RF photogun and Cherenkov decelerating system for a high power radiation source in sub-mm region
Some results of RF photogun and Cherenkov decelerating system research and design are discussed. This R&D is providing to construct a high power pulse radiation source in sub-mm region. It is well known that the conducting capillary filled by dielectric skin can be used as a Cherenkov radiation...
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| Zitieren: | RF photogun and Cherenkov decelerating system for a high power radiation source in sub-mm region / Yu.A. Bashmakov, T.V. Bondarenko, D.A. Komarov, S.M. Polozov, V.I. Rashchikov, I.S. Shchedrin, A.V. Smirnov, A.V. Voronkov, A.A. Tishchenko // Вопросы атомной науки и техники. — 2012. — № 3. — С. 92-95. — Бібліогр.: 4 назв. — англ. |
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irk-123456789-1086732016-11-14T03:02:20Z RF photogun and Cherenkov decelerating system for a high power radiation source in sub-mm region Bashmakov, Yu.A. Bondarenko, T.V. Komarov, D.A. Polozov, S.M. Rashchikov, V.I. Shchedrin, I.S. Smirnov, A.V. Voronkov, A.V. Tishchenko, A.A. Элементы ускорителей Some results of RF photogun and Cherenkov decelerating system research and design are discussed. This R&D is providing to construct a high power pulse radiation source in sub-mm region. It is well known that the conducting capillary filled by dielectric skin can be used as a Cherenkov radiation generator. One needs very short (less than 1 mm) and high brightness electron bunch to provide the coherent radiation. The short bunch can be generated by means of a photogun. The electrons should be accelerated to the energy equal to 1…4 MeV in compact section and injected to the decelerating structure. This radiation source can be used for inspection systems or as a laboratory generator. Рассмотрены некоторые результаты исследования и разработки СВЧ-фотокатода и черенковской замедляющей системы, предназначенных для генерации мощных импульсов излучения субмиллиметрового диапазона. Как известно, проводящий капилляр, покрытый изнутри слоем диэлектрика, может быть использован в качестве источника черенковского излучения. Для получения монохроматического излучения необходимо иметь очень короткий (меньше 1 мм) сгусток электронов, который может быть получен при использовании фотокатода. Электроны должны быть ускорены в короткой системе до энергии 1…4 МэВ и инжектированы в замедляющую систему. Такой источник излучения может быть использован в досмотровой системе или в качестве лабораторного генератора. Розглянуто деякі результати дослідження та розробки НВЧ-фотокатода і черенківської уповільнюючої системи, призначених для генерації потужних імпульсів випромінювання субміліметрового діапазону. Як відомо, проводячий капіляр, покритий зсередини шаром діелектрика, може бути використаний як джерело черенківського випромінювання. Для отримання монохроматичного випромінювання необхідно мати дуже короткий (менше 1 мм) згусток електронів, який може бути отриманий при використанні фотокатода. Електрони мають бути прискорені в короткій системі до енергії 1…4 МеВ і інжектовані в уповільнюючу систему. Таке джерело випромінювання може бути використано в оглядовій системі або в якості лабораторного генератора. 2012 Article RF photogun and Cherenkov decelerating system for a high power radiation source in sub-mm region / Yu.A. Bashmakov, T.V. Bondarenko, D.A. Komarov, S.M. Polozov, V.I. Rashchikov, I.S. Shchedrin, A.V. Smirnov, A.V. Voronkov, A.A. Tishchenko // Вопросы атомной науки и техники. — 2012. — № 3. — С. 92-95. — Бібліогр.: 4 назв. — англ. 1562-6016 PACS: 29.27.-A, 29.27.Bd http://dspace.nbuv.gov.ua/handle/123456789/108673 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Элементы ускорителей Элементы ускорителей |
| spellingShingle |
Элементы ускорителей Элементы ускорителей Bashmakov, Yu.A. Bondarenko, T.V. Komarov, D.A. Polozov, S.M. Rashchikov, V.I. Shchedrin, I.S. Smirnov, A.V. Voronkov, A.V. Tishchenko, A.A. RF photogun and Cherenkov decelerating system for a high power radiation source in sub-mm region Вопросы атомной науки и техники |
| description |
Some results of RF photogun and Cherenkov decelerating system research and design are discussed. This R&D is providing to construct a high power pulse radiation source in sub-mm region. It is well known that the conducting capillary filled by dielectric skin can be used as a Cherenkov radiation generator. One needs very short (less than 1 mm) and high brightness electron bunch to provide the coherent radiation. The short bunch can be generated by means of a photogun. The electrons should be accelerated to the energy equal to 1…4 MeV in compact section and injected to the decelerating structure. This radiation source can be used for inspection systems or as a laboratory generator. |
| format |
Article |
| author |
Bashmakov, Yu.A. Bondarenko, T.V. Komarov, D.A. Polozov, S.M. Rashchikov, V.I. Shchedrin, I.S. Smirnov, A.V. Voronkov, A.V. Tishchenko, A.A. |
| author_facet |
Bashmakov, Yu.A. Bondarenko, T.V. Komarov, D.A. Polozov, S.M. Rashchikov, V.I. Shchedrin, I.S. Smirnov, A.V. Voronkov, A.V. Tishchenko, A.A. |
| author_sort |
Bashmakov, Yu.A. |
| title |
RF photogun and Cherenkov decelerating system for a high power radiation source in sub-mm region |
| title_short |
RF photogun and Cherenkov decelerating system for a high power radiation source in sub-mm region |
| title_full |
RF photogun and Cherenkov decelerating system for a high power radiation source in sub-mm region |
| title_fullStr |
RF photogun and Cherenkov decelerating system for a high power radiation source in sub-mm region |
| title_full_unstemmed |
RF photogun and Cherenkov decelerating system for a high power radiation source in sub-mm region |
| title_sort |
rf photogun and cherenkov decelerating system for a high power radiation source in sub-mm region |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| publishDate |
2012 |
| topic_facet |
Элементы ускорителей |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/108673 |
| citation_txt |
RF photogun and Cherenkov decelerating system for a high power radiation source in sub-mm region / Yu.A. Bashmakov, T.V. Bondarenko, D.A. Komarov, S.M. Polozov, V.I. Rashchikov, I.S. Shchedrin, A.V. Smirnov, A.V. Voronkov, A.A. Tishchenko // Вопросы атомной науки и техники. — 2012. — № 3. — С. 92-95. — Бібліогр.: 4 назв. — англ. |
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Вопросы атомной науки и техники |
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ISSN 1562-6016. ВАНТ. 2012. №3(79) 92
RF PHOTOGUN AND CHERENKOV DECELERATING SYSTEM
FOR A HIGH POWER RADIATION SOURCE IN SUB-MM REGION
Yu.A. Bashmakov1,2, T.V. Bondarenko1, D.A. Komarov1,3, S.M. Polozov1, V.I. Rashchikov1,
I.S. Shchedrin1, A.V. Smirnov4, A.V. Voronkov1, A.A. Tishchenko1
1National Research Nuclear University MEPhI, Moscow, Russia;
2P.N. Lebedev Physical Institute of RAS, Moscow, Russia;
3Research & production corporation “Toriy”, Moscow, Russia;
4Radia Beam Technologies, Santa Monica, CA 90404
E-mail: smpolozov@mephi.ru
Some results of RF photogun and Cherenkov decelerating system research and design are discussed. This R&D
is providing to construct a high power pulse radiation source in sub-mm region. It is well known that the conducting
capillary filled by dielectric skin can be used as a Cherenkov radiation generator. One needs very short (less than
1 mm) and high brightness electron bunch to provide the coherent radiation. The short bunch can be generated by
means of a photogun. The electrons should be accelerated to the energy equal to 1…4 MeV in compact section and
injected to the decelerating structure. This radiation source can be used for inspection systems or as a laboratory
generator.
PACS: 29.27.-A, 29.27.Bd
1. INTRODUCTION
The problem of the introscopy is very actually at pre-
sent. The detection of weapons, explosives, drags and
fissionable materials is the main aim of introscopy. The
introscopy of cargo transport is strongly difficult com-
paratively of the passenger’s one. The gamma, electron or
neutron facilities are used for introscopy at present in-
cluding cargo introscopy. The compact electron or ion
gun or accelerator is the base element of such facilities.
It’s necessary to have an electron linac (or two accelera-
tors) which can derive the beam with energy variation in
3-5 times for cargo introscopy for example. This is no
easily task. All of gamma, electron or neutron facilities
have a number of great disadvantages as needs of the
enviermental shielding and activation of the cargo.
New generation of introscopy facilities with low ac-
tivation are under design now. The using of THz region
radiation is one of possible methods. The radiation in
sub-mm region is comletely safe as being not ionizing.
But compact and effective sources in sub-mm region are
absent in present. High radiation power can be gener-
ated using large accelerator (linear or synchrotron) and
free electron laser (FEL) but such facilities are not com-
pact. Traditional vacuum (as traveling wave tubes, car-
cinotrons. klinotrons, orotrons) and solid state (OLED,
resonant tunnel diodes) generators not give power
higher than 1 W.
Indeed the design of compact and effective generator
is very actual aim. Such facilities can be used not only
in introscopy system as well as in biology, medicine,
chemistry, solid state physics, radio astronomy, home-
land security, environment monitoring, spintronics, ad-
vanced spectroscopy, and plasma diagnostics [1].
2. GENERAL PRINCIPLE OF THZ
RADIATION GENERATOR
The design of THz (or sub-mm) radiation source is
one of possible needs of photo guns. The THz generator
based on Cherenkov or Smith-Parcell was proposed
early [1]. The short electron bunch with MeV energy
and special decelerating system was discussed. The ra-
diation in ps and sub-ps bands can be generated using
this scheme.
The operating principle of proposed generator is
based on using of the Cherenkov irradiator and undula-
tor is not necessary here. Such generator was proposed
by B.M. Bolotovskiy in 1961. The coherent Cherenkov
radiation can be generated using short and well colli-
mated electron bunches which must also have ps or sub-
ps duration and 100…200 μm transverse size. Such
bunch can be formed using a photo cathode and com-
pact accelerating system providing high acceleration
gradient. The laser system driving the photo gun pro-
posed must generate short laser pulses or a series of
pulses. The bunch (or bunch packet) should be acceler-
ated to the MeV energy and injected into especial irra-
diating capillary channel in which electromagnetic ra-
diation will induced. Two types of capillary channel
could be used in the proposed generator. The conduct-
ing (metal) capillary coated inside by dielectric can be
used. The slow-wave structure can be made as a cor-
rugated channel or grating surface also.
The possibility of high brightness, high density
bunch focusing to the sub-wavelength dimension (so
called microbunch) is one of advantages of this opera-
tion principle.
The schematic layout of sub-mm pulse generator is
shown in Fig.1. The main components of generator as
photo cathode and laser system, accelerating system,
MW power generator, capillary system will discussed in
this paper. The beam dynamics in the photo gun and
THz radiation generation will be studied also.
Fig.1. General layout of the THz generator
ISSN 1562-6016. ВАНТ. 2012. №3(79) 93
3. PHOTOCATHODE AND LASER SYSTEM
Photocathodes used in photoinjectors are commonly
made from metal (as copper or magnesium). Metal
photocathode is more useful because of easiness in
manufacturing and mounting in cell and long lifetime.
The Me cathodes above work in UV-band of light
source so the laser system must to provide UV-band
0.1…1 ps duration pulses. To achieve the desired elec-
tron energy on the exit of photocathode, pulses must
also have energy of about 0.1…10 mJ. Most fitting laser
systems are based on mode locked Ti:Sapphire, Ar or
Nd:YLF lasers. These lasers can provide laser pulses in
710…920 nm range. The β-barium borate frequency
doubler is used to shift laser operating frequency to UV-
range. The pumping system is mostly based on Ar laser.
4. ACCELERATING STRUCTURE
The accelerating structures consisting of 1.6 cell of
disk-loaded waveguide (DLW) (Fig.2), 3 cells and 2
half-cells of DLW, 7 cells and 2 half-cells of DLW and
accelerating structure based on traveling wave resonator
(TWR) based on 7 cells and 2 half-cells of DLW (Fig.3)
have been considered for short electron bunch accelera-
tion and its electrodynamics characteristics compared.
The results of the study are discussed in [2] in detail.
Let us represent the accelerating structures construction
and main study results briefly.
Fig.2. General view of 1.6 cell accelerating system
Fig.3. General view of TWR based on 7 cells and
2 half-cells accelerating system
Most widely used normal conducting accelerators
are based on 1.6 cells DLW. It is operate in standing
wave mode. Electromagnetic characteristics comparison
of 1.6 cell structure and traveling wave structures was
done to investigate the possibility of development more
effective structures with lower possibility of electrical
breakdown.
All accelerating structures were designed using simi-
lar construction and parameters. Structures were com-
puted for 2856 MHz RF operating frequency. Mode
with / 2μ = π phase shift per cell is the operating mode
of 3 cells and 2 half cells, 7 full cell and 2 half cell and
TWR structures because this structures are operating in
traveling wave mode. The 1.6 cell structure operates in
standing wave regime and μ = π was chosen as an op-
erating mode. All structures are characterized with posi-
tive normal dispersion.
Resonant frequency of the structure was set to the
desired value by means of cell radius variation. Iris pro-
file was made with rounding to eliminate the possibility
of breakdown. This was done to reduce the electric field
in the window’s aperture because of high-rate accelerat-
ing fields (up to 100 MV/m) in structures. Photocathode
will be arranged in half-cell’s sidewall, therefore accel-
erating field on the cathode endwall surface must be as
high as possible. The ratio of iris window to the wave-
length is set to 0.1. This value is a trade-off between the
wish to get maximum amplitude of accelerating field
and to exclude probable beam loss on the iris. Perform-
ance of the structures was also increased by rounding of
shells edges. The rounding radius value was chosen to
provide the highest possible shunt impedance and
Q-factor.
Structure’s power input was organized using stan-
dard S-band waveguide with 72×34 mm cross-section
was attached to the structures through the coupling dia-
phragm. Output of high order modes is connected sym-
metrically to the RF power input for better coupling and
also to reduce the electromagnetic field asymmetries.
Output of high order modes is designed in form of eva-
nescent waveguide [3]. Waveguide’s cross-section
matches sizes of coupling diaphragm. Full cell with RF
port and output of high order modes forms the wave
converter for 1.6 cell structure. The wave converters are
formed by half cells with attached RF ports and outputs
of higher type’s waves for all traveling wave structures.
The wave converter is designed to minimize reflections.
RF ports are jointed with the rectangular waveguide
and the power is fed to the resonator through the direc-
tional coupler in the TWR. If the electrical path length
of the structure equals to the full number of wavelengths
then the magnitude of the wave inside the ring is maxi-
mum and magnitude of the wave that is coming to the
coupled load is minimal. The MW generator must work
on the matched load all the time. The optimal operation
regime of the structure is critical mode. In this regime
part of RF power is fed into the accelerating system
through the directional coupler and fills in the power
resistance losses in the resonators sidewalls. If the struc-
tures reflecting coefficient is insignificant, the TWR
electrical field magnitude is many times more than the
magnitude of feeding wave.
The results of electrodynamics characteristics study
of all three designed structures are shown in Table 1.
ISSN 1562-6016. ВАНТ. 2012. №3(79) 94
Table 1
Main characteristics of the accelerating system models
Parameters 1.6 cell
3 cells
and 2
half cells
7 cells
and 2
half cells
TWR
Operating
mode π π/2 π/2 π/2
Structure
length, mm 77.6 105 210 210
Accelerating
field on axis E0,
kV/m (1 kW
input power)
312.3 103.8 107.3 321.9
Q-factor 16530 9290 10800 10800
Rshunt, MOhm/m 57.9 23.3 27.2 27.2
5. BEAM DYNAMICS SIMULATION IN
ACCELERATING STRUCTURES
Beam dynamics simulation in designed accelerating
structures was done using BEAMDULAC-BL code de-
signed in laboratory DINUS of NRNU MEPhI [4]. The
simulation was done with the following beam parame-
ters: injection energy Winj = 10 keV, output energy
1 MeV, beam pulse current I = 5 A, beam pulse charge
Q = 0.1 nC, beam initial radius r = 200 μm. This beam
parameter’s values are similar as most of photoinjectors.
The main aim of investigation was to achieve the value
of acceleration field magnitude that will provide accel-
eration of electron beam to 1 MeV [2], minimal energy
required by high intensity sub-mm radiation source.
Beam dynamics investigation results shows that 1.6
cell DLW structure can provide electron beam accelera-
tion to the energy 1 MeV with P = 1.5 MW of RF power
fed to the system. This result is in the good agreement
with experimental data. Accelerating structure based on
3 full cells and 2 half cells provide beam acceleration to
1 MeV with 10 MW of RF power, 7 full cells and 2 half
cells structure – with 4 MW of RF power. Consideration
of TWR accelerating system shows best results in accel-
erating electron beam to 1 MeV – with 500 kW of RF
power fed into the structure.
The beam size preservation and focusing can be re-
alized in accelerator using longitudinal magnet field. It
should be reminded that sub-mm beam dimensins are
necessary for Cherenkov THz generator.
Table 2
Results of beam dynamics simulation in designed models
Parameters 1.6 cell 3 cells and
2 half cells
7 cells and
2 half cells TWR
PE /0λ 1037 345 367 1102
E0, MV/m 10.4 9.1 6.5 6.9
P, MW 1.5 20.0 4.0 0.5
6. RF GENERATOR
As it is clear from Table 2, 0.5 MW of RF power is
necessary to realize 1 MeV energy gain for TWR struc-
ture and 1.5 MW for 1.6 cell standing wave photo gun.
Klystrons or magnetrons produced by Federal state uni-
tary enterprise Research & Production Corporation
"Toriy" are available for this aim. The pulse magnetron
MI-475 has the following characteristics: RF pulse
power up to 2 MW, average power 2.5 kW, pulse dura-
tion 4 μs. This is sufficient for TWR designed. The kly-
stron as KIU-168 can be used if the higher power is
necessary: RF pulse power up to 6 MW, average power
6 kW, pulse duration 7 μs.
7. THZ RADIATION GENERATION
Two types of channels could be used to generate the
radiation: the conducting (metal) capillary coated by
dielectric inside (Fig.4,a) and the corrugated channel or
grating surface also (Fig.4,b). The inner channel radius
d/2 should be comparable to the wavelength, i.e. can be
smaller than 1 mm. The structure period t (the distance
between of diaphragms) in the second case should also
be comparable to the wavelength.
Fig.4. The conducting (metal) capillary coated inside
by dielectric (a) and the corrugated channel or grating
surface (b)
Presence of the channel inside dielectric in which
the radiating particle moves, leads to suppression of the
radiation on high frequencies. It is suppose to use a di-
electric tube in practically interesting cases as a radiator.
The intensity of generated radiation can be significantly
increased if a high current modulated electron beam
consisting of short bunches with the period of longitudi-
nal modulation less or of the order of the radiated wave
length is used. If the dielectric tube is located in a metal
tube (waveguide) radiation will be limited by volume of
this tube. And, as consequence, width of lines on which
radiation is raised, will considerably decrease and a
maximum wavelength mλ will be limited by the critical
value crλ ( crm λ≤λ ). So the Cherenkov radiation is
considered to be one of the more effective ways for THz
radiation generation [1].
The other mechanism to produce THz radiation is
Smith-Purcell effect. Smith-Purcell radiation arises
when charged particles move near a grating or any peri-
odical structure. The total losses for Smith-Purcell ra-
diation might be estimated as
NW cωα≈ h , (1)
ISSN 1562-6016. ВАНТ. 2012. №3(79) 95
where N – number of diffraction grating elements; α –
the fine structure constant (α ≈ 1/137); h – Planck’s
constant and
h
c
c
βγ
≈ω (2)
is the cut-off frequency.
CONCLUSIONS
The scheme of high power radiation generator in
sub-mm region was proposed. This radiation source is
based on RF photogun and Cherenkov or Smith-Parcell
decelerating system. Some results of generator compo-
nents (RF photo gun and accelerating system, laser, RF
power supply system) design and beam dynamics simu-
lation were discussed.
REFERENCES
1. A.V. Smirnov. A High Performance, FIR Radiator
Based on a Laser Driven E-Gun, ISBN 978-1-
60456-720-5, 2008.
2. T.V. Bondarenko, S.M. Polozov. Photoinjector Ac-
celerating System for sub-mm High-Power Pulse
Source // Problems of Atomic Science and Technol-
ogy. Series “Nuclear Physics Investigations”. Real
journal, p.53-57.
3. A. Anisimov, et al. / Proc. of RuPAC-2010, p.328.
4. T.V. Bondarenko, et al. / Proc. of HB 2010, p.123.
Статья поступила в редакцию 23.09.2011 г.
СВЧ-ФОТОКАТОД И ЧЕРЕНКОВСКАЯ ЗАМЕДЛЯЮЩАЯ СИСТЕМА ДЛЯ МОЩНОГО
ГЕНЕРАТОРА ИЗЛУЧЕНИЯ СУБМИЛЛИМЕТРОВОГО ДИАПАЗОНА
Ю.А. Башмаков, Т.В. Бондаренко, Д.А. Комаров, С.М. Полозов, В.И. Ращиков,
И.С. Щедрин, А.В. Смирнов, А.В. Воронков, А.А. Тищенко
Рассмотрены некоторые результаты исследования и разработки СВЧ-фотокатода и черенковской замед-
ляющей системы, предназначенных для генерации мощных импульсов излучения субмиллиметрового диа-
пазона. Как известно, проводящий капилляр, покрытый изнутри слоем диэлектрика, может быть использо-
ван в качестве источника черенковского излучения. Для получения монохроматического излучения необхо-
димо иметь очень короткий (меньше 1 мм) сгусток электронов, который может быть получен при использо-
вании фотокатода. Электроны должны быть ускорены в короткой системе до энергии 1…4 МэВ и инжекти-
рованы в замедляющую систему. Такой источник излучения может быть использован в досмотровой систе-
ме или в качестве лабораторного генератора.
НВЧ-ФОТОКАТОД І ЧЕРЕНКІВСЬКА УПОВІЛЬНЮЮЧА СИСТЕМА ДЛЯ ПОТУЖНОГО
ГЕНЕРАТОРА ВИПРОМІНЮВАННЯ СУБМІЛІМЕТРОВОГО ДІАПАЗОНУ
Ю.А. Башмаков, Т.В. Бондаренко, Д.О. Комаров, С.М. Полозов, В.І. Ращиков,
І.С. Щедрін, О.В. Смірнов, А.В. Воронков, О.О. Тіщенко
Розглянуто деякі результати дослідження та розробки НВЧ-фотокатода і черенківської уповільнюючої
системи, призначених для генерації потужних імпульсів випромінювання субміліметрового діапазону. Як
відомо, проводячий капіляр, покритий зсередини шаром діелектрика, може бути використаний як джерело
черенківського випромінювання. Для отримання монохроматичного випромінювання необхідно мати дуже
короткий (менше 1 мм) згусток електронів, який може бути отриманий при використанні фотокатода. Елек-
трони мають бути прискорені в короткій системі до енергії 1…4 МеВ і інжектовані в уповільнюючу систе-
му. Таке джерело випромінювання може бути використано в оглядовій системі або в якості лабораторного
генератора.
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