Electron dynamics in RF sources with a laser controlled emission
Photoemission radiofrequency (RF) electron sources are sources of electron beams with extremely high brightness. Beam bunching processes in such devices are well studied in case when laser pulse duration is much lower of rf oscillation period. At the same time photoemission RF guns have some merits...
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| Cite this: | Electron dynamics in RF sources with a laser controlled emission / I.V. Khodak, V.A. Kushnir, V.V. Mitrochenko, S.A. Perezhogin // Вопросы атомной науки и техники. — 2001. — № 5. — С. 166-168. — Бібліогр.: 9 назв. — англ. |
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nasplib_isofts_kiev_ua-123456789-790102025-02-09T10:16:07Z Electron dynamics in RF sources with a laser controlled emission Динамика электронов в высокочастотных источниках с лазерным управлением эмиссией частиц Khodak, I.V. Kushnir, V.A. Mitrochenko, V.V. Perezhogin, S.A. Photoemission radiofrequency (RF) electron sources are sources of electron beams with extremely high brightness. Beam bunching processes in such devices are well studied in case when laser pulse duration is much lower of rf oscillation period. At the same time photoemission RF guns have some merits when operating in 'long-pulse' mode. In this case the laser pulse duration is much higher of rf oscillation period but much lower of rise time of oscillations in a gun cavity. Beam parameters at the gun output are compared for photoemission and thermoemission cathode applications. The paper presents results of a beam dynamics simulation in such guns with different resonance structures. Questions connected with defining of the current pulse peak value that can be obtained in such guns are discussed. 2001 Article Electron dynamics in RF sources with a laser controlled emission / I.V. Khodak, V.A. Kushnir, V.V. Mitrochenko, S.A. Perezhogin // Вопросы атомной науки и техники. — 2001. — № 5. — С. 166-168. — Бібліогр.: 9 назв. — англ. 1562-6016 PACS numbers: 29.25.Bx, 41.75.Lx https://nasplib.isofts.kiev.ua/handle/123456789/79010 en Вопросы атомной науки и техники application/pdf Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Photoemission radiofrequency (RF) electron sources are sources of electron beams with extremely high brightness. Beam bunching processes in such devices are well studied in case when laser pulse duration is much lower of rf oscillation period. At the same time photoemission RF guns have some merits when operating in 'long-pulse' mode. In this case the laser pulse duration is much higher of rf oscillation period but much lower of rise time of oscillations in a gun cavity. Beam parameters at the gun output are compared for photoemission and thermoemission cathode applications. The paper presents results of a beam dynamics simulation in such guns with different resonance structures. Questions connected with defining of the current pulse peak value that can be obtained in such guns are discussed. |
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Khodak, I.V. Kushnir, V.A. Mitrochenko, V.V. Perezhogin, S.A. |
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Khodak, I.V. Kushnir, V.A. Mitrochenko, V.V. Perezhogin, S.A. Electron dynamics in RF sources with a laser controlled emission Вопросы атомной науки и техники |
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Khodak, I.V. Kushnir, V.A. Mitrochenko, V.V. Perezhogin, S.A. |
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Khodak, I.V. |
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Electron dynamics in RF sources with a laser controlled emission |
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Electron dynamics in RF sources with a laser controlled emission |
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Electron dynamics in RF sources with a laser controlled emission |
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Electron dynamics in RF sources with a laser controlled emission |
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Electron dynamics in RF sources with a laser controlled emission |
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electron dynamics in rf sources with a laser controlled emission |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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2001 |
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Electron dynamics in RF sources with a laser controlled emission / I.V. Khodak, V.A. Kushnir, V.V. Mitrochenko, S.A. Perezhogin // Вопросы атомной науки и техники. — 2001. — № 5. — С. 166-168. — Бібліогр.: 9 назв. — англ. |
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Вопросы атомной науки и техники |
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ELECTRON DYNAMICS IN RF SOURCES WITH
A LASER CONTROLLED EMISSION
I.V. Khodak, V.A. Kushnir, V.V. Mitrochenko, S.A. Perezhogin
National Science Center 'Kharkov Institute of Physics & Technology'
1, Akademicheskaya St., NSC KIPT, 61108 Kharkov, Ukraine
e-mail: kushnir@kipt.kharkov.ua
Photoemission radiofrequency (RF) electron sources are sources of electron beams with extremely high brightness.
Beam bunching processes in such devices are well studied in case when laser pulse duration is much lower of rf os-
cillation period. At the same time photoemission RF guns have some merits when operating in 'long-pulse' mode. In
this case the laser pulse duration is much higher of rf oscillation period but much lower of rise time of oscillations in
a gun cavity. Beam parameters at the gun output are compared for photoemission and thermoemission cathode ap-
plications. The paper presents results of a beam dynamics simulation in such guns with different resonance struc-
tures. Questions connected with defining of the current pulse peak value that can be obtained in such guns are dis-
cussed.
PACS numbers: 29.25.Bx, 41.75.Lx
1 INTRODUCTION
Modern experimental investigations using relativis-
tic electron beams require accelerators with high bright-
ness beams to be applied. An injector system is paid
great attention when designing such accelerators. Re-
cently, injector systems based on radio frequency elec-
tron sources (RF guns) are researched highly [1, 2]. RF
gun is realized by a pillbox supplied by RF power of
~106 W. A cathode is placed on the one of face walls. Its
emitting surface is in RF field with strength of
~(107-108) V/m.
Current pulse duration in thermionic RF guns is ap-
proximately equal to RF power pulse duration. The sec-
ond case is featured by wide application of RF guns
with current pulse duration much lower than RF oscilla-
tion period (τp<<c/f0), for instance, for S band ~10−
11 sec. It's obviously that field strength in this case is not
varied during the current pulse. This circumstance per-
mits to obtain high brightness beams [3]. Beam shaping
is more complicated when current pulse duration in RF
guns is much higher of RF oscillation period but much
lower of rise time of oscillations in a gun cavity (c/f0<<τ
p<<Q/πf0) [4]. Current density of the emission from a
cathode in this case (the same as in thermionic case) is
varied during current pulse time following the Schottky
law. Besides, there are some limitations for generation
intense electron beams. It is assumed here the finite
stored RF energy value in the cavity. Oxide cathodes,
that are good photoemitters, applied as a photocathode
gives the possibility to design multipurpose electron
sources. Electron dynamics in such RF guns is not
enough researched in spite of their advantages when
producing intense electron beam with microsecond and
nanosecond current pulse duration.
The purpose of this work is the obtaining of infor-
mation about particle dynamics features and electron
bunch shaping in 'short-pulse' RF guns with photoemis-
sion oxide cathode. The main used method is the com-
puter simulation of electron dynamics using PARMELA
code [5].
2 CALCULATION RESULTS
Due to oxide photocathode can be applied in both
thermoemission and photoemission mode, let's consider
differences in electron dynamics in these cases. These
differences are defined by the dependence of current
density for photoemission and thermoemission on elec-
tric field strength (effect Schottky). The density of pho-
toemission and thermoemission current in presence of
external electric field on a cathode surface can be writ-
ten respectively as [6]:
[ ]2
002
0 4/
2
π εωννϑ tSineEehh
k
AJ kф +−=
−
−=
k
kT
kT kT
tSineEe
TAJ 02
0
4/
exp
π εωϕ
ς
where Ек is maximum electric field strength on a cath-
ode, V/m,
Тк is cathode temperature,
hν is energy of quantum of laser radiation, eV,
hν0 is minimum energy of quantum, corresponding to
power ability of a single-photon photoemission (coin-
cides with work function for metals), eV,
ϕТ is work function, eV,
ω=2πf0,
к is Boltzmann constant,
ε0 is permittivity,
А0 is Richardson constant,
е is electron charge,
ϑ and ς are dimensionless constants featuring cathode
surface properties.
It follows from the analysis of given above expres-
sions that time dependence of emission current density
in these cases will be differed during accelerating half-
period of RF oscillations. So, electric field increasing
from 0 to 30 MV/m causes the emission current density
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.
Серия: Ядерно-физические исследования (39), с. 167-168.
166
mailto:kushnir@kipt.kharkov.ua
increasing in 30% for the photoemission case (h
ν = 3.49 eV, hν0 = 2.0 eV). Such field increasing for the
thermoemission Ba-Ni cathode (Tk = 1000K) causes the
thermoemission density increasing in 11 times. It should
be noted that given estimations are valid in absence of
limitation by space charge ('3/2' law) and cathodes oper-
ate in saturation mode. However the difference in time
dependence of current density can cause differences in
phase-energy distributions of beams. In order to exam
this statement we had computer simulation of beam dy-
namics for the single cavity RF gun described in [7].
Resonance system of the gun is the cylindric E010 cavity
with fundamental frequency and quality factor of
2797 MHz and 1.3⋅104 respectively. The cavity is RF
power supplied (up to 2 MW) through the coupling win-
dow in its element. It was assumed for the simulation
that averaged over oscillation period current is equal for
the photoemission and thermoemission (1.8 A). And it
was supposed also that there is steady state for thermoe-
mission and current pulse duration is c/f0<<τp<<Q/πf0
for photoemission. These conditions correspond to the
given field approximation that is used in PARMELA
code. Space charge limitation effect was taken into ac-
count in calculations because of it takes places during
the part of oscillation period when the current value ob-
tained after Langmuir formula is lower of photoemis-
sion (or thermoemission) saturation current. Electric
field strength averaged over the cavity length was taken
of 30 MV/m in both cases. Obviously it requires differ-
ent RF power level to be supplied to obtain equal field
strength in different gun operating modes. Fig. 1 and
Fig. 2 show energy and phase particle distributions re-
spectively.
0 0.2 0.4 0.6 0.8 1
0
20
40
60
80
100
Energy (MeV)
N
1
2
Fig. 1. Electron energy spread at the single-cavity
RF gun output. 1 – photoemission, 2 – thermoemis-
sion.
- 2 0 0 - 1 0 0 0 1 0 0 2 0 0
0
1 0
2 0
3 0
4 0
5 0
6 0
7 0
8 0
9 0
P h a s e ( ° )
N
1
2
Fig. 2. Electron phase spread at the single-cavity
RF gun output. 1 – photoemission, 2 – thermoemis-
sion.
Differences in particle energy distribution for differ-
ent emission cases noted in simulation results were ob-
served experimentally when the gun was researched be-
ing a part of the linac LU-60 injector system [8]. In par-
ticular, by the beam deflection in the injector deflecting
system it was noted the electron energy distribution den-
sity is considerably higher of 'photo' electrons.
Main simulation results are summarized in Table 1.
One can see that photoemission electron beam has the
lower phase and energy spread comparatively with ther-
moemission electron beam. Hence, photoemission elec-
tron beam has lower longitudinal emittance Because of
better angular performances it causes the considerable
difference in the beam brightness. Pointed differences
have influence primarily on particle phase-energy distri-
butions (Fig. 3) that is important for the application of
additional bunch phase compression systems based on
the non-isochronism of particle motion, for instance, α-
magnet.
Table 1
Parameter Thermo Photo
Supplied RF power, MW 1.3 0.570
Normalized emittance (rms),
mm mrad
17.4 16.0
Maximum energy, MeV 0.91 0.91
Average energy, MeV 0.51 0.57
Energy spread width for 70%
of particles, %
64 55
Bunch phase length for 70%
of particles, degree
53 43
Output current averaged over
period, А
1.19 1.3
Bunch peak current, А 11 15
Normalized brightness for
95% of particles, 109 А/м2
4.5 7.3
It was noted during the simulation the strong depen-
dence of output beam parameters on electric field
strength and its distribution. It is obviously that these
performances can't be changed actually due to hardware
specialties in the single cavity gun. Therefor resonance
system of multipurpose RF gun (capable to operate in
both with thermoemission and photoemission cathode in
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.
Серия: Ядерно-физические исследования (39), с. 167-168.
167
broad supplying RF power range) has to have flexibly
tuned resonance system. Two-cavity RF gun [9] can be
taken as the example of such resonance system. The
idea of multipurpose radiofrequency electron source
was realized here for the first time. The gun has tools
for tuning fundamental frequency in each cavity. It per-
mits to vary amplitude ratio in cavities η = Ezmax1/Ezmax2
in range of 0.53…2.34 without changing fundamental
frequency totally. We researched the dependence of out-
put beam parameters on field strength and parameter η
by computer simulation both in photoemission and ther-
moemission modes.
Fig. 3. Phase-energy electron distribution at the sin-
gle-cavity RF gun output. 1 – thermoemission,
2 - photoemission.
Simulation results confirmed the main feature of the
single-cavity gun that is expressed in follows: the pho-
toemission electron beam has lower emittance, bunch
phase length and particle energy spread and, hence,
higher brightness. It was shown that the possibility to
adjust field strength distribution along the cavity axis
permits both to change particle energy and to optimize
the system relatively the given beam parameter. For in-
stance, to obtain minimum of beam normalized emit-
tance (16 mm⋅mrad) under average field strength of
20 MV/m one should set η = 1.25. To obtain minimum
of phase length (50° for 70 % of particles one should set
η = 0.82 and to obtain minimum of energy spread (19 %
for 70 % of particles one should set η = 1.52.
The finite value of electromagnetic field energy
stored in resonance system of RF gun is the factor limit-
ing maximum pulse charge in laser driven RF guns with
current pulse duration that is significantly lower of time
constant of the resonance system. It is obviously that en-
ergy of electron beam removed by particles out of the
resonance system can’t be higher of this value. Maxi-
mum charge value qmax at the gun output can be ex-
pressed as following:
averWf
PQ
q
⋅
⋅
≤
0
0
max 2π ,
where Q is unloaded quality factor of the cavity;
P is power dissipated in cavity walls, W;
Waver is average electron energy at the gun output,
eV;
f0 is operating frequency, Hz.
For typical values of Q0 = 104, Р = 1 MW,
f0 = 3 GHz, Waver = 0.5 MeV peak pulse charge can
reach 3 µC. This means that current pulse with duration
of ~10-8 sec can have amplitude of hundreds amperes. It
should be noted that energy transferred to particles stay-
ing in the cavity is not taken into account here. This en-
ergy value, as a rule, is not higher of 20 % of energy
transferred to electrons of main beam. Its value can be
defined enough accurate using computer simulation of
electron dynamics in a gun.
3 CONCLUSION
Analysis of computer simulation results permits to
conclude the following:
1.Photoemission electron beam has higher bright-
ness comparatively with thermoemission electron
beam for equal field strength in the same gun.
2.Oxide cathode application in the same RF gun per-
mits to realize thermoemission and photoemission
modes simultaneously. This gives the possibility to
obtain at RF gun output simultaneously two beams
differed by current pulse duration and phase-energy
distribution.
REFERENCES
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Preprint / Laboratorie de l'Accelerateur Linearie; RT 98-
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2.V.A.Kushnir. High-frequency electron gun – current
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8.A.N.Dovbnya, V.F.Ziglo, V.F.Koliorov et. al. Experi-
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ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.
Серия: Ядерно-физические исследования (39), с. 166-168.
168
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