MURI CONSORTIUM on

COMPACT, PORTABLE PULSED POWER

Consortium Team Members:

University of Southern California, Martin Gundersen, P.I.

University of Missouri-Columbia, William Nunnally

Texas Tech University, James C. Dickens, Andreas A. Neuber, and

Hermann Krompholz

Research Concentration Areas:

- III-V photoconductive and junction switching devices

- Super-emissive cathode switches

- Liquid breakdown for high voltage switching and energy storage

Purpose and Goals of the USC-Texas-Missouri MURI Consortium

To explore new methodologies for III-V and other device switching leading

to true optical hybrid architectures w/ vastly reduced size/weight.

To study super-emissive gas phase switching, and liquid switching

to advance understanding of underlying physics (such as the plasma-cathode interaction that enable super-emissive switches)

To apply the recent advances in optoelectronics and in electronic device design, growth, & performance to key components necessary for future compact, repetitive, portable pulsed power.

The USC-TTU-UM MURI team offers:

- Advanced university test capabilities TTU

- Liquid breakdown & switching experience TTU

- Photoconductive, bulk III-V switching UM, TTU

- Super-emissive cathode switching USC

- III-V junction pulsed power switching UM, USC

- Advanced III-V materials infrastructure USC

Size Comparison

BLT175

High Power Thyratron

42

95B L T1 7 5

Compact Pulse Power Photo-SwitchesUniv. of Missouri (Columbia)

Payoff: Improved lifetime Higher current capability Optimum High voltage, high

current switch Switching capability

1 GW/cm3 of material

Approach: Linear Photo-switch Increase optical absorption depth

by using long wavelength & interband doping

Reduce current density in GaAs & increase max current

Increase holdoff voltage by using multiple, stacked wafers &

conducting layers

Reduce optical closure energy Opportunity:

Picosecond closure, jitter

High Voltage, high current potential

Limited lifetime due to large current density in bulk, contacts

Current density limited by optical depth

Optical Waveguide

Optical ClosureEnergy

Electrode

Electrode

Semiconductor Material

Bulk Cu:Si:GaAs Photo-Switches

Semiconductor Switch SimulationsTexas Tech University

• Research Goals– Understand the behavior of

photoconductive switches (eg- GaAs) at 4

to 30 kV/cm – Computational studies of

breakdown and “lock-on”

• Approach – Collective impact ionization theory – Ensemble Monte Carlo simulations

• Personnel– Prof. Charles W. Myles, Physics– Ken Kambour, PhD Student

• Payoff– High-power solid state

switches

Photoconductive Semiconductor

Switch

GaAs phonon cooling rate vs. carrier temperature.

Energy balance must occur in steady state. Thus, the Joule heating rate (dashed) must equal the phonon cooling rate (solid). However, the carrier temperature corresponds to a density which is too low to sustain a filament. Thus, the quasi-equilibrium assumption is not valid.

Breakdown in Liquid NitrogenTexas Tech University

• New lab apparatus will examine breakdown voltages of 200 kV.

• Focus: phenomenological picture of surface flashover and volume breakdown

• Evaluate LN2 as isolating material in cryogenic compact PP devices.

• Possible use of LN2 as switching medium

Dielectric sample submerged in LN2.

Early flashovers are across center (middle).

After conditioning, discharge occurs at outer edge (bottom).

Photodiode

InstaSpec Camera

Level Monitor

Liquid N2

0.1 V/A

0.2 V/A

0.1 V/A

Voltage

Torr

Over PressureSafety

VacuumPump

OptoElectronic III-V Switches: The “SIT”University of Southern California

• The USC-SIT is a vertical GaAs FET• Advantageous mobility & band gap

make it a candidate for high speed & high hold-off voltage switching

• Can be fabricated in optically gated stacks to simplify triggering

• Will also examine II-VI, and other III-V’s.

Optical trigger for SIT stack

R G

RG

RG

VGSN

VGS2

VGS1

GROUND

VA

R L

LASER/LED

SIT1

SIT2

SITN

Gate Gate

Drain

Source

+ _

V GS

R G-V

GS

Optical stackof SITs withsimple LEDtrigger p

νn

V

V

DS

GS

Photons

R

RG

D

+

+

SI T

ν -GaAs

n -GaAs+

Pitch

xj

Lgs

L ss

Source

Gate

Drain

Source Source

Gate Gate

Source

Lsd

n -GaAs+

p - GaAs+

Silicon NitrideLT-MBE GaAs

AlAs

GaAs SIT (Static Induction Thyristor).Recessed gate configuration.

Integrated OptoElectronic SIT

Super-Emissive Cathode Switches“BLT” & “Pseudospark”

• Lower required power & parts-count make BLT attractive for “portable’ app’s

• Super-emissive cathode– 10,000 A/cm2, over 1cm2

• Stand-off voltage higher than thyratron’s • Very high rate of current rise (>1011 A/sec)• 100-kV forward voltage, 25 to >100kA peak

current, 1250-MW peak output power

Comparison of Thyratrons to BLT

Size Comparison

BLT175

High Power Thyratron

42

95B L T1 7 5

Model P (W) Wgt (gr) I (kA) Dia. (“)

1802 110 20 2 4

HY 5 190 50 5-104.5

HY 7 1660 400 40 7

BLT175 2 2 40�1.75

Standby Reservoir HOLLOW ANODE

HOLLOW CATHODE

FLASHLAMPfor triggering

3 mm electrodeseparation

University of Southern California

USC Pseudospark and BLT Switches:Comparison with Thyratron

Low pressure ( 0.1-0.5 torr)10's of kV, ~2-100 kA

Paschen Curve

X

BLT, thyratron

(pressure x d)

spark gap

High Voltage Hold-off Mechanism

Anode

Grid,grounded

Cathode

Insulator

Cathode

shield

Cathode Reservoir

Hydrogen Thyratron

Anode-grid separation3 mm for

high hold-off

Mo Anode

Mo Grid

Cathode (heatedthermionic)

Back-lighted thyratron,PseudosparkAnode-cathode separation3 mm for high hold-off

insulatorplasma

Mo Anode

Mo Cathode

Transition from “non-explosive” to “explosive” occurs nearly instantaneously, when

ne satisfies -->

Delay changes from seconds to nanoseconds when ne changes by ~ 2 For Tungsten --> -313 cm 105×≈cr

en

ne≥necr=

2ε0eUc

Eccr

β ⎛ ⎝ ⎜ ⎞

⎠

2Delay time of explosion

of cathodic micro-protrusions versus

plasma density (tungsten, 10 kV).

"Model for explosive electron emission in a pseudospark

superdense glow” A. Anders, S. Anders and M. A. Gundersen, Phys. Rev. Lett. 71 (3), 364 (1993). "On electron emission from pseudospark cathodes", A. Anders, S. Anders and M. A. Gundersen, J. Appl. Phys. (1984)

Hollow-CathodeEmission

BLT Switch

HollowCathoderegion

Anode

3 mm

plasma

cathode

Super-EmissionTransition

Transition from BLT Hollow Cathode mode (center) to Super-emissive mode.Hollow cathode plasma results in a virtual anode in close proximity to cathode.

Extremely Fast Transition from Hollow Cathode Emission to Super-Emission

Pseudospark Pulse Generator

• Used for corona assisted ignition• 70 kV peak amplitude• 1 Hz repetition rate• 50 ns pulse width• Long life

Primary pulse

30 kV 60 ns FWHM

Secondary pulse into load

200 A53 kV

Work in progress