Download Super-emissive cathode switches - Electrobionics

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 plasmacathode 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:
Size
- Advanced university test capabilities TTU
Comparison
- Liquid breakdown & switching experience TTU
- Photoconductive, bulk III-V switching UM, TTU
- Super-emissive cathode switching USC
BLT
- III-V junction pulsed power switching UM, USC
175
- Advanced III-V materials infrastructure USC
4 2
BL T
1 7 5
High Power Thyratron
9 5
Compact Pulse Power Photo-Switches
Univ. of Missouri (Columbia)
Bulk Cu:Si:GaAs Photo-Switches
Optical Waveguide
Electrode
Semiconductor Material
Optical Clos ure
Energy
Electrode
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

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
 Payoff:
 Improved lifetime
 Higher current capability
 Optimum High voltage, high
current switch
 Switching capability
 1 GW/cm3 of material
Semiconductor Switch Simulations
Texas 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
•
Photoconductive
Semiconductor
Switch
Payoff
– High-power solid state
switches
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 Nitrogen
Texas 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
InstaSpec Camera
Torr
Photodiode
Over Pressure
Safety
Liquid N2
Voltage
Level
Monitor
0.1
V/A
0.1
V/A
0.2
V/A
Vacuum
Pump
Dielectric sample
submerged in LN2.
Early flashovers are across
center (middle).
After conditioning, discharge
occurs at outer edge
(bottom).
OptoElectronic III-V Switches: The “SIT”
University of Southern California
Pitch
•
•
•
•
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.
V
V
GS
R
GS
Ga te
j
+
n -Ga As
 -GaAs
Lg s
L
p +- GaAs
n+-Ga As
sd
Dra in
GaAs SIT (Static Induction Thyristor).
Recessed gate configuration.
L
V
G
DS
SITN
Gate
Source
+ _
Gate
V
R
GSN
G
Drain
SIT2
V
Opti acl stack
of SITs with
simp el LED
trigger
R
GS2
Integrated
OptoElectronic SIT
RD
G
LASER/LED
SIT
SIT1
V
R
GS1
Photons
G
GROUND
Optical trigger for SIT stack
LT-MBE Ga As
AlAs
Source
Ga te
Ga te
L ss
A
R
-V
x
Silico n Nitride
Source
Source
Source
p+

+
n
R
V
GS
G
Super-Emissive Cathode Switches
“BLT” & “Pseudospark”
University of Southern California
• 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
Size
Comparison
42
BL T
1 7 5
95
BLT
175
High Power Thyratron
Comparison of Thyratrons to BLT
Model
P (W)
Standby
Wgt (gr) I (kA)
Reservoir
1802
110
20
2
HY 5
10
190
4.5
50
5-
HY 7
BLT175
Dia. (“)
4
3 mm
electrode
separation
HOLLOW ANODE
HOLLOW
CATHODE
1660
400
40
7
2
2
40
•
1.75
FLASHLAMP
for triggering
USC Pseudospark and BLT Switches:
Comparison with Thyratron
Low pressure (0.1-0.5 torr)
10's of kV, ~2-100 kA
Hydrogen
Thyratron
Paschen Curve
U
bd
BLT, thyratron
~200 V
III
(pressure x d)
Anode-grid
separation
3 mm for
high hold-off
Mo Anode
Insulator
Mo Grid
Grid,g rounded
Cathode
C athode
shi eld
(heated
C athode
thermionic)
C athode
Spark
Glow
II
~1 Pa*cm
I
spark gap
dense glow
Vacuum b.d.
HV glow
X
Anode
Reservoir
Mo Anode
IV
pl asma
p*d
High Voltage Hold-off Mechanism
in sula tor
Back-lighted thyratron,
Pseudospark
Anode-cathode separation
3 mm for high hold-off
Mo Cathode
Extremely Fast Transition from Hollow
Cathode Emission to Super-Emission
Transition from “non-explosive” to
“explosive” occurs nearly
2
cr
instantaneously, when

2

E

ne necr  0  c
ne satisfies -->
eUc  
108
107
Delay time of explosion
of cathodic microprotrusions versus
plasma density
(tungsten, 10 kV).
106
105
4
e
dt
Delay changes from seconds to
nanoseconds when ne changes by ~ 2
For Tungsten --> n cr  51013 cm -3
(ns)
10
103
2
10
101
100
10 -1
0
0.2
0.4
0.6
ne (10
0.8
19
1
1.2
1.4
-3
m )
"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)
Pseudospark Pulse Generator
Primary pulse
30 kV
60 ns FWHM
Secondary pulse into load
•
•
•
•
•
Used for corona assisted ignition
70 kV peak amplitude
1 Hz repetition rate
50 ns pulse width
Long life
Work in progress
53 kV
200 A