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