Ion beam space charge neutralization using for beam intensity increase in linacs
As it is well known, the space charge is the main factor limiting the beam intensity in ion bunchers and low energy linacs. It can be declared that the limit low energy beam current is achieved or close now. But it must be enlarged up to 300…1000 mA for the same purposes as neutron generators, accel...
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irk-123456789-1086762016-11-15T03:02:17Z Ion beam space charge neutralization using for beam intensity increase in linacs Polozov, S.M. Динамика пучков As it is well known, the space charge is the main factor limiting the beam intensity in ion bunchers and low energy linacs. It can be declared that the limit low energy beam current is achieved or close now. But it must be enlarged up to 300…1000 mA for the same purposes as neutron generators, accelerating driven systems and other. It is provide to discussion about new acceleration and focusing methods which can to be used for this facilities. There are two ways to increase ion beam intensity: to enlarge the beam’s cross section and to use the space charge neutralization. The second way of the limit beam current enlargement is more discussable. It is known three (or more?) ideas for beam space charge neutralization: (i) neutralization using plasmas, ionized residual gas or electron cloud; (ii) so-called “funneling” method; (iii) simultaneous acceleration of positive and negative ions in the same bunch. Some results in beam space charge neutralization will discussed for RFQ, DTL, UNDULAC. Как принято считать, влияние объемного заряда пучка является основным фактором, ограничивающим интенсивность ионных пучков в линейных ускорителях на небольшие энергии. Можно утверждать, что в настоящее время в ускорителях на небольшие энергии достигнут (или вскоре будет достигнут) предел по току пучка. Для увеличения тока ионного пучка до 300…1000 мА, что требуется для некоторых приложений, таких как нейтронные генераторы или ядерные установки, управляемые ускорителем, существуют два основных пути: увеличение поперечного сечения пучка и использование нейтрализации влияния объемного заряда. В настоящее время второй путь обсуждается все более активно. Известно три (или более) способа нейтрализации влияния объемного заряда: использование плазмы, ионизованного остаточного газа или электронного облака; метод «сложения» пучков; ускорение ионов с разным знаком в одном сгустке. Некоторые результаты исследования динамики «нейтрализованного» ионного пучка в линейных ускорителях с ПОКФ, ускорителях Альвареца, линейных ондуляторных ускорителях представлены в данной работе. Як прийнято вважати, вплив об'ємного заряду пучка є основним чинником, що обмежує інтенсивність іонних пучків у лінійних прискорювачах на невеликі енергії. Можна стверджувати, що в даний час у прискорювачах на невеликі енергії досягнута (або незабаром буде досягнута) межа по струму пучка. Для збільшення струму іонного пучка до 300...1000 мА, що потрібно для деяких додатків, таких як нейтронні генератори або ядерні установки, керовані прискорювачем, існують два основних шляхи: збільшення поперечного перерізу пучка і використання нейтралізації впливу об'ємного заряду. В даний час другий шлях обговорюється все більш активно. Відомо три (або більше) способи нейтралізації впливу об'ємного заряду: використання плазми, іонізованого залишкового газу або електронної хмари; метод «складання» пучків; прискорення іонів з різним знаком в одному згустку. Деякі результати дослідження динаміки «нейтралізованого» іонного пучка в лінійних прискорювачах з ПОКФ, прискорювачах Альвареця, лінійних ондуляторних прискорювачах представлені в даній роботі. 2012 Article Ion beam space charge neutralization using for beam intensity increase in linacs / S.M. Polozov // Вопросы атомной науки и техники. — 2012. — № 3. — С. 131-136. — Бібліогр.: 36 назв. — англ. 1562-6016 PACS: 29.17.w, 29.27.Bd http://dspace.nbuv.gov.ua/handle/123456789/108676 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Динамика пучков Динамика пучков Polozov, S.M. Ion beam space charge neutralization using for beam intensity increase in linacs Вопросы атомной науки и техники |
| description |
As it is well known, the space charge is the main factor limiting the beam intensity in ion bunchers and low energy linacs. It can be declared that the limit low energy beam current is achieved or close now. But it must be enlarged up to 300…1000 mA for the same purposes as neutron generators, accelerating driven systems and other. It is provide to discussion about new acceleration and focusing methods which can to be used for this facilities. There are two ways to increase ion beam intensity: to enlarge the beam’s cross section and to use the space charge neutralization. The second way of the limit beam current enlargement is more discussable. It is known three (or more?) ideas for beam space charge neutralization: (i) neutralization using plasmas, ionized residual gas or electron cloud; (ii) so-called “funneling” method; (iii) simultaneous acceleration of positive and negative ions in the same bunch. Some results in beam space charge neutralization will discussed for RFQ, DTL, UNDULAC. |
| format |
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| author |
Polozov, S.M. |
| author_facet |
Polozov, S.M. |
| author_sort |
Polozov, S.M. |
| title |
Ion beam space charge neutralization using for beam intensity increase in linacs |
| title_short |
Ion beam space charge neutralization using for beam intensity increase in linacs |
| title_full |
Ion beam space charge neutralization using for beam intensity increase in linacs |
| title_fullStr |
Ion beam space charge neutralization using for beam intensity increase in linacs |
| title_full_unstemmed |
Ion beam space charge neutralization using for beam intensity increase in linacs |
| title_sort |
ion beam space charge neutralization using for beam intensity increase in linacs |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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2012 |
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Динамика пучков |
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http://dspace.nbuv.gov.ua/handle/123456789/108676 |
| citation_txt |
Ion beam space charge neutralization using for beam intensity increase in linacs / S.M. Polozov // Вопросы атомной науки и техники. — 2012. — № 3. — С. 131-136. — Бібліогр.: 36 назв. — англ. |
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Вопросы атомной науки и техники |
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1837026800280010752 |
| fulltext |
ISSN 1562-6016. ВАНТ. 2012. №3(79) 131
ION BEAM SPACE CHARGE NEUTRALIZATION USING FOR BEAM
INTENSITY INCREASE IN LINACS
S.M. Polozov
National Research Nuclear University - Moscow Engineering Physics Institute,
Moscow, Russia
E-mail: smpolozov@mephi.ru
As it is well known, the space charge is the main factor limiting the beam intensity in ion bunchers and low en-
ergy linacs. It can be declared that the limit low energy beam current is achieved or close now. But it must be
enlarged up to 300…1000 mA for the same purposes as neutron generators, accelerating driven systems and other. It
is provide to discussion about new acceleration and focusing methods which can to be used for this facilities. There
are two ways to increase ion beam intensity: to enlarge the beam’s cross section and to use the space charge neu-
tralization. The second way of the limit beam current enlargement is more discussable. It is known three (or more?)
ideas for beam space charge neutralization: (i) neutralization using plasmas, ionized residual gas or electron cloud;
(ii) so-called “funneling” method; (iii) simultaneous acceleration of positive and negative ions in the same bunch.
Some results in beam space charge neutralization will discussed for RFQ, DTL, UNDULAC.
PACS: 29.17.w, 29.27.Bd
1. INTRODUCTION
Production of high intensity ion beams in a linac is a
challenging task of contemporary accelerator physics
and technology. Such accelerators can be employed in
nuclear energetic, neutron sources, thermonuclear syn-
thesis as well as in other applications. The RFQ struc-
tures are usually used as the buncher of linac. The cur-
rent in the RFQ can be limited by the losses due to in-
fluence of the space charge fields. Therefore, the maxi-
mum proton beam current achieved in the RFQ is
120…150 mA [1]. As it is well known, the space charge
is the main factor limiting the beam intensity in ion
bunchers and low energy accelerators. We can say that
the limit low energy beam current is achieved or close
now. But it must be enlarged up to 300…1000 mA for
same uses. It is provide to discussion about new accel-
eration and focusing methods which can to be used for
this facilities. There are two ways to increase ion beam
intensity: to enlarge beam’s cross section and to use
space charge neutralization. The aperture of accelerator
and the necessary RF potential on electrodes should be
enlarged in first case. The ribbon or hollow ion beam
acceleration can be used as an alternative method of
beam current enlarging.
The second way to limit beam current enlargement
is more discussable. It is known three (or more?) ideas
for beam space charge neutralization: (i) neutralization
using plasmas, ionized residual gas or electron cloud;
(ii) so-called “funneling” method; (iii) simultaneous
acceleration of positive and negative ions in the same
bunch. The idea of beam space charge neutralization by
means of electron cloud was proposed and analytically
studied in [2, 3]. It was shown that electron cloud can
really provide to the proton or heavy ion partially neu-
tralization. The neutralization of Coulomb field influ-
ence by means of plasma lenses is widely used in beam
transport lines [4]. More interest results were analyti-
cally shown and experimentally verified by number of
research groups [5-9] for bunched and continuous pro-
ton and ion beams. The ionized residual gas influence
was studied in all noted experiments. It was shown that
influence of ionized gas can provide to beam emittance
decreasing.
2. FUNNELING TECHNOLOGY
The term “funneling” we can find in 30-yars old re-
ports [10, 11]. The LAMPF DTL linac long time works
in LANL uses the funneling (but not use this term) [12,
13]. The previously accelerated to 200 MeV H+ and H-
beams were injecting in last section of LAMPF linac
and simultaneously accelerated to 800 MeV. The accel-
eration was provided in different (opposite) reference
phases and bunches of H+ and H- ions were spatially
separated. The systems for beam bunching and low en-
ergy acceleration with funneling were proposed later in
LANL [14] and Frankfurt University [15-17] using RFQ
or magnetic quadrupole lenses. In these linacs funneling
is used to increase the total beam current. The four stage
funneling scheme was presented in [18]. Frequency
multiplying is necessary in funneling method if only
positive or negative ion beam are accelerates. The linac
with very high current can be used for designing fusion
technologies facilities or spallation neutron sources
[19]. Other bunching and acceleration mechanism can
be realized in case when the positive and negative ions
were accelerated simultaneously.
3. SIMULTANEOUS ACCELERATION OF
POSITIVE AND NEGATIVE IONS IN RFQ
As it is well known, RFQ linacs are more useful for
low energy proton and ion beams bunching and accelera-
tion. This linac was proposed by V.A. Teplyakov and
I.M. Kapchinskiy [20] and the beam dynamics in RFQ
was studied by many authors [21-24]. Popular codes as
simplest LANL PARMTEQ or more accurately
DYNAMION [21] and LIDOS [25] uses for numerical
beam dynamics simulation in RFQ linacs.
It was shown by numerical simulation that the total
beam flux is lower and beam transverse emittance de-
creases in case of simultaneously acceleration of H+ and
H- ions [26]. The decreasing of output beam flux seems
very strange result and can be caused by specific model
used for simulation. The space-charge forces in these
models are calculated by assuming that the charge dis-
tribution is periodic and treating by following a separate
group of particles for each beam. In case when the two
beams have equal input parameters the problem is sim-
plified by following only the positive ions.
ISSN 1562-6016. ВАНТ. 2012. №3(79) 132
The results of simulation and experiential study of
simultaneously acceleration of O+ and O- ions were rep-
resented in [27]. It was shown that the total beam flux
can be sufficiently (approximately 1.8 times) increased
using funneling method. Analysis of beam dynamics
shows that in RFQ or DTL the intensity of the ion beam
can be made twice as higher by simultaneous accelera-
tion of ions with opposite charge signs. The accelerating
force in these linacs is proportional to the charge of the
ion. Oppositely charged ions are bunched and acceler-
ated in the different phases of the accelerating wave.
Two bunches (one with a positive and another one with
a negative charge) become separated and weakly inter-
act with each other after the initial part of the buncher.
In this case the phase separation of the bunch is large
and the space charge neutralization can’t be achieved.
The intensity of the ion beam can be made twice as
higher therefore. These results were confirmed in gen-
eral by numerical simulation [27-28].
Thus the simulation shows that the total beam flux
can be only twice enlarged in RFQ using simultaneous
negative and positive ions acceleration. Note that the
simulation results [26-29] were observed using modified
PARMTEQ code. The distribution of ions and Coulomb
fields was calculated separately for positive and nega-
tive ions on 2D grid. The full field is calculates by su-
perposition that is not all correct for two beam accelera-
tion because the beams of oppositely charged particles
are overlapping in buncher.
Different results were done by A.P. Durkin using
LIDOS code. It was shown [30] that the current trans-
mission coefficient can be “significantly (up to 10%)
diminishes”. These differences provide us to necessary
of more detail investigation of simultaneous negative
and positive ions acceleration in RFQ.
4. USING BEAMDULAC CODE FOR DUAL
BEAM DYNAMICS SIMULATION IN RFQ
The BEAMDULAC code is developing in MEPhI
for self-consistent beam dynamics investigation in RF
linacs and transport channels [31-32]. The 2D and 3D
ion beam dynamics can be studied by means of this
code. The motion equation for each particle is solved in
the external fields and the inter-particle Coulomb field
simultaneously. The BEAMDULAC code utilizes the
cloud-in-cell (CIC) method for accurate treatment of the
space charge effects. The charge density is deposited on
the grid points using the CIC technique. To determine
the potential of the Coulomb field, the Poisson equation
is solved on the grid with periodic boundary conditions
at both ends of the domain in the longitudinal direction.
The aperture of the channel is represented as an ideally
conducting surface of rectangular or circular cross-
section. Therefore the Dirichlet boundary conditions are
applied at transverse boundaries of the simulation do-
main. In such an approach, the interaction of the bunch
space charge with the accelerating channel boundaries is
taken into account. This allows consideration of the
shielding effect, which is sufficiently important for
transverse focusing in the narrow channel. The fast Fou-
rier transform (FFT) algorithm is used to solve the Pois-
son equation on a 3D grid. The Fourier series for the
space charge potential obtained can be analytically dif-
ferentiated, and thus each component of the Coulomb
electrical field can be found as a series with known co-
efficients. Accordingly in our code, the space charge
field can be calculated with the same precision as the
Coulomb potential.
Fig.1. Current transmission coefficient versus initial
beam current for proton and dual beam. The simulation
was done using BEAMDULAC code for RFQ linac with
parameters [29]
Fig.2. Main parameters of RFQ channel
Fig.3. Current transmission coefficient (a) and output
transverse emittance (b) versus initial beam current
for proton and dual beam
b
a
ISSN 1562-6016. ВАНТ. 2012. №3(79) 133
The code modification was provided for the investi-
gation of multi ion beam dynamics [33]. The Coulomb
field calculation was updated mainly. The modification
of space charge distribution calculation and algebraic
equation for Fourier coefficients was provided for multi
ion beam self-consistent dynamics simulation in espe-
cial BEAMDULAC-2B code version.
At first the results obtained by Y. Ogury in [29]
were verified: it was shown that the current transmission
coefficient is lower that he predicates. The limit beam
current for such linac is equal to 80 mA approximately
(Fig.1). Next the dual beam dynamics in this linac was
simulated using BEAMDULAC-2B code. It was shown
that the total beam flux can achieve 200 mA (Fig.1).
This value not confirms the simulation results which
were done by Y. Ogury using modified PARMTEQ
code. Note that the very low value of initial transverse
emittance was used in [29].
It can be expected that the differences between the
results are caused by non correct Coulomb field influ-
ence treatment in PARMTEQ code. The main differ-
ences should be observer in bunching part of linac in
which the H+ and H- bunches will overlap. The abstract
RFQ linac with long buncher and conventional dynamic
matching part at front end of linac was proposed to
study of dual beam bunching process. The main pa-
rameters of this linac are shown in Fig.2. The limit pro-
ton beam current for this linac is also equal to 80 mA
and up to 400 mA for dual beam (Fig.3). It is also clear
from Fig.3,b that the limit beam flux is defined by non
linear effects in the beam: the transverse emittance
grows nonlinearly if the beam flux is higher than any
limit value. The process of dual beam bunching in RFQ
linac with described above parameters is shown in Fig.4
for different beam fluxes. It is clear that positive and
negative ions are interacting appreciably in bunching
part of linac and interaction is stronger for largest cur-
rent. This interaction partially compensates the Cou-
lomb field influence and the limit beam flux can be 4-5
lager than limit current for proton or H- beam. But this
interaction provides to more intensive halo formation if
the flux is lager than any limit value.
Fig.4. Dual beam bunching process: longitudinal and transverse beam spaces in different channel cross-sections.
Current of each particles type: I=0 (left), I=100 mA (middle), I=250 mA (right)
ISSN 1562-6016. ВАНТ. 2012. №3(79) 134
It is also interest to study the beam dynamics in case
when the initial beam currents are not equally for pro-
tons and H-. This case is illustrated in Fig.5 and 6.
Fig.5. Current transmission coefficient (a) and output
transverse emittance (b) versus of the ratio of initial
beam fluxes of H- and protons, I+=100 mA
Fig.6. Current transmission coefficient (a) and output
transverse emittance (b) versus of the ratio of initial
beam fluxes of H- and protons, I++|I-|=150 mA
The current transmission coefficient Kt (a) and out-
put transverse emittance E (b) versus of the ratio of ini-
tial beam fluxes of H- and protons I-/I+ are shown in
Fig.5 in case when proton beam current is fixed and
equals to I+=100 mA. The same dependences are shown
in Fig.6 in case when initial beam flux is fixed and
I++|I-|= 150 mA. The transmission coefficient of H- ions,
Kt
(-), in the dual beam is approximately equal to the
transmission coefficient for the single H- beam with
current I=|I(-)|-|I(+)|. The Kt
(+) for H+ ions increases and
Kt
(-) for H- decreases when the ratio of |I(-)|/|I(+)| enlarges.
The beam with smaller current has the smaller output
emittance. The simulation shows that in “quasi-neutral”
beam current transmission coefficients for H+ and H- are
closely, even in case when I- and I+ differs significantly.
Fig.7. Current transmission coefficient versus initial
beam current in RFQ for bunched dual beam
Finally it is interest to study the bunched dual beam
dynamics in RFQ. It was shown that bunches of H+ and
H- are interact but the interaction is weakly compara-
tively not bunched beam. The current transmission ver-
sus initial beam flux is shown in Fig.7 for bunched
beam. It was shown that the longitudinal interaction is
observing and the bunch phase size will smaller for
largest currents. This effect explains the limit flux value
enlargement.
5. DUAL BEAM DYNAMICS SIMULATION
IN DTL
The DTL (Alvarez type) linac is the classical system
for ion beam acceleration in energy range
0.5…100 MeV. One of DTL linacs was the first accel-
erator in which protons and H- ions were successfully
accelerated simultaneously (LAMPF, LANL). The pa-
rameters of LAMPF DTL linac are unavailable but it is
known that the operation current is 100 mA for protons
and I+=100 mA, I-≈80 mA in dual mode. It is interest to
verify this result for other DTL linac. The first tank of
LINAC4 for CERN SPL linac was R&D in MEPhI,
ITEP and VNIIEF (Sarov). The parameters of this DTL
linac can be founded in [31], the input/output energy is
equal to 3/10 MeV and operation current 40 mA.
The simulation was done using especially designed
version of BEAMDULAC_DTL-2B code. It was shown
that bunches of H+ and H- interact weakly and space
charge influence not compensates but transverse emit-
tance and beam envelope will some smaller for dual
beam.
b
a
a
b
ISSN 1562-6016. ВАНТ. 2012. №3(79) 135
6. ACCELERATION OF POSITIVE AND
NEGATIVE IONS IN THE SAME BUNCH
In a conventional RF linac the beam is accelerated
by a synchronous wave of the RF field. An alternative
method of ion acceleration can be realized if the oppo-
sitely charged ions will bunched and accelerated in the
same bunch. The structure where such acceleration
mechanism can be realized was proposed by
E.S. Masunov and called linear undulator accelerator
(UNDULAC) [34, 35]. The acceleration mechanism in
UNDULAC is similar to the acceleration mechanism in
an inverse free electron laser (IFEL), where the electron
beam is accelerated by a ponderomotive force. In
UNDULAC the beam bunching, acceleration and focus-
ing are realized in the accelerating force which is driven
by a combination of two non-synchronous waves (two
undulators). As it has been shown, one of the undulators
must be of the RF type, the second one being, option-
ally, of magnetic (UNDULAC-M), electrostatic
(UNDULAC-E) or RF (UNDULAC-RF) types. The
accelerating structure of UNDULAC can be realized as
an interdigital H-type (IH) periodic resonator with drift
tubes. As it is well known the ponderomotive force is
proportional to charge of ion squared. It is possible to
bunch and to accelerate the positive and negative ions
simultaneously in the same bunch by means this prop-
erty. As two examples, the equation of ion motion is
ϕβπλ=τβ 2sin)/()2/(d/d 10
22 EEmce ,
ϕβπλ=τβ cos)2/()2/(d/d 00
22 oEEmce for
UNDULAC-E.
Here β is the ion velocity, τ=ωt is the dimensionless
time, λ – the length of wave, e and m – the ion charge
and mass, ϕ – the phase of particle in accelerating com-
bined wave, E0 and E1 are the amplitudes of base and
first RF field spatial harmonics in periodical resonator,
oE0 is the amplitude of electrostatic undulator field. The
analysis of numerical simulation results shows that the
limit dual beam flux value is very high: about 4 A for
UNDULAC-RF and 20 A for UNDULAC-E [36]. Note
that this flux value is unachievable for contemporary
accelerator technology because the limit beam current
of modern ribbon ion sources is achieves 1 A approxi-
mately. The beam power could be equal to 10 MW
when the total beam flux is equal to 10 A and the output
beam energy is 1 MeV. This is impossible for modern
RF generators.
CONCLUSIONS
The efficiency of space charge neutralization for ion
limit beam current enlargement was discussed. It was
shown that this mechanism can be very effective for ion
bunchers as RFQ. The high accuracy codes are need for
dual beam dynamics simulation and correct physical
interpretation should be done for all results.
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Статья поступила в редакцию 25.09.2011 г.
ISSN 1562-6016. ВАНТ. 2012. №3(79) 136
ИСПОЛЬЗОВАНИЕ НЕЙТРАЛИЗАЦИИ ВЛИЯНИЯ ОБЪЕМНОГО ЗАРЯДА ПУЧКА
ДЛЯ ПОВЫШЕНИЯ ИНТЕНСИВНОСТИ ПУЧКОВ В ЛИНЕЙНЫХ УСКОРИТЕЛЯХ
С.М. Полозов
Как принято считать, влияние объемного заряда пучка является основным фактором, ограничивающим
интенсивность ионных пучков в линейных ускорителях на небольшие энергии. Можно утверждать, что в
настоящее время в ускорителях на небольшие энергии достигнут (или вскоре будет достигнут) предел по
току пучка. Для увеличения тока ионного пучка до 300…1000 мА, что требуется для некоторых приложе-
ний, таких как нейтронные генераторы или ядерные установки, управляемые ускорителем, существуют два
основных пути: увеличение поперечного сечения пучка и использование нейтрализации влияния объемного
заряда. В настоящее время второй путь обсуждается все более активно. Известно три (или более) способа
нейтрализации влияния объемного заряда: использование плазмы, ионизованного остаточного газа или
электронного облака; метод «сложения» пучков; ускорение ионов с разным знаком в одном сгустке. Неко-
торые результаты исследования динамики «нейтрализованного» ионного пучка в линейных ускорителях с
ПОКФ, ускорителях Альвареца, линейных ондуляторных ускорителях представлены в данной работе.
ВИКОРИСТАННЯ НЕЙТРАЛІЗАЦІЇ ВПЛИВУ ОБ'ЄМНОГО ЗАРЯДУ ПУЧКА ДЛЯ ПІДВИЩЕННЯ
ІНТЕНСИВНОСТІ ПУЧКІВ У ЛІНІЙНОМУ ПРИСКОРЮВАЧІ
С.М. Полозов
Як прийнято вважати, вплив об'ємного заряду пучка є основним чинником, що обмежує інтенсивність
іонних пучків у лінійних прискорювачах на невеликі енергії. Можна стверджувати, що в даний час у при-
скорювачах на невеликі енергії досягнута (або незабаром буде досягнута) межа по струму пучка. Для збіль-
шення струму іонного пучка до 300...1000 мА, що потрібно для деяких додатків, таких як нейтронні генера-
тори або ядерні установки, керовані прискорювачем, існують два основних шляхи: збільшення поперечного
перерізу пучка і використання нейтралізації впливу об'ємного заряду. В даний час другий шлях обговорю-
ється все більш активно. Відомо три (або більше) способи нейтралізації впливу об'ємного заряду: викорис-
тання плазми, іонізованого залишкового газу або електронної хмари; метод «складання» пучків; прискорен-
ня іонів з різним знаком в одному згустку. Деякі результати дослідження динаміки «нейтралізованого» іон-
ного пучка в лінійних прискорювачах з ПОКФ, прискорювачах Альвареця, лінійних ондуляторних приско-
рювачах представлені в даній роботі.
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