Duration of multipacting processes and discharges in the linac of ions
It is experimentally shown that multipactor processes may be as harmful as other of parasitic discharges and cause significant disturbance in resonator electrodynamic characteristics of an accelerating structure. The disturbance may be evaluated by pulse-shape distortions of a reference rf voltage...
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
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irk-123456789-966512016-03-19T03:02:53Z Duration of multipacting processes and discharges in the linac of ions Lobzov, L.D. Shulika, N.G. Shulika, O.N. Belan, V.N. Теория и техника ускорения частиц It is experimentally shown that multipactor processes may be as harmful as other of parasitic discharges and cause significant disturbance in resonator electrodynamic characteristics of an accelerating structure. The disturbance may be evaluated by pulse-shape distortions of a reference rf voltage impulse. Control over duration of multipactor processes within diode gaps of a linac is effected by varying parameters of a self-sustained oscillation system formed by two independent positive feedback circuits (PFC). If two synchronous rf voltage pulses of given amplitude superpose in the accelerating structure, there occurs an impediment to multipactor processes. As this takes place, multipactor processes display minimal duration and do not affect acceleration stability. Експериментально показано, що мультипакторнi процеси можуть досягати розмiрiв паразитних мультипакторних розрядiв i досить довгостроково порушувати електродинамiчнi характеристики прискорювача. Порушення характеристик оцiнюється по перекручуванню форми контрольного ВЧ-iмпульсу напруги резонатора. Регулювання тривалiстю мультипакторних процесiв i розрядiв у прискорюючiх зазорах лiнiйного прискорювача iонiв Н-типу здiйснюється змiною параметрiв результуючих ВЧ-напруг, збуджених iмпульсної автоколивальною системою iз двома контурами з незалежними позитивними зворотними зв’язками. У випадку накладення в прискорювальнiй структурi двох синхронних ВЧ-напруг номiнальних амплiтуд, розвиток мультипакторних процесiв придушується. При цьому тривалiсть процесiв мiнiмальна i їхнiй вплив на стiйкiсть характеристик прискорювача вiдсутнiй. Экспериментально показано, что мультипакторные процессы могут достигать величин паразитных мультипакторных разрядов и весьма длительно нарушать электродинамические характеристики ускорителя. Нарушение характеристик оценивается по искажению формы контрольного ВЧ-импульса напряжения резонатора. Регулирование длительности мультипакторных процессов и разрядов в ускоряющих зазорах однорезонаторного линейного ускорителя ионов Н-типа, производится измене- нием параметров результирующих ВЧ-напряжений, возбуждаемых импульсной автоколебательной системой с двумя контурами с независимыми положительными обратными связями. В случае наложения в ускоряющей структуре двух синхронных ВЧ-напряжений номинальных амплитуд, развитие мультипакторных процессов подавляется. При этом длительность процессов минимальна и их влияние на устойчивость характеристик ускорителя отсутствует. 2009 Article Duration of multipacting processes and discharges in the linac of ions / L.D. Lobzov, N.G. Shulika, O.N. Shulika, V.N. Belan // Вопросы атомной науки и техники. — 2009. — № 5. — С. 154-158. — Бібліогр.: 20 назв. — англ. 1562-6016 PACS: 29.17.+w. http://dspace.nbuv.gov.ua/handle/123456789/96651 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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| language |
English |
| topic |
Теория и техника ускорения частиц Теория и техника ускорения частиц |
| spellingShingle |
Теория и техника ускорения частиц Теория и техника ускорения частиц Lobzov, L.D. Shulika, N.G. Shulika, O.N. Belan, V.N. Duration of multipacting processes and discharges in the linac of ions Вопросы атомной науки и техники |
| description |
It is experimentally shown that multipactor processes may be as harmful as other of parasitic discharges and cause
significant disturbance in resonator electrodynamic characteristics of an accelerating structure. The disturbance
may be evaluated by pulse-shape distortions of a reference rf voltage impulse. Control over duration of multipactor
processes within diode gaps of a linac is effected by varying parameters of a self-sustained oscillation system formed by
two independent positive feedback circuits (PFC). If two synchronous rf voltage pulses of given amplitude superpose
in the accelerating structure, there occurs an impediment to multipactor processes. As this takes place, multipactor
processes display minimal duration and do not affect acceleration stability. |
| format |
Article |
| author |
Lobzov, L.D. Shulika, N.G. Shulika, O.N. Belan, V.N. |
| author_facet |
Lobzov, L.D. Shulika, N.G. Shulika, O.N. Belan, V.N. |
| author_sort |
Lobzov, L.D. |
| title |
Duration of multipacting processes and discharges in the linac of ions |
| title_short |
Duration of multipacting processes and discharges in the linac of ions |
| title_full |
Duration of multipacting processes and discharges in the linac of ions |
| title_fullStr |
Duration of multipacting processes and discharges in the linac of ions |
| title_full_unstemmed |
Duration of multipacting processes and discharges in the linac of ions |
| title_sort |
duration of multipacting processes and discharges in the linac of ions |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| publishDate |
2009 |
| topic_facet |
Теория и техника ускорения частиц |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/96651 |
| citation_txt |
Duration of multipacting processes and discharges in the linac of ions / L.D. Lobzov, N.G. Shulika, O.N. Shulika, V.N. Belan // Вопросы атомной науки и техники. — 2009. — № 5. — С. 154-158. — Бібліогр.: 20 назв. — англ. |
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Вопросы атомной науки и техники |
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| fulltext |
DURATION OF MULTIPACTING PROCESSES AND
DISCHARGES IN THE LINAC OF IONS
L.D. Lobzov, N.G. Shulika, O.N. Shulika, V.N. Belan∗
National Science Center ”Kharkov Institute of Physics and Technology”, 61108, Kharkov, Ukraine
(Received July 6, 2009)
It is experimentally shown that multipactor processes may be as harmful as other of parasitic discharges and cause
significant disturbance in resonator electrodynamic characteristics of an accelerating structure. The disturbance
may be evaluated by pulse-shape distortions of a reference rf voltage impulse. Control over duration of multipactor
processes within diode gaps of a linac is effected by varying parameters of a self-sustained oscillation system formed by
two independent positive feedback circuits (PFC). If two synchronous rf voltage pulses of given amplitude superpose
in the accelerating structure, there occurs an impediment to multipactor processes. As this takes place, multipactor
processes display minimal duration and do not affect acceleration stability.
PACS: 29.17.+w.
1. INTRODUCTION
Usually the issue of multipactor processes and dis-
charges between electrode surfaces in a linac res-
onance structure is addressed when the structure
undergoes RF excitation. However, to obtain rea-
sonable electrodynamic characteristics at the rated
structure frequency it is essential to eliminate all ad-
verse effects caused by secondary electron emission
from the electrodes [1-8].
There exists a number of methods for impediment
to multipactor discharge, namely, profile modification
of accelerating electrodes [9]; the use of special elec-
trode coating [10]; the usage of an auxiliary electrode
[11]; application of additional DC voltage [12], etc. It
should be remarked that any of the methods above-
mentioned results in substantial deterioration in elec-
trodynamic parameters of an accelerating structure
and makes its design rather complicated.
Integration of a stabilized generator operating at a
different frequency into RF power system of an accel-
erating structure reduces multipactor discharge influ-
ence on the structure but makes it difficult to archive
required working parameters of the linac [13].
It is also possible to impede multipactor discharge
influence using a countercircuit [14]. Although the
voltage generated by the countercircuit has the same
operating frequency, its phase differs by 180◦ from
the main RF voltage phase. This adds complexity to
the generation module of the accelerator. Moreover,
to prevent the decrease in total voltage magnitude
caused by inverse feedback, additional amplifiers are
required. Hence, the RF module becomes rather ex-
pensive and complicated in design and operation.
At present, a very effective technique for multi-
pactor discharge suppressing has been proposed at
the NSC KIPT [15]. It holds a certain advantage
over methods mentioned, namely, there is no need to
incorporate modifications either in accelerating struc-
ture design or its fabrication and assembling technol-
ogy. The rated voltage for RF field excitation is a
combination of in-phase voltages generated by two in-
dependent reactive circuits powered by a common RF
power source. As this takes place, operating mode of
the source is nominal. The change in any parameter
of the positive feedback circuit makes it possible to
investigate temporal effects of multipactor discharge
and processes on linac electrodynamic characteristics.
The purpose of this paper is to study multipactor
discharge and its duration in a single-cavity multigap
ion linac.
2. BASICS
Electron motion equation in variable electric field
d2x
dt2
=
e
m
· E sin(ωt + ϕ), (1)
where e is the electron charge, m-electron mass, E-
electric field intensity, ω-cyclotron frequency, and ϕ is
the initial phase. Upon integrating (1) under assump-
tion x=d (d being a diode gap length) and t = T/2
(T = 2π/ω) the solution takes the form [16]:
d =
eE
mω2
· {π cosϕ + 2 sin ϕ}. (2)
Taking into account that Ed = U , the expression (2)
can be rewritten as
eU |(π cosϕ + 2 sin ϕ)| = m(ωd)2. (3)
This formula shows coupling between the voltage,
phase and frequency providing an electron could cross
the diode gap d in half-period of RF voltage.
∗Corresponding author E-mail address: belan@kipt.kharkov.ua
154 PROBLEMS OF ATOMIC SCIENCE AND TECHNOLOGY, 2009, N5.
Series: Nuclear Physics Investigations (52), p.154-158.
By introducing a dimensionless parameter
ξ = eU/[m(ωd)2/2] the expression (3) can be rewrit-
ten as
ξ =
∣∣∣∣
(π
2
· cosϕ + sin ϕ
)−1
∣∣∣∣ . (4)
Referring to Fig.1 that presents a plot of de-
pendences (4), the parameter ξ is varying from
0.536 to 0.636 as the initial phase ranges from 0◦
to 65◦. The minimum ξmin = 0, 536 is observed at
ϕ = 32.5◦. A point that should be mentioned is that
discharge stability occurs over the phase range be-
tween 0◦ and 32.5◦ where the derivate is negative.
Fig.1. Phase-dependence of the dimensionless
parameter ξ
3. EXPERIMENTAL PROCEDURE AND
RESULTS
Fig.2 depicts a chart for a multigap linac with two
independent positive feedback circuits (PFC). Self-
oscillations of PFC1 embrace the power source 1 and
accelerating structure 3, whereas PFC2 includes the
power source 1 along with the feeder 2. It should
be pointed out that multipactor processes affect the
PFC1 parameters directly.
RF phase and amplitude controllers 4 provide
phase and amplitude adjustment in both PFC1 and
PFC2. The adder 5 summarizes RF voltages gener-
ated by either circuit. Output voltages from PFC1
and PFC2 therewith define resultant fields of the
structure.
The accelerating structure 3 used for experiments
has been partially described in [17]. It is a periodic
resonant H-type facility for proton and deuteron ac-
celeration; it operates at the frequency f = 100 MHz
with a high quality factor Q0 ≈ 5000.
Three-step cascade amplification channel serves
as a power source. The final cascade is designed to
use a pulse triode GI 24A as a basis. The total ampli-
fication coefficient for RF power source ranges from
100 to 200 ( Kamp ≈ 100...200).
The acceleration channel consists of 17 elec-
trodes (making the number of gaps 16) so placed
that π-type axisymmetric electric field is gener-
ated. Each electrode (a so called drift tube) is
designed as a metallic tube; the tube length and
distance between them increase towards the accel-
erating channel exit. At the beginning of the accel-
erating channel the distance between the first and
second drift tubes makes 1.353 cm and increases up
to 1.753 cm between the second and third one while
at the end of the channel three last drift tubes are
placed 6.120 cm and 6.422 cm apart, respectively.
Fig.2. RF excitation system of single-cavity ion
linac: 1 - RF power amplifier, 2 - coaxial feeder,
3 - multigap periodic structure of single-cavity
accelerator, 4 - RF phase and amplitude controller,
5 - RF power adder.
PFC1 - The Contour with the plus feedback of the
amplifier channel with RF fields of the accelerating
structure, forming its self-oscillatory RF excitation.
PFC2 - The Contour with the plus feedback of the
amplifier channel with TEM waves of the coaxial
feeder, forming exterior self-excited RF structure
Electric field across annular electrode flanks changes
its polarity (i.e. alternates) twice a period. The
voltage range over which multipactor processes oc-
cur is calculated using the expression (3) and reads
(100...124.6)V and (2370...2810)V across the first
and last gap, correspondingly.
It is calculated that accelerating electric field
strength across the fist and last gap is 30 kV/cm,
while this characteristic averaged over all the gaps
is 60 kV/cm. Hence, the field coefficient for the first
and last gap makes 0.5. As this takes place, rated
voltage of reference RF pulses is Ures = 8, 5 V and
Ures = 17 V for proton and deuteron resonant accel-
eration, respectively.
Coupling cell size for the PFC1 is specified by
its amplification coefficient in the range (0.1...0.15)
Kamp at a small RF field disturbance (less than 1%).
Similar small field disturbances over the feeder govern
the size for PFC2 coupling element.
The ratio Upfc2 ≥ 0, 25 Upfc1 between PFC1 and
PFC2 voltages generated simultaneously is chosen
under condition of PFC1 being dominant circuit [17].
Control over operational phase of positive feed-
back in either circuit makes it possible to establish
traveling-wave mode in both the excitation and feeder
channels. As this takes place, the amplitude of in-
cident and reflected waves determines conditions for
resonant input resistant consistency with feeder wave
impedance and the amount of incoming RF power.
As RF field alteration takes place in each gap si-
155
multaneously, electron current measured at the accel-
erating structure output reflects the same secondary
electron processes that occur across the initial part of
the structure. This fact allows us to study secondary
electron current by means of Faraday cup intended
for accelerated ion diagnostics.
Snapshots on Fig.3 present conjugated RF pulse
oscillograms which have been observed for the same
resultant accelerating voltage Ures (lower curves) and
secondary electron current Isee (upper curves) emit-
ted from edges of electrode flanks facing the accel-
erator exit. These diagrams differ from one another
other because the second pair of curves has passed
through a simple energy analyzer.
Fig.3. Conjugated current Isee (upper curves)
and voltage Ures (lower curves) diagrams measured
across the penultimate accelerating gap.
Referring to Fig.3, the current amplitude has sev-
eral spikes on both the leading and trailing edges
(we marked these spikes with arrows). Hence, the
multipactor processes are observed not only on the
RF voltage impulse front end, but on its rear end as
well. It is also obvious that secondary electron cur-
rent pulses differ in magnitude and length.
As evident from Fig.3(a), low voltage amplitude
that corresponds to leading and trailing edges of the
pulse is responsible for the secondary electron cur-
rent spikes. As these spikes appear discretely from
impulse main body, it is possible to measure elec-
tron energy by means of a thin foil placed before the
Faraday cup. The material choice for the foil and
its thickness is dictated by specific requirements on
absorption of low-energy electrons and partial decel-
eration of high-energy particles.
Low-energy electrons are fully absorbed by the
aluminum target of thickness 5 µm, as may be seen
from Fig.3(b). The decrease in high-energy secondary
electron current in the mean portion of pulse and res-
onance spikes (I ∼ du/dt, [18]) does not exceed 20%.
It follows from these results that the pattern in
which high-frequency voltage amplitude is changing
does not affect the initial growth in electron number
across the diode gaps of the accelerating structure. In
other words, the multipactor processes under attenu-
ating RF fields of low amplitude are governed by the
same secondary electrons that are emitted at electric
field rise.
It is also possible to investigate the influence of
voltage amplitude Upfc1 and Upfc2 on time evolu-
tion of multipactor process if a resonant accelerat-
ing structure features the high quality factor Q along
with relatively slow RF voltage build-up.
The voltage Upfc2 induced by the inductive feed-
back loop is adjusted by tilting the loop’s plane in
relation to magnetic field lines of the coaxial feeder.
Taking the slope of 0 and 90◦ degrees as the initial
and final position, correspondingly, it is convenient
to express the amplitude of induced RF voltage as
Upfc2 = Upfc2 nom · sin ϕ (α is the tilt angle).
If step control of positive feedback coefficient of
PFC2 runs in backward direction (from 90◦ down to
0◦ degrees), it makes possible to reverse the nature
of secondary electron processes. To put it differently,
a decrease in voltage amplitude can also cause an
avalanche-like increase in secondary electron number
across electrodes in the initial part of the accelerating
structure.
Phase relation for each oscillation circuit stays at
nominal value. The amplitude of modulation pulse
generated at the final stage of RF power source de-
fines the value of peak voltage produced by either
circuit.
General matching of input resonant structure re-
sistance to wave feeder impedance is governed by
minimum of reflected wave amplitude and standing
wave coefficient Csw . This allows exerting additional
manual control over resonant accelerator character-
istics. If the PFC2 coupling element is parallel to
magnetic field lines or has a small tilt (of less than 20
degrees), the induced voltage is rather low and stable
multipactor discharge is retained. Also a white-bluish
glow is observed across two initial accelerating gaps.
As the increasing voltage Upfc2 reaches the value
Upfc2 = Upfc2 nom · sin 20◦ or higher, shape-distorted
voltage pulses are beginning to register by the oscil-
loscope with initial time delay (td) and varying am-
plitude along the pulse length.
Referring to Fig.4a-d, as the voltage Upfc2 builds-
up in step-like manner, excitation time delay drops
from td = 50 µs to zero. The pulse frontline clearly
exhibits distortions which are typical for a multi-
pactor discharge. It is also evident that the pulse
changes its shape and the leading step dwindles in
the process [19,20].
At the end of the pulse plateau where decrease
rate of resultant voltage amplitude is maximal, the
conditions for secondary electron growth are breaking
down that, in its turn, leads to multipactor discharge
suppression.
With further increase in the PFC2 coupling coef-
ficient along with resultant electric field magnitude
growth, the influence of multipactor discharges weak-
ens. This is obvious from the difference in the pulse
156
leading-edge duration (Fig.4e-g). It is also believed
that total distortion of electrodynamic characteris-
tics of the accelerating structure is reduced as well.
Fig.4. Presents an
oscillogram sequence
showing pulse-shape
and pulse-duration dis-
tortions as the voltage
Upfc2 increases from
zero up to Upfc2nom.
RF voltage pulses
of the accelerator
cavity that undergoes
multipactor processes
influence at rated
PFC1 parameters
and PFC2 voltage
step-controlled: (a)-(f)
pulses at fixed tilt angle
in the range from 20◦
up to 70◦ (every 10◦);
(g)-RF voltage pulse
at tilt angle higher
than 70◦ (up to 90◦).
Horizontal scale is
50mks per graduation;
vertical scale is 2V per
division
As the voltage Upfc2 continues to grow
and at some moment meets the condition
Upfc2 = Upfc2 nom · sin 70◦, the multipactor discharge
starts to die down till it ceases to exist. Pulse shape
in Fig.4(g) provides clear evidence to this process. In
this case, increase in energy of electrons bombard-
ing the electrodes results in decrease in secondary
emission coefficient.
RF field energy transferred by the electrons deter-
mines general electron load and -factor value of the
accelerating structure. Complete shape of RF pulse
holds no visual distortions of electrodynamic parame-
ters and resonant characteristics of the linac cavity.
The oscillograms provide an experimental verifi-
cation of high efficiency of this method of multipactor
discharges suppression.
4. SUMMARY
Experiments confirm the possibility to control the
duration of multipactor discharge in a diode gap of
single-cavity accelerating structure by two RF volt-
ages generated by different independent circuits.
The duration of multipactor processes and dis-
charges is defined by generation time of RF voltage
induced by the positive feedback circuit with the ac-
celerating structure integrated. Without additional
independent reaction circuit (Upfc2 = 0) multipactor
discharge can exist for a long time.
Minimal multipactor discharge duration is deter-
mined by resultant RF voltage generated by two inde-
pendent positive feedback circuits, PFC1 and PFC2,
with proper positive feedback coefficients.
Experimental results obtained point to the fact
that there exist new possibilities for studies into mul-
tipacting phenomenon and development of highly ef-
ficient RF structures which are not affected by mul-
tipactor discharges.
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ДЛИТЕЛЬНОСТЬ МУЛЬТИПАКТОРНЫХ ПРОЦЕССОВ И РАЗРЯДОВ В
ЛИНЕЙНОМ УСКОРИТЕЛЕ ИОНОВ
Л.Д. Лобзов, Н.Г. Шулика, О.Н. Шулика, В.Н. Белан
Экспериментально показано, что мультипакторные процессы могут достигать величин паразитных
мультипакторных разрядов и весьма длительно нарушать электродинамические характеристики
ускорителя. Нарушение характеристик оценивается по искажению формы контрольного ВЧ-импульса
напряжения резонатора. Регулирование длительности мультипакторных процессов и разрядов в
ускоряющих зазорах однорезонаторного линейного ускорителя ионов Н-типа, производится измене-
нием параметров результирующих ВЧ-напряжений, возбуждаемых импульсной автоколебательной
системой с двумя контурами с независимыми положительными обратными связями. В случае нало-
жения в ускоряющей структуре двух синхронных ВЧ-напряжений номинальных амплитуд, развитие
мультипакторных процессов подавляется. При этом длительность процессов минимальна и их влияние
на устойчивость характеристик ускорителя отсутствует.
ТРИВАЛIСТЬ МУЛЬТИПАКТОРНИХ ПРОЦЕСIВ I РОЗРЯДIВ У ЛIНIЙНОМУ
ПРИСКОРЮВАЧI IОНIВ
Л.Д. Лобзов, М.Г. Шулика, О.М. Шулика, В.М. Бєлан
Експериментально показано, що мультипакторнi процеси можуть досягати розмiрiв паразитних
мультипакторних розрядiв i досить довгостроково порушувати електродинамiчнi характеристики
прискорювача. Порушення характеристик оцiнюється по перекручуванню форми контрольного
ВЧ-iмпульсу напруги резонатора. Регулювання тривалiстю мультипакторних процесiв i розрядiв
у прискорюючiх зазорах лiнiйного прискорювача iонiв Н-типу здiйснюється змiною параметрiв
результуючих ВЧ-напруг, збуджених iмпульсної автоколивальною системою iз двома контурами
з незалежними позитивними зворотними зв’язками. У випадку накладення в прискорювальнiй
структурi двох синхронних ВЧ-напруг номiнальних амплiтуд, розвиток мультипакторних процесiв
придушується. При цьому тривалiсть процесiв мiнiмальна i їхнiй вплив на стiйкiсть характеристик
прискорювача вiдсутнiй.
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