Download DEVELOPMENT OF TABLETOP 50 kV ION ACCELERATOR

DEVELOPMENT OF TABLETOP 50 KV ION ACCELERATOR
Raj Kumar, Rajeev Ahuja, C. P. Safvan, Amit Roy, IUAC, New Delhi- 110 067
Abstract
A low energy accelerator is designed, fabricated,
assembled & tested at IUAC with the concept of having a
compact table top type machine. The accelerator is
specially developed for students & faculty at the
undergraduate & postgraduate level. Students will get a
chance to leverage their existing theoretical knowledge in
physics to explore ion source, bending/analyzing magnet,
analyzed - ion, molecular & neutral beams at different
energies, vacuum systems, beam transport, energy
selection, building an accelerator etc. Faculty & students
can conduct experiments using alpha, proton & hydrogen
beams up to 50 kV without any radiation hazard. These
would include, but not be restricted to positive ion
implantation, ion induced fluorescence studies, particle
detector experiments, ion scattering experiments,
production of neutral atomic species in beam form etc.
The accelerator consists of a cold cathode penning ion
generator capable of generating gaseous beams upto 350
µA with acceleration voltage of 50 kV. An electrostatic
quadrupole triplet, permanent magnet based analyzing
magnet, beam scanner etc have been designed &
developed in-house for the accelerator. The accelerator
components are controlled & monitored through a
programmable logic controller.
INTRODUCTION
Development of low energy accelerator was started in
IUAC with a concept of having stand alone tabletop type
machine requiring minimum components & utilities. Cold
cathode penning ion generator has been designed &
fabricated in-house by IUAC which can deliver gaseous
beams like, Alpha, Proton, hydrogen, argon, nitrogen etc
up to 350 µA. The source is assembled inside nylon
cylinder to provide isolation of 50 kV. The source
consists of a small permanent magnet, stainless steel
cylindrical anode, feed thru's, mild steel cathode body
with face plate, extractor etc. This has been tested for
maximum current of 400 µA. Stability test at 350 µA has
been carried out for 24 hrs.
An electrostatic quadrupole triplet has been designed
and fabricated for use in above accelerator. It has poles of
100, 185, 100 mm long with 56 mm radius. The
quadrupole has a total length of 421 mm with a 60 mm
aperture. It has been designed to use a maximum of 3 kV
for focusing of 50 keV beam at 450 mm distance.
An analyzing magnet has been assembled by using
pallets of permanent magnets to get a stable field of up to
2110 Gauss as required for accelerator. The poles of
magnet have been fabricated by using 25 mm thick mild
steel plates. Field shorting plates mounted on a common
plate having vertical movement arrangement using a
common hand wheel has been used to vary the magnetic
field for beam energy and mass selection. Field variation
of 900 to 2110 Gauss has been achieved.
Figure 1: IUAC's 50 kV Ion Accelerator
All parts of the accelerator have been assembled on a
common stand with vacuum system having a 400 lpm
turbo molecular pump, rotary pump, vacuum gauges
controllers etc. Penning ion generator in a grounded
extractor mode has been directly mounted onto the inlet
flange of the quadrupole triplet. A small experimental
chamber has been installed with an isolation valve after
the magnet. It has a faraday cup, suppressor, beam profile
viewer, target mounting ladder etc. The overall physical
size of the accelerator is 1600x1400x1500 mm and is
shown in Figure 1.
The accelerator has been satisfactorily tested for beam
energy range and currents as tabulated below.
Beam
H+
H2+
He+
He2+
Beam energy range and currents obtained
Energy (keV)
Field (Gauss) Current (µA)
23.4 - 46.8
891 - 1230
4.45
14 - 46.8
1007 - 1758
52
14 - 33.7
1398 - 2110
43.5
37.5 - 91.8
1130 - 1748
0.16
SUB SYSTEMS OF THE ACCELERATOR
Penning Ion Generator (PIG) Ion Source
A cold plasma based PIG ion source[1] is designed,
developed and used with IUAC's 50 kV ion accelerator.
The ion source is capable of delivering up to 350 μA of
stable current. In a penning ion generator the electrons
oscillate between two cathode electrodes inside
cylindrical anode. The plasma production is improved by
providing an axial magnetic field using a permanent
magnet having a surface field density of around 3.5 kG.
The ion source is assembled in a nylon housing and
connected to a 50 kV power supply. The required anode
voltage for the source is derived from the main 50 kV
supply. A copper tube of 1/8 inch diameter is tightened on
the cathode body with an adapter and connected to a
precision needle valve mounted on the high voltage
divider plate to supply the source gas to the source. The
complete assembled view of PIG ion source is shown in
Figure 2.
stable field having variation from 900 to 2110 Gauss has
been achieved.
Figure 4: Bending Magnet Assembly
Figure 2: Ion Source Assembly
Electrostatic Quadrupole Triplet
An electrostatic quadrupole triplet has been designed
and fabricated for use in the above accelerator. It has a
stainless steel vacuum jacket of 250 mm diameter with
nine 2.75” ConFlat ports for power connections and an
8” ConFlat port for mounting the turbo molecular pump.
The quadrupole elements are 100, 185 and 100 mm long
with a radius of 56 mm providing 60 mm aperture for the
beam. It is designed to focus a 50 keV beam at 450 mm
away from its exit port by applying up to 3 kV on the
poles. Glassman make 3 kV power supply modules are
used to power the quadrupole lens. These power supply
modules are controlled & monitored by a programmable
logic controller (PLC) through analogue input/output
modules. The assembled view of the internal components
of the quadrupole is shown in Figure 3.
Beam Scanner
Beam scanner for the accelerator has been developed to
get uniform implantation throughout the target surface.
Glassman power supply modules of 3 kV have been used
for the scanner. By applying voltages across a set of
parallel plates, the beam is made to scan the surface of the
target, thereby eliminating any non uniformity in the
dosage received in the target The power supplies are
controlled through PLC analogue outputs to get triangular
voltage on the scanner plates. Discharge resistors are used
across scanner plates to get symmetric triangular voltages.
The beam scanner assembly is shown in Figure 5.
Figure 5:Electrostatic Beam Scanner
Vacuum System
Figure 3: Quadrupole Triplet Inner Assembly
Permanent Magnet Based Bending Magnet
A bending magnet has been designed, fabricated &
assembled by using pallets of permanent magnet of size
25x50x12 mm having surface flux density of 3.5 kG. The
pallets are stacked between two 25 mm thick rectangular
MS plates acting as poles. Orientation of all the
permanent magnet pallets & stacks have been kept the
same. Four vertically moving field shorting plates have
been mounted onto a third MS plate for varying the field.
This third MS plate moves the field shorting vertical
plates between the magnet poles to vary the field that is
seen by the ion beam. A common hand wheel is used for
movement of the third plate holding the vertical shorting
elements to provide field continuous variability.
The magnet has beam pipe of cross section 60x75 mm
with a bending radius of 200 mm. Homogeneous &
One turbo molecular pump of 400 litres per minute is
mounted on the quadrupole triplet vacuum jacket with
backing of rotary pump to provide vacuum to the
accelerator. Vacuum in the range of 2.0x10-7 bar is
achieved by this pump.
ACCELERATOR PERFORMANCE
CURVES
The accelerator has been tested for various ion beams,
energies and currents. The current is varied by varying the
gas flow into the PIG source using a needle valve. The
beam energy is varied by varying the high voltage power
supply and simultaneously adjusting the magnetic field to
select the desired ion species.
Proton beam is extracted by injecting hydrogen gas into
the ion source. Figures 6 and 7 show the accelerator
output as a function of pressure at a constant energy and
as a function of beam energy at a constant pressure
respectively.
Figure 6: Current vs Pressure at a constant energy
Figure 10: Beam current vs time in hrs.
PROPOSED USES OF THIS
ACCELERATOR
Figure 7: Current vs beam energy at a constant pressure
Helium beam is extracted by injecting helium gas into
the ion source. Fig. 8 & 9 shows accelerator output as a
function of pressure at a constant energy and as a function
of beam energy at a constant pressure respectively for He+
ions.
Figure 8: Current vs Pressure at a constant energy
This ion accelerator has been specially designed &
developed for educational institutions. UG & PG students
and faculty will get a chance to leverage their existing
theoretical knowledge in Physics to explore:
 Ion source
 Bending magnet
 Analyzed-ion beams, Molecular beams &
Neutral beams at different energies
 Vacuum systems
 Beam transport
 Energy selection
 Beam Scanner
 Building an Accelerator etc.....
Students & faculty can conduct experiments using
alpha, proton and hydrogen beams up to 50 kV without
any radiation hazard for following.
 Positive ion implantation
 Ion induced fluorescence studies
 Particle detector experiments
 Ion scattering experiments
 Production of neutral atomic species in beam
form etc ….
ACKNOWLEDGMENT
We acknowledge with thanks the support provided by
D Kanjilal in the completion of this project.
REFERENCES
[1] Joshua L. Rovey, Brandon P. Ruzic, and Thomas J.
Houlahan, Rev. of Sci. Instr. 78 (2007) 106101.
Figure 9: Current vs beam energy at a constant pressure
ACCELERATOR STABILITY TESTS
Tests for beam current stability are carried out for
longer durations. Fig.10 shows stability test for 14 hrs. It
is observed that the current stability depends mainly on
the vacuum stability. It is clear from Figure 10 that the
vacuum stabilizes after 3-4 hrs from the start of the
machine and current also stabilizes with the vacuum and
remains very stable for longer times.