icops 2011 june
TRANSCRIPT
MAGNESIUM BASED PHOTOCATHODES FOR BACK-LIGHTED THYRATRON
Esin B. SozerDr. Chunqi Jiang
Dr. Martin A . Gundersen
ICOPS Chicago, IL July 2011
Outline
Pseudospark switches and back-lighted thyratrons
(BLTs)
A look at previously used cathodes for BLTs
Experimental setup and procedures for Mg
integrated BLT
Results
Discussion
Summary
Introduction: Pseudospark Switches
• Low pressure (<1 Torr), glow-like discharge operation
• Cold cathode
• High voltage (typ. 30 kV)
• High peak current (1-100 kA)
• High current rate of rise (1012A/s)
• Reasonable lifetime (~109 shots) and repetition rate (1 kHz)
Optically triggered pseudospark switch: Back-Lighted Thyratron (BLT)
Back-Lighted Thyratron and Optical Triggering
• Complete isolation of trigger from ground & high voltage potentials => scalability
• Fiber optic coupling
Cathode
Material
(Work
function)
Trigger Source
Wavelength
(Photon
Energy)
Light
Energy
For
Triggering
Delay
(ns)Jitter (ns)
Molybdenum
(4.3-4.6 eV)
222 nm
(5.60 eV)10 μJ [1] 78 0.4
Molybdenum
(4.3-4.6 eV)
308 nm
(4.03 eV)4.4 mJ [1] 78 0.4
Molybdenum
(4.3-4.6 eV)
266 nm
(4.67 eV)7 mJ [3] 85 8
Brass
(4.7-4.8 eV [4])
308 nm
(4.03 eV)
12- 50 mJ
[2]*
150 -
2000 ~15
Brass
(4.7-4.8 eV [4])
308 nm
(4.03eV)
12-50 mJ
[2]*40-80 ~10
Back-Lighted Thyratron (BLT)
[1] C. Braun et al., 1988
[2] Pitchfordet al., 1995
[3] C. Jiang et al., 2005
[4] J. W. Bond, et al., 2008
* Two different cathode hole diameters used. 3 and 6 mm, electrode spacing: 4 mm.
Materials with higher
photosensitivity
Higher electron emission
Better switch parameters
Less light energy for triggering
Mg work function = 3.66 eV
5 MΩ16 nF 3 Ω
Probe 2 (x1000)Switch Current
Probe 1 (x1000)Switch Voltage-HV
Experimental Setup and Procedures
13 cm
266 nm
mirror
photodiode
UV
Laser
UV lensf = 25 cm
Mg foil
Hold-off Voltage and Peak Current
Delay and jitter
Lifetime
λ= 266 nmpulsewidth= 5 nsenergy per pulse= 3 mJ
0
1
2
3
4
5
6
7
8
0.2 0.4 0.6 0.8 1 1.2
Pe
ak C
urr
en
t (k
A)
Pressure (Torr)
Peak Current
without Mg foil with Mg foil
Self-ignited Breakdown
Peak Currents >6 kA and
Hold-off Voltage >30 kV
The self-ignited breakdown characteristics are independent of presence of Mg foil.
0
5
10
15
20
25
30
35
0.2 0.7 1.2
Ho
ld-o
ff V
olt
age
(kV
)
Pressure (Torr)
Hold-off Voltage
without Mg foil with Mg foil
Switch DelayEffect of pressure
Switch Voltage= 4.1 kV
Order of magnitude shorter delay is observed with Mg foil.
Both delay and jitter decreases as the pressure increases.
Typical waveformPressure: 0.6 TorrCathode: plain copper
delay
0
5
10
15
20
0.2 0.4 0.6 0.8 1 1.2
De
lay
(μs)
Pressure (Torr)
without Mg foil with Mg foil (x10)
0
0.5
1
1.5
2
2.5
0.2 0.4 0.6 0.8 1 1.2
De
lay
(μs)
Pressure (Torr)
Switch Delay: Effect of Field
• Delay is at least three times shorter when Mg is present.
Delay measurements with switch voltage at 65 % of self-breakdown voltage at a given pressure:
with Mg foil without Mg foil
Pressure (Torr)
Delay (μs) @ 0.65 VBD
Delay (μs) @ 4.1 kV
Percent difference
Delay (μs) @ 0.65 VBD
Delay (μs) @4.1 kV
Percent difference
0.4 0.28 0.43 35 0.84 14.84 94
0.6 0.17 0.32 47 1.13 6.58 83
0.8 0.15 0.26 42 0.65 3.88 83
17.4 kV 14.3 kV 8.6 kV 4.1 kV
Discussion: Contributing electron emission mechanisms
• Photoemission
– hν – ΦMg>hν – ΦCu
– Larger number of electrons per pulse
– Less dependence on field as (hν – Φ0) gets larger compared to
Effect of the same field
enhancement is approximately 10%
of the photoemission for Cu whereas
it is only around 2% for Mg.
b =
β = 1 (Parallel plate)
E*= 500 V/cm
~ 0.01 eV
hν – ΦMg = 1.02 eV
hν – Φcu = 0.17 eV
Time-dependent heat conduction equation
• Thermionic emission (Field enhanced):– How much the laser can heat the surface of the cathode?
– At which temperature thermionic emission is comparable to photoemission?
Im= Laser Intensity (W/m2)R = ReflectivityK= Thermal conductivity (W/Km)κ =K/ρcρ=Mass density (kg/m3)c=specific heat (J/kgK)T0=initial temperature (293 K)
Temperature (K)
Mg Cu
Laser heating (7.5 ×107 W/cm2) 832 535
Melting point 923 1358
Discussion: Contributing Electron Emission Mechanisms
Thermionic emission is comparable to photoemission only above ~1700 K
R* ~ 0.85
*Gesell, et al., 1973
A: constant T: Temperature (K)JRLD=Current density (A/cm2)K=Boltzmann’s constant (J/K)ϕ= Work function (eV)
Calculated using QE=10-7
Richardson-Laue-Dushmannrelation for temperature-limited
emission
Summary
We have shown that the delay of the BLT switch can bereduced three times or more when low work functionmaterials like magnesium are integrated to its cathode. The best delay measured is 150 ns, at 800 mTorr, 8.6 kV switchvoltage with Mg foil integrated cathode.
Effect electric field enhancement was found to be morefor Cu cathode without Mg foil than for with Mg foil.
This result is in agreement with the estimations basedon photoemission to be the dominating mechanism foremission of seed electrons.
No significant degradation in delay and jitter was observedafter 106 shots at moderate current levels (~2 kA).
Lifetime measurementsMg foil after 106
shots
SEM photos: courtesy of Yung-Hsu Lin