17. 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 b = β = 1 (Parallel plate) E*= 500 V/cm ~ 0.01 eV hν – ΦMg = 1.02 eV hν – Φcu = 0.17 eV Effect of the same field enhancement is approximately 10% of the photoemission for Cu whereas it is only around 2% for Mg.
18. Discussion: Contributing Electron Emission Mechanisms Thermionic emission (Field enhanced): How much the laser can heat the surface of the cathode? At which temperature thermionic emission is comparable to photoemission? Richardson-Laue-Dushmann relation for temperature-limited emission Time-dependent heat conduction equation Im= Laser Intensity (W/m2) R = Reflectivity K= Thermal conductivity (W/Km) κ =K/ρc ρ=Mass density (kg/m3) c=specific heat (J/kgK) T0=initial temperature (293 K) 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 R* ~ 0.85 *Gesell, et al., 1973 Thermionic emission is comparable to photoemission only above ~1700 K
19.
20. The best delay measured is 150 ns, at 800 mTorr, 8.6 kV switch voltage with Mg foil integrated cathode.
21. Effect electric field enhancement was found to be more for Cu cathode without Mg foil than for with Mg foil.
22. This result is in agreement with the estimations based on photoemission to be the dominating mechanism for emission of seed electrons.
Thank you to everybody for being here, today I will talk a
Here is quick outline of my talkI will first describe what a pseudospark switch is and why we care about this type switches, then I will summarize prior work on BLT from cathodes perspective and explain our motivation for this work, nest I will give details of our particuler experimental setup and procedures and conclude with results/discussion and a summary
Pseudospark switches are low-pressure, high voltage (typ. 10-50 kV), high current (typ. 1-100 kA), thyratron-like plasma switches, and BLTs are optically triggered versions of pseudosparks . They operate with a diffuse glow-like discharge unlike spark gaps, but still reach very high current densities, this gives the advantage of much longer lifetime and higher repetition rate. The key to this operation characteristic of pseudosparks is their hollow cathode geometry. In the figure you can see a BLT with its side-cut, you can see the cylindrical hollow cathode and hollow anode are with bore holes on their faces. The bore hole is in order of anode-cathode gap, in our case 3 mm each. BLTs optical triggering has further advantages of electrical isolation of triggering source, allowing substantial scalability of the switch. Mini-BLTs built at USC Pulsed power lab are 20 times smaller than a commercial pseudospark.
Here is the photo from experimentYou can see helium plasma operation of BLT, the cylindrical electrodes are located.. You can also see low-inductance configuration capacitor bank and high voltage probe.If we look at prior work on BLTs from cathodes perspective, cathodes, light sources and total light energy used with corresponding delay and jitter of the switches. I have to say these are the only parameters affecting the switch delay and jitter, however still gives an idea about prior performance of the BLT with relevant triggering parameters. One thing I want to get your attention on this table is that in this experiments as the difference btw photon energy and the work function of the material increases requirement for light energy decreases. Our motivation for this project is looking at what happens if we lower the work function of the material instead of increasing the photon energy. Our QE measurements and research on cathode literature led us to Mg as the lowest work function metal photocathode.
We conducted three types of measurements: ….Here is a side view of our switch with electrical circuit schematic, the cylindrical electrodes seen are made up of oxygen free copper. Here is a cross-sectional view showing parallel planes of anode and cathode with central bore holes. The anode-cathode gap is 3 mm as well as the central bore hole diameters. High voltage capacitor bank is charged through a charging resistors and high voltage DC source. The discharge path of the switch is through the 3 ohm, damping resistors allowing us to measure the current through the circuit. The setup is composed of this for hold-off voltage and peak current measurements, conducted in self-ignited breakdown mode. For delay and jitter measurements as well as the lifetime, we need an optical triggering source….
sqrtaMg=2.25e-4 ev^-1 from old datasqrtaCu=2.32e-4
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