(141) 77
https://doi.org/10.46813/2022-141-077
RF MODULATED THERMIONIC ELECTRON GUN
V.V. Mytrochenko, V.F. Zhyglo, V.A. Kushnir
National Science Center “Kharkov Institute of Physics and Technology”,
Kharkiv, Ukraine
E-mail: kushnir@kipt.kharkov.ua
We present design and simulation results on the electron source for S-band technological electron linac. This
device is a combination of an RF gun and a DC diode gun. The gun consists of a thermionic cathode, which is
mounted on the wall of the TM010 cavity that is under negative high voltage potential relatively to an anode. The
particles dynamics has been studied by simulation. At an average particle energy of 95 keV, the beam pulse current
is 0.44 A, the emittance does not exceed 4.8 mmmrad, and 70% of all electrons are concentrated in the 72° phase
range.
PACS: 29.20.Ej, 07.77.Ka
INTRODUCTION
One of the elements that determine the output beam
quality in electron linacs is the electron source - the
electron gun. If the conventional diode electron gun is
used, the formation of electron bunches, occurs in a
bunching system and in first accelerating section.
Obviously, the requirements for an electron source
are determined by the accelerator purpose and the
requirements for the beam parameters at its output. In
particular, in accelerator based free electron lasers,
radiofrequency (RF) guns with cathodes of various
types were designed. Thus, in the most widely used
photocathode RF guns, precision beams with extremely
high brightness are obtained [1, 2]. Although the
thermionic RF guns are much simpler than the
photocathode guns; in particular, they do not require the
use of a laser system, they are suitable to obtain even
femtosecond bunches using non-isochronous
compression device [3].
Currently, intensive research is being carried out on
electron sources that would combine the advantages of
RF guns and DC guns with different cathodes. For
example, a DC gun with photoemission cathode has
been designed, built, and commissioned at Cornell
University [4]. The high-voltage (350 kV) DC diode
gun contains the GaAs photocathode, irradiated with the
1 ps duration laser pulses at pulse repetition rate of
1.3 GHz. This makes it possible to obtain at the injector
output bunch charge of 77 pC with a normalized
emittance of 0.53 μm. Paper [5] describes a combined
gun with thermionic CeB6 cathode. The cathode is
installed in a complex form stripline (stripline loop).
Two high voltage 1 ns pulses propagating in strip-loop
towards each other created electric field on the cathode.
Emitted electrons pass through the aperture of the line
outer conductor and accelerated in the RF resonator.
Such gun, according to simulations, makes it possible to
form beams with an emittance, close to the thermal level
of 0.3 mmmrad.
The various possibilities to obtain both pulsed and
RF modulated beam near the cathode can be realized by
using a triode electron gun. In the most used accelerator
schemes the nanosecond pulses are applied to the grid.
The pulse repetition rate is equal to one of
subharmonics of f0. Then, pulsed beam generated in the
gun is compressed by subharmonic bunchers. The
advantage of such schemes for the formation of bunches
is the bunches production with a large charge. Such a
bunching scheme is often used in synchrotron injector
accelerators. An example is the linear accelerator-
injector for ALBA synchrotron [6] with a triode 90 kV
electron gun. In one of operating modes of this
accelerator, pulses with duration of 1ns are applied to
the gun grid. Pulse duration of 1 ns corresponds to of
180at the sixth subharmonic of the accelerator's
operating frequency. Further, 0.25 nK bunch is
compressed by bunchers to duration of 17 ps. A similar
scheme of bunch formation is used in compact
accelerators for various purposes including FEL [7–9].
In a triode gun, the beam RF modulation can also be
carried out. This approach was first applied in the
accelerator FELIX [10]. A microwave signal with
frequency of 1 GHz (3rd subharmonic of the linac
operating frequency) is applied to the grid of the 100 kV
gun. By choosing ratio between the constant bias value
and the RF field amplitude on the grid, one can control
the bunch length at the output of the gun. It is possible
to provide modulation at higher linac operating
frequency using CPI Eimac cathode-grid assemblies.
Such a gun, for example, was developed for the
2856 MHz linac at Pohang Accelerator Laboratory [11,
12]. At the same time, the using for near-cathode
modulation of standard cathode assemblies at high
frequency have some problems caused by both the grid
configuration and the cathode-grid large capacity. The
creation of a special near-cathode resonator with an
appropriate microwave power supply system is one of
the solutions to these problems. The article presents the
results of calculation and numerical simulation of an
electron gun that contains these elements.
1. COMBINED RF/DC GUN DESIGN
The developed gun is intended for use in the
upgraded S band technological electron linac LU-10
[13]. It is an alternative version of the electron source
[14], which has already been developed and
manufactured for this accelerator. The injection system
of the linac consists of a diode thermionic gun with a
voltage of 80 kV and TM010 prebuncher. A beam with a
current of 0.4 A enters the accelerating section. The
suggested gun (Fig. 1) consists of a thermionic cathode,
mailto:kushnir@kipt.kharkov.ua
78 ISSN 1562-6016. Problems of Atomic Science and Technology. 2022. №5(141)
which is installed on the wall of the TM010 cavity and
anode, which is located at some distance from the
cavity. The electron bunches, which are produced in the
RF cavity, are rapidly accelerated in the space cavity-
anode and then injected into the accelerating section.
Thus, this device is a combination of RF gun and DC
diode gun. The RF cavity is under high voltage potential
relative to ground, so a filter that protects the
microwave power source and supply lines from high
voltage is required.
Fig. 1. Layout of the Gun
The cavity tuned to frequency of 2856 MHz. The
cavity diameter and the accelerating gap are 79.08 and
2 mm, respectively. The hole diameter for the beam exit
is 2 mm. The microwave power is supplied to the
resonator by two diametrically located waveguides,
which makes it possible to reduce the field asymmetry
in the cathode region. According to the simulation, the
loaded quality factor of the cavity is 1540, the shunt
impedance is 2.68 MΩ/m. With a high-frequency power
of 11.9 kW supplied to the cavity, the average field on
the axis is 1.7 MV/m. An anode is located at 10 mm
from the cavity. The LaB6 cathode is installed in the
traditional way for thermionic RF guns using a /2
filter. To obtain the required output beam current, the
cathode diameter was chosen to be 3 mm. Simulation
shows that at an emission density of 18 A/cm
2
, the
cathode current in the operating mode is 1.3 A, and the
current at the gun output is 0.44 A. The RF and DC
fields are shown in Fig. 2.
Fig. 2. The distribution of RF and DC field on the gun axis
It can be seen that there is only a high-frequency
field at the cathode. This makes it possible to abandon
the high-voltage gun modulator since the current pulse
duration at the gun output is determined by the RF pulse
duration. This allows to adjust easily the duration of the
pulse current using PIN diode RF switch, which is
connected in circuit of the gun RF power supply.
The gun is powered both by a highly stable DC
voltage source of 80 kV and by RF power source. A
simplified diagram of the gun high-frequency power
supply is shown in Fig. 3. Using a waveguide adjustable
directional coupler, part of the klystron power is fed
through a phase shifter, the PIN diode RF switch and
the filter to coaxial hybrid ring coupler and then through
coaxial-to-wave guide adapters to 73x10 mm
waveguides, which provide RF power to the cavity.
ISSN 1562-6016. Problems of Atomic Science and Technology. 2022. №5(141) 79
Fig. 3. Schematic representation of the gun: 1 – waveguide directional coupler; 2 – variable phase shifter and
the PIN diode RF switch; 3 – filterж 4 – coaxial hybrid ring coupler
2. FILTER
Let us consider in more detail the developed filter,
which should both ensure the transmission of RF power
with minimal losses and perform an insulator function
withstanding the voltage of 80 kV. The device (Fig. 4)
consists of input rectangular waveguide 72x34 mm, the
rectangular horn, the dielectric rod and the coaxial
adapter. In such a system, several mode transformations
take place. Thus, in the input device, the TE10 wave in a
rectangular waveguide is transformed into a hybrid
ME11 mode in the dielectric rod. At the output device,
the ME11 mode is transformed into a TE11 cylindrical
mode, which is transformed into a TEM mode in a
coaxial. The material of the dielectric rod must have low
dielectric losses (for example, Teflon, polystyrene,
quartz, or RF ceramics), good insulating properties and
sufficiently high radiation resistance. We chose quartz
(=3,75, tg=110
-4
), which has all these properties for
simulation and filter design. The diameter of the rod
regular part (48 mm) was determined to maintain the
single-mode regime in a quartz dielectric waveguide
[15].
Fig. 4. Schematic of the filter: 1 – rectangular TE10 waveguide; 2 – rectangular horn;
3 – section of dielectric waveguide; 4 – transitions from a cylindrical waveguide to coaxial line
The rod length of 220 mm ensures reliable
insulation. The main sources of microwave power losses
in such system are reflection losses, losses in metal
walls and dielectric, and radiation losses from the
surface of the dielectric rod. Analysis shows that most
of the losses are due to the presence of numerous
reflections from inhomogeneities within the junction.
The device was adjusted by sequentially changing the
dimensions of its elements. As a result, at frequency of
2856 MHz, the values S11 = -19 dB and S12 = -1.04 dB
were obtained, i.e., the standing wave ratio at the input
is 1.25 and the power loss is about 20.
3. BEAM DYNAMICS SIMULATION
The particle dynamics in the gun was simulated
using the Parmela program [16]. In modeling, it was
assumed that the initial bunch length is 360°, which
corresponds to the continuous emission of particles
from the cathode surface, and the initial energy of
electrons at the cathode is 0.15 eV. Electrons enter the
cavity only during the half-cycle of microwave
oscillations. The simulation was carried out at various
phases of the field in the cavity and at various reference
particle positions. In all cases the main beam parameters
at the gun exit changed insignificantly and difference
does not exceed of the simulation error. Therefore, we
present below the results obtained with the cavity phase
80 ISSN 1562-6016. Problems of Atomic Science and Technology. 2022. №5(141)
of 180°. In this case, the second half of the injected
bunch is accelerated, and the parameter Z0 is chosen so
that the reference particle enters the resonator at a phase
of 180°. The beam dynamic simulation showed that at
cathode current of 1.3 A, the gun output current is
0.444 A, i.e., 66% of the injected particles come back.
The energy and phase characteristics of the beam at the
gun exit are shown in Fig. 5. The average and maximum
electron energies at the gun output are 95 and 103 keV,
respectively. In this case, for 70% of the particles, the
relative energy spread does not exceed 11%. In the
longitudinal phase space, the beam is a compact bunch
and 70% of the particles are concentrated within 72°.
This provides a good particle capture rate in the
acceleration process in the accelerating section. The
spatial characteristics of the beam at the gun exit are
shown in Fig. 6.
It can be seen that the distribution of beam particles
in the transverse plane is close to symmetrical. The rms
beam diameters are Dx = 4.67 mm and Dy = 4.63 mm.
The diameters for 95% of particles in the X and Y
directions are 8.81 and 8.77 mm, respectively. The
value of the rms normalized beam emittance in different
planes is also close: x = 4.79 mmmrad and the
y = 4.70 mmmrad. The slight asymmetry in the spatial
distribution of the particle density observed in the
simulation is apparently caused by RF field asymmetry
in the cavity. The output beam is divergent (see Fig. 6),
but the divergence is such that it allows the beam to be
injected into the accelerating section without loss of
particles. The time for current to reach its steady state
depends on cavity filling time and does not exceed
0.2 μs. This value is acceptable at the required current
pulse width of 10 μs.
As is known, one of the problems in the existing
thermionic RF guns is the back bombardment of the
cathode. This effect limits the possibility of both
increasing the pulse duration and increasing the pulse
repetition rate. In our case, power of the reversed
electrons is significantly less than the cathode heating
power even at a pulse repetition rate of 600 Hz.
Therefore, the back bombarding electrons do not
significantly change the cathode temperature. Compared
to the previously developed LU-10 injection system (the
diode gun and the prebunching resonator), the cathode
microwave modulation gun has significantly better
beam parameters. In particular, the bunch duration is 2.5
times smaller.
a
b
c
Fig. 5. Energy spectrum (a), phase spectrum (b), and phase-energy distribution of electrons at the gun output (c)
Fig. 6. Distribution of electrons on the transverse XY plane (left) and on the phase XX´ plane (right)
CONCLUSIONS
The results of calculations and numerical simulation
showed the possibility of creating the electron gun for S
band linac with near cathode RF modulation. The beam
at the gun output is a sequence of bunches. The bunch
repetition frequency is equal to the linac operating
frequency. The beam parameters at the gun exit fully
satisfy the stated requirements. Its application makes it
possible to exclude the high-voltage electron source
modulator and the resonator prebuncher from the
accelerator circuit and improve the beam parameters at
the linac output. The developed gun is designed for a
specific technological accelerator, but the same
principle – the principle of near-cathode microwave
beam modulation by RF cavity can also be used in
accelerators designed for scientific research.
REFERENCES
1. D.H. Dowell. Sources of Emittance in RF
Photocathode Injectors: Intrinsic emittance, space
ISSN 1562-6016. Problems of Atomic Science and Technology. 2022. №5(141) 81
charge forces due to non-uniformities, RF and
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DOI: 10.1109/PAC.2003.1289968
Article received 12.09.2022
ТЕРМОЕМІСІЙНА ЕЛЕКТРОННА ГАРМАТА З ВИСОКОЧАСТОТНОЮ МОДУЛЯЦІЄЮ ПУЧКА
В.В. Митроченко, В.Ф. Жигло, В.А. Кушнір
Наведено результати розрахунку джерела електронів для лінійного технологічного прискорювача
електронів десятисантиметрового діапазону. Пропонована нами гармата складається з термоелектронного
катода, який встановлений на стінці ТМ010-резонатора, що знаходиться під високим негативним потенціалом
відносно анода. Методом чисельного моделювання досліджено динаміку частинок. При середній енергії
частинок 95 кeВ імпульсний струм пучка дорівнює 0,44 А, еміттанс не перевищує 4,8 мммрад і 70% всіх
електронів зосереджено в діапазоні фаз 72°.
https://arxiv.org/ftp/arxiv/papers/1610/1610.01242.pdf
https://ieeexplore.ieee.org/author/37344024400
https://ieeexplore.ieee.org/author/37353582200
https://ieeexplore.ieee.org/xpl/conhome/9054/proceeding
https://ieeexplore.ieee.org/xpl/conhome/9054/proceeding
https://doi.org/10.1109/PAC.2003.1289968
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