pseudospark - sourced electron beam for the generation of x-rays & thz radiation a.w. cross 1,...
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Pseudospark - sourced electron beam for the
generation of X-rays & THz radiation
A.W. Cross1, H. Yin1, D. Bowes1, W He1, K. Ronald1, A.D.R. Phelps1,
D. Li2 and X. Chen2
1Department of Physics, SUPA, University of Strathclyde, Glasgow, G4
0NG, UK2EECS, Queen Mary University of London, UK
UNIVERSITY OFSTRATHCLYDE
ABPAtoms, Beams & Plasmas
Compact Accelerator Workshop,18th April 2012, Cockcroft Institute, Warrington, UK
Contents• Description of pseudospark discharge (PSD)
• Pseudospark e-beam generation
- X-rays
- Coherent millimetre wave radiation
• Numerical simulation of PSD
• Experiments
- X-ray imaging
- Backward wave oscillator
- Millimetre wave klystron amplifier
• Conclusion & future work
• Spark, operating pressure (p) is large (~500 torr)- mean free path is very small and so there is very
frequent electron – neutral collisions- ionisation growth and large current density is due to
electron multiplication by electron-neutral collisions- gas breakdown is fast (ns) & discharge current is large- phenomena observed at large pressure on the RHS of
the Paschen curve• Pseudospark operating pressure is (typ 50 - 500 mtorr)- mean free path is larger than deff
- hole in centre results in spark like phenomena at lower (p)• Pseudospark is a low (p) gas
discharge that operates in a spark mode with hollow electrodes
J. Christiansen and C. Schultheiss, Z.. Phys., vol. A290, p. 35, 1979.
Generalised discharge characteristics
Introduction 1. What is a pseudospark?
A special hollow cathode discharge Low pressure (10-100 Pa, 70-700mtorr for
gap separation of several mm)
electron beam
anode
insulator
hollowcathode
deff
deff
electron beam
anode
insulator
hollowcathode
deff
(pd)min
pd [torr x cm]
VB [V]
0 2 4 6 8 10 12 14 16
200
400
600
800
1000
1200
1400vacuum breakdown
p seud osp ark reg io n
pseudosparkregion
(pd)min
pd [torr x cm]
VB [V]
0 2 4 6 8 10 12 14 16
200
400
600
800
1000
1200
1400vacuum breakdown
p seu d o sp ark reg io n
pseudosparkregion
Introduction1. What is a pseudospark? low pressure, 50 mtorr – 500 mtorr (for a gap separation
of several mm) self-sustained, transient hollow cathode discharge
occurs in special confining geometry in various gases such as nitrogen, argon, hydrogen,
xenon
Spark Pseudospark
Pressure range ~ 500 torr < 500 mtorr
Structure parallel plate parallel plate with axial hole (mm)
e-field uniform non-uniform and focused
Mean-free-path very small greater than d
Ionization e–neutral collisions field-enhanced thermionic emission,
vacuum arcs
Breakdown law f(pd) f(p2d)
Discharge occur RHS of Paschen curve
LHS of Paschen curve
Gas breakdown fast (ns) and large current rise (ns)
fast (ns) and large current rise (ns)
• 3 stages during a pseudospark discharge: a) Townsend dischargeb) Hollow cathode dischargec) Superdense glow discharge (conductive phase)
M. Stetter, P. Felsner, J. Christiansen, K. Frank, A. Gortler, G. Hunts et al, IEEE Trans Plasma Sci., vol. 23, no. 3, Special Issue on Pseudospark Physics and Applications, pp283-293, 2004
3. PS discharge can be scaled down in size (mm to mm) Intense electron beam
– point-like X-ray source– generation of coherent high power mm-wave radiation
1. Characteristics of a pseudospark Pseudospark is a low pressure gas discharge that
operates in a spark mode with hollow electrodes High quality electron beam extraction before and
during the conductive phase High current rise rate (dI/dt ~ 1011 Am-2 )
2. Applications Pulsed-power switching, can operate at high
PRFs Material processing
Time-dependent evolution of a pseudospark discharge simulated by a hybrid fluid-particle (Monte Carlo) model developed by J.P. Boeuf and L.C. Pitchford
A.W. Cross, H. Yin, W. He, K. Ronald, A.D.R. Phelps, L.C. Pitchford, Journal of Applied Physics, pp.1953-1956, 2007.
PSD Numerical Simulation
MAGIC: Particle-In-Cell and Monte-Carlo Collision (PIC-MCC)
C.K. Birdsall et al, Computer Phy. Comm 87, 1995.
PSD-2D Computational ModelMAGIC Model:Constant A-K voltage 10kV, gap d=6mmRadius of hollow cathode = 25mmRoom temperature
Insulator: 6mm thick PerspexAnode aperture: 0.5mm radiusAnode thickness: 12mm
Cathode aperture: 1.5mm radius
Argon 100mTorr
Plasma formation at 30ns
Plasma expansion at 50ns
Plasma expansion and emission at 80ns
Simulation results
Observed voltage between the anode and the cathode
Observed current at the anode aperture
PS beam experimental results
gas inlet
drift tubeto vacuumpump
hollowcathode
HV
volta
ge
pro
be
Cext
anode
Rogowski coil
insulator
P
Cathode, anode aperture diameter = 1 mmSeparation = 6 mm V = 10 kV
P = 100 mTorr I = 4 APlasma density 5 x1012cm-3
X-ray generation from a 4-gap PS discharge
gas in
voltag
e prob
e
vacuum gauge
drift tube
30M
extC
anode
to vacuum pump
cathod
ecav
ity
Rogowski coil
x ray
Rogowskibelt 1
detector
target
HV
Schematic of experimental setup
Experimental setup
Pseudospark e-beam and X-ray image
Object for X-ray image(100 micron diameter wire)
X-ray image of the objectMolybdenum target for X-ray
generation showing the beam spot
4-gap pseudospark e-beam experiments
gas inlet
drift tubehollowcathode
HV
volta
gepr
obe
Cext
anodeRogowski coil
insulator
P
collimatorwith micronaperture
camera
scintillator
vacu
umpu
mp
glass window
The cross-section image of the beam
3mm diameter anode aperture 500mm diameter anode aperture
500 mm
Quality of pseudospark-sourced electron beam pulses • High quality electron beam extraction before & during the conductive
phase - electron beam quality is decided by emittance, PS normalised rms
emittance of 18 mm mrad- brightness 1 x1011 A m-2 rad-2
Collimator
Acceleration gap
Pseudosparkchamber
Triggerelectrode
Rogowski coil
Electron beam
Voltage probe
Pumping system
Gas inAnode
Needlevalve
P
Pseudospark e-beam post-acceleration experiment
Trigger system for the pseudospark powered by a cable pulser
-400 -200 0 200 400
-40
-20
0
20
40
-100
0
100
Time [ns]
Voltage [kV] Beam current [A]
voltage at ps cathode
post-acceleration voltage
beam current
A typical record of the time-correlated pseudospark discharge voltage, beam current and the acceleration voltage pulse
Experimental setup of the PS powered by a cable pulser and beam-wave interaction investigation
BWO InteractionW-band (75 to 110)GHz
Ka-band(26.5 to 40)GHz
Advantages: a) e-beam source for THz radiation; b) simplicity (no B-field); c) compactness (table-top size);d) high power, high PRF operation
W-band Aluminium positive former - Under construction at the Univ of Strathclyde- Copper is deposited - Aluminium dissolved in alkali solution
G-band (140 to 220)GHz
H. Yin, A.W. Cross, W. He, A.D.R. Phelps, K. Ronald, D. Bowes and C.W. Robertson Physics of Plasmas, 16, 063105, 2009.
96 GHz Klystron
Pulse duration
50 ns
Vbeam 8 kV
Ibeam 15mA
Freq 96.8GHz
PIN 200mW
POUT 8.86 W
Gain 45
Efficiency
7.4 %
500 mm
100 mm
Design of the 96 GHz PS driven klystron
Construction of 96GHz Klystron
500 mm
Conclusion: • Electron beam generation and diagnostics from a 3-gap PS discharge
powered by a DC power supply. A beam was measured up to 300 A at 50kV and propagated as far as 20 cm away from the anode with no external guiding magnetic field
– Point-like X-ray source for imaging
• Beam-wave interactions were simulated with BWO structures in the W-band (75 to 110GHz) frequency range and with dielectric slow-wave
structure in Ka-band (26.5 to 40GHz)
– mm wave radiation was successfully generated in the Ka and W band
• High current conductive phase pseudospark beam from a 3-gap DC powered pseudospark was post accelerated
• Small-size beam (100mm) has been measured from both a 4-gap and single- gap DC powered pseudospark, to be used as an electron beam source for
– 200GHz BWO
– 96GHz Klystron
Future Work: • Re-entrant cavities for the klystron operating with a higher order mode will
enable the diameter of electron beam drift section to be increased
• Klystron multiplier operation at a higher harmonic will enable a lower frequency driver to be used to power the amplifier
– both these concepts will be of great benefit to industry
– result in scaling of the klystron to THz frequencies
Any questions?
Thank you for listening!
The authors would like to thank EPSRC for supporting this work
Configuration of the field-free collimator for the brightness measurement on the pseudospark
e-beam source
R
rdr
L
dW
max
2/124222
2
n 'xLRwhen;1;c/v;
R
IL2B
Normalised beam brightness
2n
2
2
I
Bn
Simulations on pseudospark e-beam post-acceleration Transport and post-acceleration of the beam were simulated by an
electromagnetic particle-in-cell (PIC) code MAGIC.
The effect of plasma densities was investigated
The influence of the presence of the plasma and its radial size
The effect of the shape of the electrodes was investigated
Magic PIC code simulations of beam propagation across the acceleration gap with infinite planar electrodes corresponding to different plasma densities
• Beam propagation with a plasma filling the channel
• Plasma density 6 x1012cm-3
• CP beam 200A, 200V
• Beam propagation, plasma filling the region up to the end of the cathode
aperture• Plasma density 1x1012cm-3
• HC beam 50A 22kV • yellow - plasma • red electrons
Beam 50A, 22kV
1E+10 1E+11 1E+12 1E+13 1E+14 1E+150
20
40
60
80
100
Plasma density (cm )
Beam current (A)
-3
beam oscillation
Beam 200A, 200V
3E+11 1E+12 3E+12 1E+13 3E+13 1E+14 3E+14 1E+150
100
200
300
400
Plasma density (cm )
Beam current (A)
-3
beam oscillation
Fig.4c Beam currentpassing through the anodevs. plasma density for beam50A, 22kV
Fig.4d Beam currentpassing through the anodevs. plasma density forbeam 200A, 200V
(2) For the beam propagation under different electrode shapes for the beam 50A, 22kV, the simulations show that: a) with no plasma in the accelerating gap, only the cathode shape has some effect on the beam transportation; b) in a plasma density of 5x1011 cm-3, both the cathode and anode shape has little effect on the beam transportation. Results of the simulations
The simulations show that the beams will propagate along the beam channel in
plasma of certain densities and comparable radial size. In plasma, the shapes of
both the cathode and anode have little effect on beam propagation.
Pseudospark e-beam further investigations
• Further study of pseudospark physics and its plasma process will enable
Potential future applications:
1) high power coherent sources of millimetre and sub-millimetre wave radiation 2) high brightness electron sources for post acceleration in the next generation of accelerators.
Electron beam current pulse vs the applied voltage pulse
from a cable pulser
-80
-60
-40
-20
0
20
40
60
80
100
120
-200 -100 0 100 200
Time (ns)
Current / A
Voltage / kV
Dispersion diagram of BWO interactiona0=1.875,d=1.75,h=0.375mm, 100kV
40
60
80
100
120
140
0 400 800 1200 1600 2000 2400 2800 3200 3600
Kz [1/m]
Fre
qu
ency
[G
Hz]
0
0.12
0.24
0.36
0.48
0.6
e-beam TE11 TM01 TM11
TM21 Magic (TM01) TM02 A2/A1
Particle-in-Cell code simulations of beam wave interaction
Particle-in-Cell code simulation of the beam wave interaction
Time-correlated electron beam pulse (green), microwave pulse (red)
and applied voltage pulse (blue)
W-band (75-110 GHz) BWO
-200
-150
-100
-50
0
50
100
150
200
250
-100 -50 0 50 100 150 200
Time (ns)
Vo
ltag
e (k
V)
-80
-60
-40
-20
0
20
40
60
80
100
Mic
row
ave
(mV
) C
urr
ent
(A)
Applied voltage Beam current Microwave pulse
1mm aperture single gap pseudospark beam measurements
gas inlet
drift tubeto vacuumpump
hollowcathode
HV
volta
ge
pro
be
Cext
anode
Rogowski coil
insulator
P
1mm Aperture, 2 Disk, 10kV
-2
0
2
4
6
8
10
12
-178 -58 62 182 302
Time (ns)
Vo
ltag
e (-
kV)
-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Bea
m C
urr
ent
(A)
Voltage
Beam Current
Four cavity klystron
Comparison of 2, 3 and 4 cavity klystron simulation
QCav.1
QCav.2
QCav.3
QCav.4
GaindB
h%
2 cavity Pin/rf=
100mW726
1600gap12=
10mm
---- ----7.3dB
Po=
0.55W
2.2
3 cavity Pin/rf=
100mW726
1600gap12 =
6mm
1600gap23=
3.3mm
----10.5Po=
1.14W
4.6
4 cavity Pin/rf=
25mW
7261600 gap12=
3.15mm
1600 gap23=
3.15mm
1600 gap34=
3.15mm
23dB Po=
5W
20
Four cavity klystron energy recovery system
Klystron interaction efficiency: 20%Recovery efficiency: 62% Total efficiency: ~40%
Work in Progress:
• More simulations
• Puffed gas feeding experiment
• Construction 200GHz microklystron driven by a pseudospark electron beam using
– Micro-electromechanical (MEMS) systems
Acknowledgements
The authors would like to thank the EPSRC for financial support of this work and the UK Faraday Partnership in high power microwaves for providing PIC MAGIC simulation package
References W Benker, J Christiansen, K Frank, H Gundel, W Hartmann, T Redel, and M Stetter,
IEEE Trans. Plasma Sci. 17, 754, 1989
H Yin, W He, G R M Robb, A D R Phelps, K Ronald, and A W Cross, Physical Review
Special Topics-Accelerators and Beams, 2, 020701, 1999
H Yin, A D R Phelps, W He, G R M Robb, K Ronald, P Aitken, B W J McNeil, A W
Cross, C G Whyte, Nuclear Instruments & Methods in Physics Research A, 407, 175,
1998
H Yin, G R M Robb, W He, A D R Phelps, A W Cross and K Ronald, Phys. Plasmas,
7, 5195, 2000
H Yin, W He, A W Cross, A D R Phelps, and K Ronald, J. Appl. Phys. 90, 3212,Oct.,
2001
H. Yin, A.W. Cross, A.D.R. Phelps, D. Zhu, W. He, and K. Ronald, J. Appl. Phys. 91,
5419, Apr., 2002
H. Yin, A.W. Cross, W. He, A.D.R. Phelps, and K. Ronald, The 2nd special edition of
IEEE Trans. Plasma Sci. Special issue on Pseudospark Phy. 32, 1, p233-239, 2004
H. Yin, A.W. Cross, W. He, A.D.R. Phelps, K. Ronald, D. Bowes, C.W. Robertson
Yhe Physics of Plasma, 2009 (in press)
Measured beam brightness from a 3-gappseudospark discharge
-100 -50 0 50 100 150 2001E+8
1E+9
1E+10
1E+11
1E+12
Time/ns
Normalized beam brightness [SI Unit]
conductive phase beam
hollow cathodephase beam
Brightness as a function of current density for various electron beam sources – the top right hand corner of the above diagram indicates the highest brightness combined with the highest current density
1.00E+06
1.00E+07
1.00E+08
1.00E+09
1.00E+10
1.00E+11
1.00E+12
1.00E+13
0 200 400 600 800 1000 1200
Current Density (A cm -2 )
Bri
gh
tnes
s (A
m-2
rad
-2)
Thermionic
Cathode
PhotocathodePseudospark
Cathode
Plasma FlareCathode
Beam current versus propagation distance
Electron beam can propagate as far as 20 cm
with no guiding B-field
0
200
400
600
800
0 10 20 30
Drift Distance [cm]
Bea
m C
urr
ent
[A]
010
203040
5060
Per
cen
tag
e o
f B
eam
Pro
pag
atio
n [
%]
Beam Current Percentage of Beam Propagation
H.Yin, IEEE Trans.Plasma Sci.,32, Special Issue on Pseudospark Physics and Applications, 2004
Experimental results
The propagation of the electron beam from a three-gap pseudospark discharge chamber was studied as a function of the length of a collimator of 3mm internal diameter. The beam was measured at 150 mm away from the anode of the pseudospark chamber. The results are shown in the following table.
Beam measured 150 mm away from the PS anodeCollimator length / mm Beam current / A Percentage of beam
transportedNo collimator 240 ±35 -
30 168 ± 20 70 %60 118 ±10 49 %90 36 ± 5 15 %
1 - Voltage probe7 - Collimator
2 - Hollow cathode8 - Rogowski coil
3 - Intermediate electrode9 - Gas inlet
4 - Insulator10 - Solenoid
5 - Anode11 - Dielectric liner
6 - CVR12 - Waveguide and horn
12
HV
1
2
3
4 56
7 8
9
10
118
Pseudospark-based Cherenkov maser experimental configuration
Pseudospark e-beam in Cherenkov interaction
• Operating voltage can be increased by using multi-gaps
-200 -100 0 100 200 300-20
0
20
40
60
80
-100
0
100
200
300
400
Time [ns]
Voltage [kV] Beam current [A]
Voltage
Beam currenthollow cathodephase
conductivephase
A typical beam pulse from a 8-gap pseudospark discharge
Voltage / kV Current / Amps
Time / ns
500 510 520 530 540 550
0
20
40
60
80
Time [ns]
Impedance
hollow cathode discharge
pseudosparkdischarge
[W]
Impedance of the pseudosparkdischarge chamber
Impedance / W
500 1000 1500 2000 2500 3000
10
20
30
40
50
60
70
k [1/m]
TM 02
TM 01
Slow space charge wave
z
The dispersion diagrams of the dielectric-lined waveguide modes TM01, TM02 and the slow space-charge wave of the pseudospark-based electron beam with parameters of 75 kV and 10 A
Frequency [GHz]
Axial wavenumber [1/m]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.710 0
101
10 2
10 3
10 4
z [ m ]
Power [ W ]
TM 01
TM 02
predicted power
measured power
Predicted output power of the TM01 andTM02 modes as a function of z
Discharge voltage
Beam current
MicrowaveOutput
Time (20ns/div)
conductivephase
hollow cathodedischarge
Typical beam current, voltage and microwave traces from the 8-gap pseudospark-based
Cherenkov maser experiments
500 1,000 1,500 2,000 2,500 10 20 30 40 50 60
kz [1/m]
f [GHz]
75kV, 20A beam
44 kV, 20 A beam TM01
MAGIC simulation
Dispersion diagram of TM01 mode calculated (dotted line) and
simulated by MAGIC code (squares) and slow space charge mode of the electron beams (75 kV,
25 A-top thin line)
Ka-band (26.5-40GHz) Cherenkov
-150-100-50
050
100150
-150 -50 50 150 250 350
Time (ns)
Vo
ltag
e (k
V),
C
urr
ent
(A)
-150-100-50050100150
Mic
row
ave
(mv)
Voltage Beam current Microw ave
Measured e-beam voltage,current and mm-wave pulse
-30 -20 -10 0 10 20 300
0.2
0.4
0.6
0.8
1
1.2
Theta (degree)
Normalized microwave output (Er component)
bench
calibration
experiment
calculation
Far field radial mode pattern scan result of the radial componentEr in the Cherenkov microwave electric field compared with the
calculation and bench calibration
25-28.6 GHz
28.6-32.2 GHz 36.4-41.8 GHz
41.8 GHz and up
63.6%
6.2%21.9%
8.2%
The Cherenkov maser results:
¨ Beam parameters: 10 A, 70 – 80 kV
¨ Mode: TM01
¨ Frequency: 25.5 – 28.6 GHz
¨ Output Power: 2 0.2 kW
¨ Gain: 29 ± 3 dB
±
Typical beam current, voltage and microwave traces from the 8-gap pseudospark-based
Cherenkov maser experiments
Discharge voltage (20kV/div) Beam current (20A/div)
Enlarged beam current
Microwave
Time (20ns/div)
conductivephase
hollow cathodephase
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