Download introduction

DESIGN AND DEVELOPMENT OF COMPACT ELECTRON GUN AND ITS
PERFORMANCE WITH COMPACT LINAC OPERATION
D. Bhattacharjee*, R. Tiwari, D. Jayaprakash, R.L. Mishra, Shiv Chandan, A.R.Tillu, R.B. Chavan,
B. Nayak, V.Yadav, S.R. Ghodke, N. Chaudhary, A. Waghmare, H. Sarukte, K.C. Mittal and L.M.
Gantayet
Electron Beam Centre, Accelerator & Pulse Power Division
BARC, Mumbai – 400085
e-mail: [email protected]
Abstract
The electron gun for the compact linac was designed
using the CST Particle Studio Software. It is a triode gun
Pierce focusing electrode of ID 23 mm and angle ~ 67.5
degrees, to give ~ 1 A of beam current at 85 kV and
having a diameter of less than 3.0 mm at a distance of 100
mm from the cathode. The cathode-focusing electrode
distance is 8 mm. The electron gun was fabricated,
assembled and tested on test bench. It was then connected
to the compact linac and beam commissioning was done.
By knowing the change in rf reflected power from no
beam condition to full beam load condition, the beam
current was estimated. Also, injection voltage was varied
to get different output radiation dose from the target.
The paper presents the compact gun design and
development, gun testing on test bench and its
performance with compact linac.
INTRODUCTION
Based on the demonstration of 9 MeV LINAC at ECIL,
in order to make the cargo scanning system more
compact, a 6 MeV, 700 W compact linac based on
magnetron as microwave generator, was developed by
APPD, BARC. The 6 MeV pencil beam electrons are
made incident on a tungsten target to produce x-rays with
a dose rate of 8 Gy/min/m. The specifications of the linac
are given in Table 1. The detailed description of the
compact linac system is given in ref. 1.
Table 1: Specifications of 6 MeV Compact Linac
Beam Energy
: 6 MeV
Peak beam current
: 160 mA
Avg. beam power
: 700 W (max)
X-ray beam focal size
: < 2 mm
X-ray dose
: 8 Gy/min/m
X-ray field size
: Std 30º cone
Pulse width
: 3.4 µs
Pulse rep. rate
: 250 Hz (max)
Length of linac
: 0.6 m
RF Frequency
: 2856 ± 2 MHz
Injection voltage
: 85 kV (max)
DESIGN ASPECTS
The initial 3 buncher RF structure cells are same for
both the 9 MeV ECIL linac and the 6 MeV compact linac.
Hence injector can be same. An injector between 50 to 85
keV can be used with a beam current of  1 A. Also, it is
known that a converging or collimating beam of smaller
beam diameter at the linac input will improve the capture
efficiency and hence the transmission of the beam. For
the compact linac, the output beam from the linac should
be of diameter ≤ 2 mm at the Tantalum target. To achieve
this, a Pierce gun design approach was attempted.
Considering all above, we had decided to design and
implement an injector of 85 keV, 1 A configuration with
a beam size less than 3.0 mm at a distance of 100 mm
from the cathode.
To design the electron gun, simulations were conducted
using the CST Particle Studio. It is a triode type
thermionic gun. It incorporates a Pierce geometry grid,
also known as focusing electrode. Studies were conducted
to obtain good beam quality, beam size and beam current
by varying (a) anode inner diameter, (b) Pierce angle of
the grid, (c) cathode-anode gap, (d) cathode-grid gap, (e)
anode shapes.
The simulations resulted in the required 85 kV triode
electron gun with Pierce focusing electrode (grid) of ID
23 mm and angle ~ 67.5 , to give ~ 1 A of beam current
and having a diameter of less than 3 mm at a distance of
100 mm from the cathode. The grid biasing is -1 kV. The
anode is also shaped with aperture of 10 mm. The
cathode-grid distance is 8 mm and the anode-cathode
distance is 30 mm. The gun is operated in the space
charge region.
Figure 1. CST Simulation of the 85 kV, 1080 mA
Pierce triode gun with -1 kV negative bias.
CATHODE AND CATHODE ASSEMBLY
The cathode is a LaB6 Pellet of diameter 10 mm and
thickness 1 mm with beam emission region of diameter 8
mm. The Lab6 pellet is indirectly heated by a coil
comprising of tantalum heat shields, tungsten heater
assembled along with lanthanum hexaboride (LaB6)
pellet (of size 10 mm diameter x 1 mm thickness) as an
electron emitter, a Pierce grid electrode and an anode.
Cathode unit and Pierce grid electrode are supported on
feedthrough flange. Anode is supported on an
intermediate adaptor flange. High voltage insulation
between cathode and anode is provided by an alumina
ceramic (99.9 %) insulator tube. An Aluminum wire of
diameter 0.5 mm was used for vacuum sealing of ceramic
isolator with metal parts and it was separately leak tested
with a leak rate of 2x10-9 mbar.l/s. Also asbestos based
SUPER 54 gaskets were used in place of Teflon gaskets
as cushion between the collar of the ceramic insulator
tube and metal split flange. The adaptor flange is
introduced in between the ceramic isolator and gun
vacuum chamber for easier maintenance of electron gun.
The whole assembly was made compact with reduced
component sizes and weight.
A filament connector assembly was designed,
fabricated and connected to the electron gun during its
integration with the linac system. It consists of delrin
holder to firmly hold the filament and grid cables, and
copper connectors which connect to the feedthroughs of
the gun.
filament made out of Tungsten wire. A filament power of
270 - 290 W raises the temperature of the LaB6 pellet
enough to emit the required current of 1 A.
LaB6 cathode is chosen because it has low thermionic
work function 2.66 eV, high melting point 2715 C, high
current density, it can work in vacuum of the order of 10 6
– 10-5 mbar. LaB6 offers the capability of long life and
orders of magnitude less sensitivity to air exposure than
conventional dispenser cathodes. In the case of mild
cathode poisoning resulting in lower beam emission, it is
observed that after conditioning the cathode to higher
filament power a few times, there is a an improvement in
the output beam current.
The cathode pellet is housed in a 0.05 mm thk.
rhenium cup which is then housed in an outer cylindrical
tantalum cup of 25 mm diameter and 0.15 mm thickness.
The pellet is held in place by two thin strips of rhenium
which are spot welded to the tantalum cup. The opening
of the rhenium and tantalum cups decides the beam
emission area of the cathode to be of diameter 8 mm or 6
mm.
The Lab6 pellet is indirectly heated by a coil filament
made out of tungsten wire of diameter 0.5 mm and
consisting of 9 turns with coil diameter of 5 mm ‘figure
2’. The gap between the tungsten wire and the pellet is
about 1.5 mm. The radial heat shields consist of two
cylindrical heat shields of tantalum (0.15 mm thk.) and
having diameters 20 mm and 25 mm. The inner
cylindrical Ta heat shield also has two Tantalum discs as
heat shields in the axial direction. The inner and outer
cylindrical heat shields are connected by Ta strips (2-3
mm wide, 0.15 mm thk.). The outer cylindrical heat shield
is connected to the ceramic base by 4 Ta strips (2-3 mm
wide, 0.25 mm thk.) and a SS 304 cup. The filament is
spot welded to two tantalum rods (diameter 2.5 mm)
which are rigidly supported on a ceramic base. These Ta
rods have ceramic sleeves on them to electrically isolate
them from the heat shields. The ceramic base sits on the
SS 304 cup. One of the Ta rods is shorted to the outer
cylindrical heat shield to maintain same potential of the
filament and the cathode.
Figure 3. Compact electron gun.
Figure 4. Comparison of electron gun sizes. The right one
is the compact electron gun for the 6 MeV compact linac.
Figure 2. Schematic of cathode assembly, fabricated
cathode assembly and filament with heat shields.
GUN CHARACTERIZATION
GUN FABRICATION AND ASSEMBLY
The electron gun test bench consists of DN-100 CF six
way chamber on which the electron gun is vertically
assembled. Side ports of the chamber are used for TMP
connection and a view port was also incorporated in the
The components of the electron gun were fabricated
which consists of the SS parts and the ceramic insulator
tube. The electron gun assembly consists of a cathode unit
2
system. A Faraday cup assembly was designed and
fabricated. It consists of a collector made of copper and
isolated from the body. Beam current can be measured
through a feedthrough on a DN 35 CF flange. The faraday
cup can be mounted on CF-100 flange and connected to
one of the port for beam collection and measurement.
The filament transformer was rated for 70 kV and also
the ceramic insulator tube withstood 72 kV insulation test.
Hence the cathode-anode gap and the cathode-grid gap
was changed to get a beam current of  1 A. Thus the
electron gun was assembled with cathode-anode distance
of 25 mm and cathode-grid distance of 9 mm. After the
assembly and vacuum leak testing, the electron gun was
put on test bench for emission testing and
characterization. The gun was conditioned to 300 Watts
of AC filament power. The I-V characteristics of the gun
were measured using an in-house solid state modulator. A
maximum beam current of 800 mA was extracted from
gun at 66 kV anode voltage and at 296 Watts of filament
power which was found repeatable. Further increase in
the anode voltage resulted in inter-electrode high voltage
arching inside gun so the anode voltage was limited to 66
kV. The reasons for the arching have been identified and
the improvements will be incorporated in future gun
assemblies. In all the above beam experiments cathode
and grid were shorted to each other. With cathode-grid
shorted we also have the values (a) 60 kV, 564 mA, 259
W and (b) 65 kV, 560 mA, 230 W.
Grid biasing experiments resulted in the following
outputs: (a) 60 kV, 232 mA, -2.8 kV, 259 W; (b) 60 kV,
872 mA, +2.8 kV, 259 W.
From the VSWR value we can back calculate the output
beam current to be around 100 mA.
Inorganic scintillation detector, especially Cadmium
tungstate (CdWO4) in form of a linear array, is used for
high energy x-ray imaging applications. Therefore some
beam trial experiments with CdWO4 pixels were carried
out. The detector pixel was placed in beam line at 4.2 m
from target and the dose rate of 0.08 Gy/min was
measured at exit of primary collimator at parameters of
2.5 MW, 25Hz, 50 kV. Detector responses were measured
by using oscilloscope.
Dose measurement was done at exit of primary
collimator at various linac parameters. The maximum
dose rate of 2 Gy/min at exit of primary collimator
(around 0.41 mm from target) was measured under the
operating parameters of 3 MW RF peak power, 25 Hz
PRF, 56 kV injection voltage & 175 A solenoid current.
On extrapolation at 250 Hz, this dose rate will be 20
Gy/min. Corresponding dose rate at 1 meter from target
will be 4 Gy/min which is sufficient for cargo imaging
applications.
Dose measurement was done at fixed linac parameters
at different angles at the exit of primary collimator. Dose
at +3cm, +6cm, +9 cm & +11 cm are 94%, 60%, 37% &
16% respectively compared to dose at the centre. Dose at
-3cm, -6cm, -9 cm & -11 cm are 85%, 70%, 44% & 13%
respectively compared to dose at the centre.
Total RF ON time during this period was 80 hours, EGun Filament ON time was 100 hours & total beam ON
time was 60 hours.
collimator focusing coil RF Window linac cavity E-gun
BEAM COMMISSIONING
The gun was assembled with the linac system and after
He leak testing (leak rate: 2 x 10-9 mbar.l/s), the system
was evacuated to a base vacuum of 2.0 x 10 -6 mbar and
then a total of 120 hours of continuous baking was done
at uniform temperature of 100 0C. Vacuum of 4.0 x 10 -7
mbar is achieved in the linac.
E-gun filament conditioning was carried out for 50
hours, with filament power being increased gradually up
to 300 W. With this, the linac was ready for beam trials.
First beam trials were carried out with the aim to
determine the dose rates produced by the linac. At RF
input power of 1.65 MW, 20 Hz PRF and injection
voltage of ~20 kV, dose rate measured at the tungsten
target was 3.1 Gy/min.
Experiments were conducted to study the effect of
various linac parameters on the measured dose rate. The
parameters included pulse repetition rate, injection
voltage and RF Power. Dose rate increase was linear with
all the three parameters one at a time.
Effect of Reflected Power with E-Gun Injection
Voltage was also seen. At 2.0 MW RF power reflected
power without beam was 45% (VSWR 5.0), which
reduced to 28% (VSWR 3.2) at 57 kV injection voltage.
Figure 6: Linac under vacuum & RF conditioning
CONCLUSION
The compact electron gun was designed for 85 kV and
performed up to 66 kV, 800 mA. Integration with the
linac system resulted in an output dose sufficient for
cargo scanning applications.
REFERENCES
[1] Shiv Chandan et al, “Commissioning and beam trials
of 6 MeV RF electronlLinac for cargo scanning”, this
conference.
3