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BUNCHER DESIGN & OPTIMIZATION FOR INDUSTRIAL RF
ELECTRON LINACS
V. T. Nimje and K. C. Mittal
Accelerator & Pulse Power Division, Bhabha Atomic Research Centre
Mumbai – 400 085
ABSTRACT
The electron linacs with output energy up to 10 MeV are very popular for industrial applications like food irradiation,
sterilization of medical products, coloration of diamonds etc. One of the applications is using such linac as an X-ray
source for cargo–inspection system, where high energy and intensity of γ-quanta are necessary to obtain clear images of
the inspected items. Hence, RF Electron accelerators that provide a fairly high capture coefficient, narrow energy
spectrum and reasonably high accelerating gradient are of particular interest. In the interest of the above, S-band 2856
MHz, 𝜋/2 mode standing wave bi-periodic RF structures for 6-10 MeV linacs have been designed, developed, and
beam tested at Electron Beam Centre, BARC. In this paper, the study of new buncher with input electron beam energy
of ~ 25 keV with optimum beam transmission, energy spread , fabrication and RF studies is described. Because of the
low injection energy, the size of the electron gun and handling of HV power supplies will be reduced. Already the
buncher has been designed, manufactured and rf tested.
INTRODUCTION
At present 10 MeV RF electron Linac is being
operated at a beam power level ~ 3 kW. The structure is
integrated and consisting of buncher as well as accelerator
section is of the standing wave, biperiodic, on axis
coupled cavity type operating in the continuous regime at
a working RF frequency ~ 2856MHz and RF input power
~ 3.0 - 4.5 MW (peak). The shape of accelerating cavity
is shown on figure 1.
FEq/2
Rco
D/2
Ro
In this paper, the optimization results of new bunching
part of the accelerating structure for electron linac are
presented. Optimization was carried out by means of
beam dynamics simulation code developed EBDYN, and
SUPERFISH, PARMELA package of programs for the
axial symmetric structures calculation [1].
F
s/2
Rci
Beam axis
c
g/2
L/2
buncher cavity has been designed in order to reduce the
injection energy from electron gun of ~ 25 keV to reduce
the size of the electron gun as well as easy operation from
the high voltage point of view. The optimum beam
transmission is as compared to the present buncher
cavity which requires an injector energy of ~ 60 - 75
keV to have optimum transmission of ~ 60 %. .
Ri
g/2
Rb
Rb
Fig. 1 : schematic diagram of ½ cavity
The rf structure was optimized for high shunt impedance
considering iris
beam aperture of 10 mm diameter, the inner and outer
corner radius Rci and Rco ~ 3.5 and 19.0 mm resp., inner
and outer nose cone radius Ri and Ro ~ 1.0 and 3.0 mm
resp. The lower shunt impedance of the bunching cavities
lead to average shunt impedance for the accelerator of 80
The rf coupling coefficient between the adjacent
cells K1 ~ 4.6 % was optimised with annular aperture of
10 mm width and 300 length at a radial distance of 26
mm from the beam axis. The linac was optimized for a
beam current of ~ 500 mA and beam energy of ~ 10
MeV. The present buncher was designed for the input
energy of ~ 50 - 85 keV from the electron gun. A new
Longitudinal Beam Dynamics in a Standing Wave
Electron Linac :
The computer code EBDYN developed has been
developed. The motion of an electron is expressed by
𝑑
𝑑𝑡
𝑚0 𝑣
1/2
𝑣 2
𝑐
= 𝑒𝐸0 cos⁡(𝜔𝑎 𝑡 + 𝜑0 )⁡
{1−( ) }
Where, e and m0 is the charge and rest mass of electron, v
and c are the velocity of electron and light respectively. φ
the phase angle and defined as zero when the RF field is
maximum. t is the time measured from the injection of
electrons into the field. E0 is the peak axial electric field
along the axial distance z and φ0 is the initial phase
position of electron.
DESIGN OF BUNCHER SECTION
Energy Gain (keV))
Estimations from the computer simulation of buncher
studies, it has been observed the length of the first
bunching cell is mainly responsible for the input beam
energy, optimum beam current and Energy spectrum at
the output of accelerator. Longitudinal particle beam
450
dynamics in the first
18 MV/m
375
buncher cavity has
been evaluated by
300
considering a constant
225
longitudinal electric
150 Leff
field
(Hard
edge
75
approx.)
along
the
0
0 4 8 12 16 20 24 28 32 36
beam axis for 12 to 30
Length along Beam axis / mm
MV/m and 31 to
Fig. 2 : Energy gain Vs.
41 mm length. In this
Length along Beam Axis
calculation
energy
gain as well as output
phase wrt to length along the beam axis for various input
phase have been evaluated and is shown in fig.2 for 36
mm length. Fig. 2 shows the longitudinal bunching after
certain length. The results are mentioned in table 1.
MV/
m in
gap
Leff/
mm
ΔE
/keV
Phase
acceptance
(Deg)
MV/
m for
SF
12
21
45
-15
-165
7
18
17
70
-15
-160
8.5
24
16.9
90
-10
-155
11.25
28
17.2
102
-5
-145
13.3
30
16.0
115
-10
-150
13.3
From the fig. 2, it is clearly observed after some length,
the spread in the output energy increases, hence, we
should limit this length as a effective gap length Leff of the
cavity. Considering above effective length as a gap
length of the cylindrically symmetric cavity, the eff. gap
length for 18 - 30 MV/m, comes out to be approx. 16 - 18
mm as shown in table 1 .
As the buncher has to be optimized for 25 keV electron
Gun,. Hence we limited the length of cavity as 36 mm
which includes 3 mm wall thick required to separate the
accelerating and coupling cell. 2 mm is the half the length
of coupling cell as well length for the nose cone to
increase the effective shunt impedance of the cavity. The
electric field pattern inside the cavity and electric field
Superfish
12.0
0.0
0
4
8
12
16
20
24
28
32
36
Length along Beam Axis ->)
Fig. 3 : E field along beam axis in 36 mm buncher
(measured with VNA and evaluated using Superfish)
The computer code EBDYN has been developed to
calculate the electron beam dynamics along the beam axis
using the electric field data evaluated by SUPERFISH.
The results for average electric field of 12 MV/m is taken
along the beam axis and input energy ~ 25 keV. The
output energy of ~ 375 keV with an energy spread of ~
400
310
350
Output Phase(Deg.)
t yields the distance z travelled by the electron inside the
cavity.
24.0
300
250
200
150
100
50
290
270
250
0
0
6
12
18
24
30
36
120
Length along Beam axis
180
240
Input Phase (Deg.)
Fig.4a,b: Energy gain and phase output after 1st Buncher
± 25 keV has been obtained at the end of first buncher
cavity, similarly the output phase spread of 400 for the
total input phase of 1300. As shown in fig. 2 which is in
comparison observed with the average uniform electric
field calculation made in the previous section and shown
in fig. 1. Hence, results obtained with the hard edge
approx. and SUPERFISH data gives approx. same results.
Extending the EBDYN computer code for 3 buncher
cavity for an input energy of 25 keV electron beam and
12, 16, 17 MV/m as an average electric field along the
beam axis in 1st, 2nd, 3rd buncher cavity. Energy gain vs.
length along the buncher and output phase for various
input rf phase is shown in fig. 5a. An output energy of
375, 900 and 1650 keV is obtained successively at the
end of 1st, 2nd, 3rd buncher cells beam energy spread
310
1800
output phase (Deg.)
. Integrating once again , the equation wrt.
Electric field (MV/m)
𝑐
electron Energy (keV)
−1/2
𝑣 2
{1 − ( ) }
along the beam axis has been evaluated for the buncher
cavity with the above gap length and total length of 36
mm using SUPERFISH computer code. The electric field
lines and field values
along the beam axis is
shown in fig. 3. For an
average electric field of
1 MV/m gives 2.02
MV at the center of the
cavity.
electron Energy (keV)
Integrating the above eqn. wrt t, the normalized energy
of electron γ can be evaluated. Where γ is the given by
1200
600
0
0
30
60
90
Length along Beam axis
120
290
270
250
110
140
170
200
230
260
input Phase (Deg.)
Fig. 5a,b : Energy gain and Phase output after 3 rd b-cell.
12
10
8
6
4
2
0
-250
-215
-180
Input Phase (Deg.)
-145
-110
New Buncher 25 keV
Output Energy At the end of each cell
12
10
6.5
80
36 mm
5.5
39 mm
45 mm
4.5
3.5
45 mm
36 mm
75
% transmission
Output Energy at the end of Each Cell
As the design of buncher section is satisfactory, the
electrons are traced for the whole accelerator section ie.,
17 cells, 3 buncher cells and 14 accelerating cells. The
maximum output energy at the end of accelerator section
found to be 12.137 MeV and average beam energy ~
11.66 MeV with an acceptable input phase lies between
-2450 to -1100. The output energy at the end of every cell
wrt. input phase is plotted in fig. 6 for the new and old
buncher resp.
To study, the output energy of electron beam wrt.
Injection energy is carried out. It is found that with
increasing the injection energy the maximum energy
remains the same but the average energy of the electron
beam decreases. Hence, one has to optimize the input
beam energy and is shown in fig. 9a &9b using
PARMELA.
Energy (MeV)
of ~ 70 keV after buncher section. Similarly, longitudinal
bunching has taken place with output phase spread of ~
450 for input phase varying from 1100 to 2500 ie., 1400 .
70
39 mm
65
60
55
50
20
40
e-gun Voltage
60
0
20
40
60
E-gun Voltage
8
6
4
Fig. 9a & 9b : Av. output beam energy wrt. E-Gun
Voltage
2
0
-240 -220 -200 -180 -160 -140 -120
Input Phase(Deg.)
Fig. 6 : Energy gain at the output of each cell
Output Phase (Deg.) -->
Similarly, the output phase wrt. cell no. is calculated for
various input phase and found to be phase spread obtained
is ~ 50 Deg. The results obtained are compared with the
old buncher having a length of 45 mm and comparison of
output phase wrt. input phase is shown in fig. 7. Both
shows phase spread ~ 500.
-60
But one can observe
-80
average energy in case of
-100
new buncher is higher as
-120
seen in fig. 6a and hence,
-140
The newly designed 36
-160
-250 -225 -200 -175 -150 -125 -100
mm buncher shows the a
Input Phase (Deg.) -->
better results.
Fig. 7 : Output phase at the end of the accelerator
Output Energy (MeV)
Similarly, the exercise was done to study the effect of
average electric field on the output energy of the electron.
It is found that for 8 MV/m the output energy is constant
from 1300 to 1700 and then it falls sharply. So, for
monoenergetic beam lower average electric field should
be favourable . But as the average electric field increases,
Then one can observe the output energy increases upto
the phase ~ 1100 and then decreases upto 1800.and then
again increases upto ~ 2500 and again decreases sharply.
This will have better bunching at and after 12 MV/m and
13
hence, to have high
8 MV/m
power
and
brems12
strahlung
generation.
12 MV/m
11
Electric field of ~ 12
16 MV/m
10
MV/m
is
more
9
favourable. This is
8
evaluated
using
90 120 150 180 210 240 270
PARMELA in fig. 8.
Input Phase (Deg.)
Fig. 8 : Output Energy at different av. E field after Linac.
After evaluating the longitudinal beam dynamics, we
have evaluated the buncher and accelerator using LANL
PARMELA code. The output energy and beam
transmission is studied. The study shows Tinj = 25 keV
is optimum for the new buncher as in fig. 9.
RF TEST using VNA
The cavity has been manufactured at CDM, BARC and
measured the electric field pattern using VNA. The
measurements are shown in fig. 10 and SUPERFISH
calculations is shown in fig. 3, matches well with the
measurements.
Fig. 10 : Electric field distribution in 6 MeV Linac.
Acknowledgement
Authors would like to thank Dr. L. M. Gantayet,
Director, Beam Technology Development Group, for his
keen interest in the low energy electron Beam
Accelerators Programme ( 6 MeV Compact Linac
Project) for Cargo Scanners. Also, one of the author
would like to thanks Shri HK Manjunatha for his keen
interest in preparing the technical drawings, Mechanical
set-ups, and RF measurements for the above work. We
would like to thanks our colleagues of EBC Kharghar for
the beam operations of the 10 MeV Linac.
REFERENCES
[1] Poisson-Superfish, Free distribution Code, LANL.
[2] Lloyd Young, AOTD, Los Alamos National
Laboratory.