Stand for RF gap breakdown strength study in magnetic field
Presented are the design and the features of the stand for experimental studies of high-frequency gap electric strength in a magnetic field along with the verification of magnetic circuit simulation of linac accelerating structure with combined alternating-phase and magnetic focusing. Надано констру...
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| Опубліковано в: : | Вопросы атомной науки и техники |
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| Дата: | 2013 |
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| Мова: | Англійська |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2013
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| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Цитувати: | Stand for RF gap breakdown strength study in magnetic field / P.A. Demchenko, Eu.V. Gussev, N.G. Shulika, O.N. Shulika, D.Yu. Zalesky // Вопросы атомной науки и техники. — 2013. — № 4. — С. 293-296. — Бібліогр.: 4 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1860208071248183296 |
|---|---|
| author | Demchenko, P.A. Gussev, Eu.V. Shulika, N.G. Shulika, O.N. Zalesky, D.Yu. |
| author_facet | Demchenko, P.A. Gussev, Eu.V. Shulika, N.G. Shulika, O.N. Zalesky, D.Yu. |
| citation_txt | Stand for RF gap breakdown strength study in magnetic field / P.A. Demchenko, Eu.V. Gussev, N.G. Shulika, O.N. Shulika, D.Yu. Zalesky // Вопросы атомной науки и техники. — 2013. — № 4. — С. 293-296. — Бібліогр.: 4 назв. — англ. |
| collection | DSpace DC |
| container_title | Вопросы атомной науки и техники |
| description | Presented are the design and the features of the stand for experimental studies of high-frequency gap electric strength in a magnetic field along with the verification of magnetic circuit simulation of linac accelerating structure with combined alternating-phase and magnetic focusing.
Надано конструкцію та характеристики стенду для експериментальних досліджень електричної міцності високочастотних зазорів у магнітному полі та перевірки результатів чисельного моделювання магнітного кола прискорювальних структур лінійних прискорювачів з комбінацією змінно-фазового і магнітного фокусувань.
Приведены конструкция и характеристики стенда для экспериментальных исследований электрической прочности высокочастотных зазоров в магнитном поле и проверки результатов численного моделирования магнитных цепей ускоряющих структур линейных ускорителей с комбинацией переменно-фазовой и магнитной фокусировок.
|
| first_indexed | 2025-12-07T18:13:08Z |
| format | Article |
| fulltext |
ISSN 1562-6016. ВАНТ. 2013. №4(86) 293
APPLICATIONS AND TECHNOLOGY
STAND FOR RF GAP BREAKDOWN STRENGTH STUDY
IN MAGNETIC FIELD
P.A. Demchenko, Eu.V. Gussev, N.G. Shulika, O.N. Shulika, D.Yu. Zalesky
National Science Center “Kharkov Institute of Physics and Technology”, Kharkov, Ukraine
E-mail: demchenko@kipt.kharkov.ua
Presented are the design and the features of the stand for experimental studies of high-frequency gap electric
strength in a magnetic field along with the verification of magnetic circuit simulation of linac accelerating structure
with combined alternating-phase and magnetic focusing.
PACS: 29.20.Ej
INTRODUCTION
The means to enhance stability of proton beam fo-
cusing in a linear resonance accelerator with alternating-
phase focusing are under study at the Institute of Plasma
Electronics and New Acceleration Methods of NSC
KIPT. It is well known that there exists a very strong
connection between longitudinal and transverse particle
dynamics in linac accelerating channels with alternat-
ing-phase focusing. As a result, effective beam emmit-
tance increases leading to current loses and activation of
linac structural units [1].
To increase the effect of proton beam focusing by
RF electric field and, therefore, to decrease an ampli-
tude of particle radial oscillations in an accelerating
channel, it is proposed to apply an external magnetic
field in gaps between drift tubes [1].
There are several ways how to produce a longitudi-
nal magnetic field in accelerating gaps between axial-
symmetric drift tubes. One of them is to use tubes made
of ferromagnetic material with high saturation induc-
tion. An outer shell of the tubes is made of copper. The
thickness of the shell should exceed the skin-layer thick-
ness at operation frequency to preserve resonator Q-
factor of accelerating structure. As this takes place, the
RF power supply of linac does not change.
A sequence of the drift tubes with ferromagnetic
core forms a magnetic circuit where magnetic field is
concentrated in accelerating gaps between the tubes. To
lessen dissipation of magnetic induction flux along the
drift tubes, it is important to ensure the ferromagnetic
material is not in the saturation mode and possesses
high relative magnetic permeability μr. It is clear that
maximum induction in a gap is always less than satura-
tion induction of a drift tube material.
Thus, the drift tubes have two functions: on the one
hand they serve as electrodes with voltage difference to
accelerate charged particles, and on the other hand they
serve as magnetic poles for additional particle beam
magnetic focusing. Fig. 1 illustrates a conceptual ver-
sion of a section with combined alternating-phase and
magnetic focusing [1].
The magnetic circuit is formed by the drift tube se-
quence and the yoke system with a coil to provide mag-
netic flux Φ, see Fig. 1. The coil is outside the vacuum
chamber with the accelerating structure.
The above design of an accelerating channel with
combined focusing was named a structure with spatially
combined alternating-phase and magnetic focusing.
Such structures can be used to accelerate proton beams
in low and medium energy range (up to 100 MeV).
Fig. 1. Section with combined alternating-phase and
magnetic focusing: 1 – resonator; 2 – drift tube;
3 – rack; 4 – vacuum chamber;
5 – magnetic conductor; 6 – coil; 7 – pole tip
EXPERIMENTAL STAND FOR STUDIES
OF RF GAP BREAKDOWN STRENGH
IN MAGNETIC FIELD
The development of an accelerating section with spa-
tially combined alternating-phase and magnetic focusing,
see Fig. 1, calls for preliminary scientific R&D work. In
particular, it is important to perform magnetic circuit
simulation for accelerating gap flux density evaluation in
a resonant section. The magnetic circuit requirements are
specified after charged particle dynamics simulation. As a
result of the magnetic circuit simulation, it is expected the
following problems will be resolved:
• the design and materials for the magnetic circuit
including drift tubes;
• the coil parameters and design;
• the coil power supply;
• coil heat generation problem and cooling;
• magnitude of magnetic ponderomotive force that
affects accelerating section elements made of ferro-
magnetic materials;
• necessary mechanical stiffness of an accelerating
section considering its deformation as a result of
magnetic field affect.
It is very important to investigate the electric
strength of RF accelerating gaps when magnetic field
generated by drift tubes with ferromagnetic core is ap-
plied. Electric field intensity in gaps defines the maxi-
mal accelerating rate.
ISSN 1562-6016. ВАНТ. 2013. №4(86) 294
To solve these problems, an experimental stand is
being designed. Also, the stand is being used for simula-
tion results verification. For the R&D cost reduction, a
simple construction of a resonant accelerating section
has been considered.
As the result of numerical simulation, the section
with two drift tubes and one accelerating gap between
them has been chosen as a test model. Low-carbon steel
(Stee l3, Stee l10) has been selected as ferromagnetic
material for magnetic circuit elements. In preliminary
experiments, the magnetically soft steel has been used
as the drift tube core material. In further experiments, it
is supposed to replace the steel with an iron-cobalt alloy
(permendure) due to its high saturation induction up to
2.2 T to increase magnetic field induction in a gap.
The stand design is presented in Fig. 2. The acceler-
ating structure is a quarter-wave coaxial resonator oper-
ating at a frequency f =100 MHz. The electrodes 1 and 2
form the accelerating gap at the cut-section of central
electrode. The coaxial resonator length is 760 mm.
Fig. 2. Experimental stand for RF breakdown strength studies in magnetic field: 1, 2 – ferromagnetic
poles/electrodes; 3 – coil; 4, 5 – magnetic shields; 6, 7 – coaxial resonator electrodes; 8, 9 – coupling loops;
10 – insulator
The coaxial parts of the resonator are formed by
copper cylindrical tubes 6 and 7. The interior volume of
the tube 7 is partially filled with ferromagnetic material
thus forming the ferromagnetic core of electrode 1. The
electrode 1 face is covered with copper layer 1 mm
thick. The electrode 2 is made of ferromagnetic material
and has 1 mm thick copper coating on the resonator
side. Conditions on copper surface of the electrodes 1
and 2 should meet the technological requirements for
drift tube surfaces in present-day linacs.
Thus, the electrodes 1 and 2 form an RF electric
field in the gap between them as well as create a focus-
ing magnetic field.
The closed magnetic circuit consists of the
pole/electrode 1, the accelerating gap, the pole/electrode
2, the cylindrical magnetic yokes 4,5 and the circular
vacuum gap between the magnetic core 5 and the
pole/electrode 1. Magnetic potential difference between
the poles is produced by the coil 3.
The coupling loops 8 and 9 provide high-frequency
electromagnetic field excitation in the resonator. The
coaxial electrode 7 is centered by the insulators 10 in-
stalled in group of three at an angle of 120° in each
resonator cross-section.
The stand is connected with the vacuum volume
through a flange where the pole 2 is installed. Pumping
occurs through apertures in the flange and the channel
along the axis, see Fig. 2. Axial apertures in the elec-
trodes 1 and 2 simulate a channel for ion beam passing.
Residual gas pressure in the chamber is 1.3⋅10-4 Pa.
Numerical simulation of the magnetic circuit pre-
sented in Fig. 2 is performed using FEMM code, ver-
sion 4.2. This software is based on the finite element
method [2] which allows one to calculate field distribu-
tion for plane and axial-symmetric geometries using
magnetization (B-H) curves for different ferromagnetic
materials.
The solution method for magnetostatics problems is
based on calculation of magnetic field vector potential
A
ur
connected with field induction B
ur
by the relation
B rot A=
ur ur
.
Fig. 3. Magnetic field topography
in a quarter-wave coaxial resonator
On the other hand, vector potential satisfies Poisson
equation A jμΔ = −
ur r
. Here j
r
(r,z) is distribution of
electric current density inducing magnetomotive force,
μ(r,z) is magnetic permeability, r,z are radial and axial
coordinates respectively.
In axial-symmetrical geometry, vector potential has
only one component A(r,z) normal to (r,z)-plane and
obeys Laplace equation ΔA=0.
z
r
ISSN 1562-6016. ВАНТ. 2013. №4(86) 295
Fig. 3. represents the distribution of magnetic field
induced by the coil with magnetomotive force of
8000 ampere-turn. Absolute values of magnetic induc-
tion are shown in color spectrum.
As it follows from the numerical simulation, see
Fig. 3, magnetic induction on the poles of accelerating
gap is less than 0.96 T. It results from low saturation
induction in steel Bs≈1.4 T. Referring to Fig. 3, it is
clear that a part of electrode 2 ferromagnetic core is near
saturation.
If we use supermendur 49К2ФА as a material for
poles 1 and 2 then surface induction could be increased
up to 1.2…1.3 T.
The distribution of magnetic induction |B| between
the accelerating gap poles along z-axis at r=18 mm is
illustrated in Fig. 4. At this radius the maximal induc-
tion is observed at corresponding points on the pole
surfaces. The poles are round-shaped to decrease both
electric and magnetic field gradients along the surface.
B, Т
z,
0.7
0.8
0
0.9
1.0
5 10
Fig. 4. Longitudinal distribution of magnetic induction
between poles at r=18 mm
As it follows from Fig. 4, magnetic induction at the
outer pole 2 is about B2≈0.85 T, then it declines mo-
notonously to about 0.75 T in the middle plane of the
accelerating gap forming a so-called field dip and then
increases again up to about B1≈0.96 T at the inner
pole 1. Non-symmetry in the curve behavior is due to
the different pole geometry. Attractive force that acts on
the inner electrode 1 is 450 Н.
In order to obtain presented magnetic induction val-
ues, it is required about 800 W power supply for the
coil.
Fig. 5 presents the radial distribution of magnetic in-
duction |B| in the middle plane between the poles. The
maximal induction value is about 0.8 T. The field dip on
the axis is caused by the pole apertures used to simulate
the channel for the ion beam transportation.
An important problem for an accelerating structure
with spatially combined alternating-phase and magnetic
focusing is electrical strength of vacuum gaps. When a
large magnetic field is applied between electrodes the
electric breakdown probability may increase. Electrical
strength could depend on magnetic induction and field
topography in an accelerating gap.
In present-day technique of charged particle linacs,
Kilpatrick criterion giving the relationship between a
field frequency f and an electric intensity E on elec-
trodes is used to evaluate the possibility of electrical
breakdown without magnetic field [3]. In case of copper
electrodes this criterion is f=1.64⋅EK
2⋅exp(-8.5/EK),
where f is field frequency (MHz); EK is maximal electric
field intensity that provides stable operation mode with-
out breakdown of drift tube gaps (MV/m) [4].
It should be emphasized that Kilpatrick criterion de-
pends on several empiric parameters based on experi-
ments. An excess factor at Ek depends on electrode sur-
face roughness, electrode contamination, vacuum condi-
tions, etc. Under modern technology conditions it could
reach up to 1.2…2 depending on electrode manufactur-
ing process and vacuum hygiene [4].
To induce an electric field between poles 1 and 2
(see Fig. 2), an external RF generator is used to excite
oscillations in the resonator at a frequency f0=100 MHz.
During numerical simulation the following parame-
ters have been calculated: a resonator Q-factor, electric
field distribution in the gap at the storage energy
W0=1 J, RF power needed to obtain electric fields which
are equal to or exceed Kilpatrick criterion.
It follows from the calculation analyses that the
resonator Q-factor is about Q≈3500 if the resonator and
electrode copper surfaces and element junctions have no
defects.
Fig. 6. shows the distribution of electric intensity be-
tween the electrodes along z-axis at r=18 mm for the
storage resonator energy W0=1 J. As this takes place,
electric field intensity reaches about Emax≈360 kV/cm on
the inner electrode surface while on the surface of the
outer electrode its value is about 260 kV/cm. The differ-
ence is due to the difference in the electrode geometry
which has been optimized to obtain magnetic field in-
duction in the gap as high as possible.
Fig. 5. Radial magnetic induction distribution
in the middle plane between the electrodes
|B|, Tesla
Length, mm
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 5 10 15 20
r, mm
B, Т 0.8
0.6
0.4
0.2
0
5 0 10 15 20
0
100
200
300
400
0 2 4 6 8 9 Z,mm
E, кV/сm
Fig. 6. Longitudinal distribution of electric field
intensity in gap between electrodes at r=18 mm
and storage resonator energy 1 J
ISSN 1562-6016. ВАНТ. 2013. №4(86) 296
Fig. 7 presents the radial distributions of the electric
field intensity |E| at tangential planes of the inner (1)
and outer (2) electrodes for the same storage resonator
energy W0=1 J. Again, the difference between two dis-
tributions is a result of the electrode geometry differ-
ence.
Kilpatrick criterion in case of copper electrodes and
field frequency f0= 100 MHz is EK≈110 kV/cm. Consid-
ering that Q = 2πf0⋅W0/Pdis (where f0 is a resonator fre-
quency, W0 is storage resonator energy, Pdis is power
dissipated in the resonator), a power supply
Pdis≈180 kW is required to generate an electric field
with intensity Emax≈360 kV/cm at W0=1 J. Since power
dissipation is proportional to electric field intensity
squared E2, an RF generator with power about 20 kW is
sufficient to meet Kilpatrick criterion conditions.
At present, an RF generator with the following pa-
rameters: output capacity up to 400 kW, pulse duration
200 μs, pulse repetition up to 10 Hz is available to the
stand developers.
CONCLUSIONS
Based on results of simulation, the design and con-
struction documentation of the stand for experimental
studies of high-frequency gap electric strength in a
magnetic field has been developed. The stand will also
be used for the verification of the results of magnetic
circuit simulation for proton linac sections with com-
bined alternating-phase and magnetic focusing.
REFERENCES
1. S.A. Vdovin, P.A. Demchenko, Ye.V. Gussev,
M.G. Shulika, O.M. Shulika. Combined Focusing in
Linear Ion Accelerator // Problems of Atomic Sci-
ence and Technology. Series “Plasma Electronics
and New Methods of Acceleration” (68). 2010, № 4,
p. 325-329.
2. D. Meeker. Finite Element Method Magnetics
FEMM 4.2, User’s Manual, 2010, 158 p.
(http://www.femm/info).
3. W.D. Kilpatrick. Criterion for Vacuum Sparking
Designed to Include RF and DC // Rev. Sci. Instrum.
1957, v. 28. p. 824-826.
4. T.P. Wangler. RF Linear Accelerators. Wiley-VCH,
2008, 450 p.
Article received 04.03.2013.
СТЕНД ДЛЯ ИССЛЕДОВАНИЙ ЭЛЕКТРИЧЕСКОЙ ПРОЧНОСТИ ВЧ-ЗАЗОРОВ
В МАГНИТНОМ ПОЛЕ
П.А. Демченко, Е.В. Гусев, Н.Г. Шулика, О.Н. Шулика, Д.Ю. Залеский
Приведены конструкция и характеристики стенда для экспериментальных исследований электрической
прочности высокочастотных зазоров в магнитном поле и проверки результатов численного моделирования
магнитных цепей ускоряющих структур линейных ускорителей с комбинацией переменно-фазовой и маг-
нитной фокусировок.
СТЕНД ДЛЯ ДОСЛІДЖЕНЬ ЕЛЕКТРИЧНОЇ МІЦНОСТІ ВЧ-ЗАЗОРІВ У МАГНІТНОМУ ПОЛІ
П.О. Демченко, Є.В. Гусєв, М.Г. Шуліка, О.М. Шулiка, Д.Ю. Залеський
Надано конструкцію та характеристики стенду для експериментальних досліджень електричної міцності
високочастотних зазорів у магнітному полі та перевірки результатів чисельного моделювання магнітного
кола прискорювальних структур лінійних прискорювачів з комбінацією змінно-фазового і магнітного фоку-
сувань.
0
100
200
300
400
0 8 16 24 32 40 r, mm
E, кV/сm
2
1
Fig. 7. Radial distribution of electric field intensity
at inner (1) and outer (2) electrodes
|
| id | nasplib_isofts_kiev_ua-123456789-112155 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1562-6016 |
| language | English |
| last_indexed | 2025-12-07T18:13:08Z |
| publishDate | 2013 |
| publisher | Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| record_format | dspace |
| spelling | Demchenko, P.A. Gussev, Eu.V. Shulika, N.G. Shulika, O.N. Zalesky, D.Yu. 2017-01-17T19:32:54Z 2017-01-17T19:32:54Z 2013 Stand for RF gap breakdown strength study in magnetic field / P.A. Demchenko, Eu.V. Gussev, N.G. Shulika, O.N. Shulika, D.Yu. Zalesky // Вопросы атомной науки и техники. — 2013. — № 4. — С. 293-296. — Бібліогр.: 4 назв. — англ. 1562-6016 PACS: 29.20.Ej https://nasplib.isofts.kiev.ua/handle/123456789/112155 Presented are the design and the features of the stand for experimental studies of high-frequency gap electric strength in a magnetic field along with the verification of magnetic circuit simulation of linac accelerating structure with combined alternating-phase and magnetic focusing. Надано конструкцію та характеристики стенду для експериментальних досліджень електричної міцності високочастотних зазорів у магнітному полі та перевірки результатів чисельного моделювання магнітного кола прискорювальних структур лінійних прискорювачів з комбінацією змінно-фазового і магнітного фокусувань. Приведены конструкция и характеристики стенда для экспериментальных исследований электрической прочности высокочастотных зазоров в магнитном поле и проверки результатов численного моделирования магнитных цепей ускоряющих структур линейных ускорителей с комбинацией переменно-фазовой и магнитной фокусировок. en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Приложения и технологии Stand for RF gap breakdown strength study in magnetic field Стенд для досліджень електричної міцності ВЧ-зазорів у магнітному полі Стенд для исследований электрической прочности вч-зазоров в магнитном поле Article published earlier |
| spellingShingle | Stand for RF gap breakdown strength study in magnetic field Demchenko, P.A. Gussev, Eu.V. Shulika, N.G. Shulika, O.N. Zalesky, D.Yu. Приложения и технологии |
| title | Stand for RF gap breakdown strength study in magnetic field |
| title_alt | Стенд для досліджень електричної міцності ВЧ-зазорів у магнітному полі Стенд для исследований электрической прочности вч-зазоров в магнитном поле |
| title_full | Stand for RF gap breakdown strength study in magnetic field |
| title_fullStr | Stand for RF gap breakdown strength study in magnetic field |
| title_full_unstemmed | Stand for RF gap breakdown strength study in magnetic field |
| title_short | Stand for RF gap breakdown strength study in magnetic field |
| title_sort | stand for rf gap breakdown strength study in magnetic field |
| topic | Приложения и технологии |
| topic_facet | Приложения и технологии |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/112155 |
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