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Abstract—This paper presents a new radio frequency (RF)
excited plasma-cathode electron beam (EB) gun design and
experimental results at a frequency of 84 MHz. The design offers
the following benefits over thermionic-cathode triode EB guns:
unlimited cathode lifetime and as a result reduced maintenance
costs; no requirement for a grid electrode, avoiding beam
aberration; and rapid beam pulsing. The construction of the
diode gun was completed and the results of this work
demonstrate that the electron beam can be switched on in 200 ns
and off within 800 ns. Electrons were extracted from a 2-3 mm
plasma chamber and then accelerated by an electric field applied
in a vacuum chamber at 10
-5
to 10
-6
mbar producing a collimated
electron beam. The ionized gas used was nitrogen at a 0.5 to 1 bar
pressure. The EB gun has been operated at –60 kV accelerating
potential and has produced beams of up to 3.2 kW power
continuous wave (CW). Modulation of the RF signal was used to
control the beam power. Details of the design features of the
plasma device are given and evidence of the advantages over
conventional EB guns is provided through empirical results.
Index Terms—Amplitude modulation, electron sources, plasma
devices, pulsed power systems, vacuum electronics.
I. INTRODUCTION
HERMIONIC
cathode electron beam (EB) guns are a
widely used electron gun type for vacuum electron
devices (such as klystrons, gyrotrons, betatrons and free
electron lasers), x-ray generators and material processing
equipment (e.g. welding and melting) [1]. The emitter is
usually a metal cathode that is heated up until the electrons
gain sufficient energy to leave the surface, which are then
accelerated by a high gradient electric field to form a beam.
However, cathode lifetime is limited due to material
evaporation [2, 3] and erosion such that frequent maintenance
is required. The beam current from diode electron guns can
only be changed slowly (i.e. of the order 100 ms) due to the
thermal inertia of the cathode. Consequently, grids are often
used to control beam current more rapidly, but the grid
electrode in an EB triode gun introduces beam aberration [3,
Manuscript received October 30, 2013.
This work was supported in part by TWI Ltd.
David R. Smith, is with the Electronic & Computer Engineering
Department, Brunel University, Uxbridge, UB8 3PH, UK. (e-mail:
david.smith@brunel.ac.uk
).
Colin Ribton is with the Electron Beam Processes Section at TWI Ltd,
Granta Park, Great Abington, Cambridge, CB21 6AL, UK. (e-mail:
colin.ribton@twi.co.uk
).
Sofia del Pozo is with Brunel University and TWI Ltd. (e-mail:
sofia.delpozo@affiliate.twi.co.uk
).
Fig. 1. Photograph showing interior of the plasma EB gun vacuum housing: a
RF signal is applied to a first antenna 1, and this induces a current in the
second antenna 2, which resonates with a parallel capacitor 6. The resonant
voltage is applied across the plasma chamber. The gas fed into the chamber is
controlled by a needle valve 3. The gun is mounted on a high voltage insulator
5 at -60 kV. The free electrons are extracted and accelerated towards the
anode 4 (at 0 V potential).
4], which for some electron devices may affect the beam
characteristics and device operation. Rapid pulsing of the
beam in an EB triode gun requires complex and expensive
electronic control of the grid electrode voltage and, for
materials processing, pulsing at transition times below 1 ms is
not generally available.
N. Rempe et al. [5] have presented a plasma cathode EB gun
using a DC signal for the plasma excitation. The gun operates
at up to –60 kV accelerating voltage and 12 kW power.
However, rapid beam pulsing is not practicable due to the
requirement for the cable and plasma chamber capacitances to
be charged and discharged, which necessitates a high
frequency response, high current drive, elevated at the
accelerating voltage.
This paper presents work on a plasma-cathode [6-8] gun
design resonating at a frequency of 84 MHz and capable of
producing beams of up to 3.2 kW power constant wave (CW).
Electrons are extracted from a plasma source instead of an
emissive material surface allowing electron beam parameters
to be stable over a long time [6]. Plasma-cathode guns have
much longer lives than guns with standard cathode assemblies
[9]. The plasma-cathode provides solutions to the main
problems encountered in thermionic-cathode EB guns [vide
supra]. Fig. 1 is a photograph of the EB gun apparatus
developed.
A Novel RF Excited Plasma Cathode Electron
Beam Gun Design
Sofia del Pozo, Graduate Member, IEEE, Colin N. Ribton, and David R. Smith
T
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4
6
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Key features of the plasma electron source design are given
in Section II. Section III describes the methodology of the
work. Section IV gives a summary of the results obtained in
switching tests at -25 kV accelerating voltage and 84 MHz
resonant frequency, and the plasma electron source is
compared to other electron sources. Section V contains
conclusions and future work.
II. D
ESIGN CONCEPT
The plasma EB gun consists of a plasma chamber used as a
cathode or electron source, an RF plasma generation unit, and
a particle-accelerating unit for extracting the electrons from
the plasma chamber and accelerating them to form a beam.
Fig. 2. Circuit schematic of the plasma gun showing electrical circuit and
electrodes [10].
Fig. 3. Schematic cross-section of the EB gun design showing an electron
beam (P) extracted from the plasma chamber (1) and accelerated towards a
work piece (W) [10].
Fig. 2 shows a circuit schematic of the plasma cathode EB
gun and Fig. 3 is a schematic cross section of it, both of the
figures comprising [10]: a plasma chamber (1) provided with
an inlet (2) for the controlled ingress of gas (G), a hollow
electrode (3) and a diaphragm (4) with an aperture for the
extraction of electrons from it; a radiofrequency (RF) plasma
generation unit consisting of an RF power supply (5) (up to
50W) inductively coupled to the plasma chamber by a pair of
resonant circuits (RC) (RC1 is the transmitter and RC2 is the
receiver) tuned to resonate at the same resonant frequency; an
electron accelerating unit consisting of a DC power source (6)
applying a high voltage between the plasma chamber and an
accelerating electrode (7) on lines (ii) and (iii) respectively.
The plasma chamber, accelerating unit and the resonant
circuits are within a housing (8) at a pressure of 10
-5
mbar, in
order to maintain electrical isolation between the electrodes.
However, the pressure inside the plasma chamber is higher
(0.5 to 1 mbar). A needle valve controls the ingress of a steady
low flow of gas into the chamber. Since the plasma chamber is
connected to the gun housing directly through the diaphragm
hole, the diameter of this aperture plays an important role in
determining the pressure in the gun housing. Quantity flow
rates through the inlet and the aperture are therefore preferably
arranged such that the chamber pressure remains substantially
constant [10]. The gun housing is constantly being evacuated
by a turbo molecular pump (9) and a backing pump to keep it
at the right vacuum level.
The resonant circuits are configured as parallel LC circuits.
Both the first antenna (10) and the second antenna (11) are
single turn inductors. The quality factor of the coupled
resonant circuits is high enough (at least 500) so that the
voltage of the induced RF signal is substantially higher than
the output voltage on RC1. The values of the parallel
capacitors were chosen so that the system is at its resonance
frequency. This capacitance can be changed by using the
tuning mushroom in the gun.
The plasma chamber contributes to the capacitance of the
second resonant circuit. When a plasma is excited within the
chamber, the loading of the secondary circuit will increase and
this will lower the circuit’s Q factor, lowering the plasma
voltage but making more current available for sustaining the
plasma. [10]
Fig. 4. Half-section of the EB gun device showing RC2, plasma chamber and
-60 kV electrode.
Fig. 4 illustrates a half-section of the EB gun body showing
RC2, the plasma chamber and the –60 kV electrode. Different
materials were used for the diaphragm and the plasma
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chamber in order to get higher density of secondary electrons.
The upper wall of the chamber and the diaphragm are formed
of conductive material, and the RF voltage is applied between
them. The side walls between them is formed of an insulating
material (e.g. boron nitride or alumina ceramic).
III. M
ETHODOLOGY
Several configurations of EB gun have been investigated. The
work presented here used a configuration illustrated in Fig 5
with gun parameters given in Table 1.
Several gases including air, neon and nitrogen, were tested
for the generation of the plasma. Inert gases were favored so
as to avoid chemical reaction with the metal parts of the
plasma chamber. Noble gases were found to be readily
ionized, but it was important to contain the plasma within the
plasma chamber as ionization in the electron accelerator
region could lead to high voltage breakdown between the EB
gun and the anode [11, 12].
Fig. 5. –25 kV test set up, showing Faraday cup and resistor to measure the
beam current extracted from the plasma.
The ability of the EB gun to produce a rapidly pulsed beam
was tested using the configuration shown in Fig. 2. A Faraday
cup was used to collect the beam current. Fig. 5 illustrates the
test set up. The RF excitation of the plasma was modulated,
the envelope of the RF being compared with the beam current
generated by the gun.
IV. E
XPERIMENTAL RESULTS
A. Test at –25 kV accelerating voltage and 84 MHz resonant
frequency.
Fig. 6. Test results at –25 kV acceleration voltage, showing input RF voltage
applied to RC1 and beam current from the plasma.
Fig. 7. The electron beam is switched on in 200 ns Acceleration voltage in the
test is –25 kV and the resonant frequency is 84 MHz.
Fig. 8. The electron beam is switched off in 800 ns by modulation of the RF
signal.
Figs. 6, 7 and 8 present the results obtained from the EB gun
in a test at –25 kV accelerating voltage, 5×10
-5
mbar gun
TABLE I
VALUES OF THE MAIN PARAMETERS INVOLVED IN THE PLASMA-CATHODE EB
GUN OPERATION
RF output power amplitude
up to 50 W
Resonant frequency
84 MHz
Induced RF signal
1-10 kV
Diaphragm aperture diameter
0.1 -1 mm
Plasma chamber diameter
<12 mm (2-3 mm)
Accelerating voltage
25 kV–60 kV
Plasma chamber pressure
0.1 mbar to 1 mbar
Gas fed pressure
1 bar
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housing pressure, and 0.5 mbar plasma chamber pressure.
First, a beam current of more than 3 mA was extracted from
the plasma. Second, the results illustrate that the beam current
can be controlled by modulation of the RF signal. Fig. 7 shows
that the beam current is switched on in 200 ns using RF
amplitude modulation. Fig. 8 shows that the beam current is
switched off in 800 ns. The minimum time to switch off is
limited by the time for the plasma to extinguish and by the
frequency of the RF signal [10].
B. Comparison of the RF excited plasma-cathode with
conventional electron sources.
By using RF excitation to generate the plasma electron
source, the plasma parameters can be rapidly changed
compared to conventional control of diode EB guns, and
materials processing triode guns. This leads to quicker and
more accurate control of the beam parameters, not only over
DC plasma-source guns, but also over thermionic-cathode
guns. In some applications the beam is required to be switched
on and off quickly and this would be feasible in less than 1
microsecond in an easy and inexpensive way. There is no
waiting time for a metal filament to cool down, as in
thermionic emitters; and no capacitance to be discharged as in
DC excited plasma-source guns. As a result, the welding cycle
is shortened [9]. An RF off-the-shell power supply can be used
since 50 to 100 W are needed. The use of an RF signal would
offer additional benefits such as beam power control by
amplitude modulation and average beam power control by
pulse-width modulation.
However, minimum beam pulse duration is determined by
the plasma lifetime. At atmospheric pressure this is around 20
ns since the ionized species are recombined with ambient gas
molecules. At low pressure this time can be several
milliseconds. The plasma chamber diameter has been kept
small, so that the ions collide against the walls and recombine
more quickly, leading to the plasma extinction when the
excitation has been stopped. Preferably, the plasma chamber
dimensions are selected to be less than the average path length
of the ions in the plasma, in order to ensure that the plasma
lifetime is short after the RF excitation is stopped.
As a result, there is no grid electrode needed to control the
beam current, and the gun can be implemented as a diode,
producing high integrity particle beams. In addition, the power
supply and control units are simpler and less expensive than
for conventional EB guns. This may contribute to a reduction
in size of the equipment. Since cathode is not damaged due to
particle bombardment as with metallic filaments, it might be
possible to operate the device at higher pressures, and thus to
have a simpler pumping system for evacuating the EB gun
chamber.
In thermionic cathodes, filament thickness and surface
properties change during the cathode lifetime. In addition,
when the filament is replaced differences in the beam quality
and the device performance can become apparent. Using a
plasma cathode as the electron source avoids these problems
and provides an unlimited cathode life that has no wear giving
a repeatable and uniform performance, which is very
important in material processing [1].
V. C
ONCLUSION
A plasma cathode EB gun with a number of novel features
has been presented. Since there is no filament to wear out,
maintenance and beam distortion problems have been
substantially reduced compared with thermionic electron guns.
Modulation of the RF power can be used to control the beam
intensity, such that a grid electrode is not required, avoiding
beam aberration. Moreover, the plasma cathode EB gun can
readily produce rapidly modulated beam powers utilizing
much less complex power supply systems.
Applications of the RF plasma EB gun are cutting, welding,
curing, melting, additive layer manufacturing, drilling, and gas
treatment, among others.
A need to understand the plasma that is generated in the
apparatus was identified and work on the plasma diagnosis is
ongoing. A separate setup for the study of the RF generated
plasma was developed.
R
EFERENCES
[1] H. Schultz, Electron Beam Welding. Abington Publishing, Cambridge,
UK, 1993.
[2] O. Richardson, Science, 38(967), pp. 57-61.
[3] G Fiksel et al., Plasma Sources Sci. Technol., 5, pp.78-83, 1996.
[4] J. Pierce, Theory and design of electron beams, 2nd ed. New York,
1954.
[5] N. Rempe et al., Welding and Cutting, 11(2), 2012.
[6] S.I. Belyuk et al., Russian Physics Journal, 44(9), pp. 987-995, 2001.
[7] A.S. Bugaev et al., Hardware Laser and Particle Beams, 21, p. 139,
2003.
[8] S. Yu. Kornilov et al., Instrum. Exper. Tech., 52(3), pp. 406, 2009.
[9] S.I. Belyuk et al., “Electron-beam welding guns with plasma cathodes,”
Avt. Svarka, no. 1, pp. 49-50, 1974.
[10] C.N. Ribton et al., “Plasma source apparatus and method for generating
charged particle beams”, UK Patent Application, Patent GB1210607.6,
2012.
[11] E.M. Oks, Plasma Sources Sci. Technol. 1, 249, 1992.
[12] S. Humphries, Charge Particle Beams, p.321, 1990.
[13] Tuohimaa T, Liquid-Jet-Target Microfocus X-ray Sources: Electron
Guns, Optics and Phase-Contrast Imaging. Royal Institute of
Technology. Stockholm, Sweden, 2008.
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Sofia del Pozo (GSMIEEE’12) was
born in Toledo, Spain, in 1990. She
received the B. Eng. degree with honors
in industrial engineering in electronics
from Castilla-La Mancha University,
Toledo, Spain, in 2011, and the M. Sc.
degree in advanced engineering design
from Brunel University, London, UK, in
2012.
She is currently pursuing the Ph.D. degree in electronic and
computer engineering with Brunel University and she is based
for her research at The Welding Institute (TWI) Ltd,
Cambridge, UK. She has been working on the design,
development and testing of a thermionic-cathode electron
beam gun for the treatment of marine engine emissions as part
of the DEECON project (FP7 programme). Her research
interests include electron beam gun design, plasmas as
electron sources, FE analysis, plasma diagnosis, and
spectroscopy.
Ms. del Pozo is a student member of the Institute of Physics
(AMInstP) and the Welding Institute (AWeldI), and she was
awarded the ‘Alumni Prize for Advanced Engineering Design’
for her Masters project by Brunel University in 2013.
Colin N. Ribton graduated from The
University of Nottingham in 1984 with
a joint honors degree in pure and
applied physics. He joined TWI’s
Electron Beam group in 1985. In 2001
he left TWI to join a company
developing novel antenna technologies,
where he became Vice President of
Application Engineering. He re-joined
TWI in 2003.
His roles at TWI have involved him in the computer
modeling of electron optics and high voltage components, the
design of high voltage power supplies, the design and
optimization of radiation shielding, real-time control system
architecture, and the design of digital and analogue
electronics. In particular, this has been involved in the
development of equipment and processes to manufacture
major components in power generation, nuclear, aerospace
and medical applications. He is presently a Technology
Consultant in the EB group where he is active in promoting
EB technology for new applications.
Mr. Ribton is a Chartered Physicist (CPhys), a Chartered
Engineer (CEng), a Member of the Institute of Physics
(MInstP), and a Member of the Welding Institute (MWeldI).
David R. Smith, received the
M.Phys. honours degree in
physics with space science and
systems from the University of
Kent, Canterbury, Kent, U.K., in
2000, and the Ph.D. degree from
the University of Leicester,
Leicester, U. K., in 2004. His
Ph.D. thesis was entitled
“Radiation damage in charge-
coupled devices.”
He has continued to do research in the field of radiation
dosimetry and radiation damage in silicon devices and is
currently a Senior Lecturer in the School of Engineering and
Design at Brunel University, Uxbridge, U.K. He has published
a number of papers on the effects of proton damage on CCD
operation, in particular the generation and characteristics of
bright pixels exhibiting “random telegraph signal” behaviour,
and the effects of radiation damage on the operational
characteristics of electron-multiplication CCDs.
Dr. Smith is a Member of the Institute of Physics, a Fellow
of the British Interplanetary Society and a Fellow of the
Higher Education Academy.
