EMP Device

Apart from working in an optics lab at the moment, I'm not sure what brought this line of inquiry on. But here it is, in its unpolished glory. I make no promises on the veracity of any ideas presented herein. These are my very hastily researched ideas.

The Motivation/General Idea:

• Create a rapid, high amplitude oscillation of charged particles (electrons). Possibly high enough amplitude to generate a destructive impulse, similar to the electromagnetic impulse generated by a nuclear warhead. There do exist such non-nuclear devices and are already employed by the military. (Source).
• Exploit current laser technology to shorten the time-width of the EM impulse from hundreds of microseconds to tens of nanoseconds.
• Have this impulse be generated without the use of explosives (making it cleaner).
• Preferably make this device inexpensive and ruggedly operable.
  

Preliminary Concepts:

• Stimulated Emission
• Cerenkov Radiation
• CO2 laser
• Photo-Electric Effect
• Frank-Tamm Formula
• Smith-Purcell Effect
  

Method Outline:

1. The photo-electric effect gives us a method to liberate electrons from a metallic surface.
2. The work function of the metallic surface acts as a high-pass filter for photon frequencies. High pass in the sense that only photons of sufficiently high frequency will liberate electrons, the remainder are reflected.
3. CO2 lasers output in the infrared. They commonly use gold plated reflectors, the work function for which is around 5 eV (Source). The work function can be reduced by altering the E-field across the metal. You can use this fact, coupled with a light source of known wavelength, to measure the work function of a metal.
4. CO2 lasers can be "Q-switched" to produce a pulsed laser with peak pulse energies in the Gigawatt region (commercially). The duration of these pulses can be as small as 10s of nano-seconds.
5. There are a number of ways to pump CO2 lasers and I'm not sure if this is the best means, but I thought it would be cool if we used the Cerenkov effect to provide a broad-band photon source. This would give us an incentive to make the lasing cavity double as a weak linear particle accelerator. Negatively charge the cavity walls to center an electron beam onto a Smith-Purcell grating. So long as some of the photons coming off the Cerenkov cell induce stimulated emission in the CO2, we're okay. As an added bonus, if the frequency content of the light is sufficiently high to overcome the already negatively biased cavity walls, it will cause electrons showers to either excite the Cerenkov device further or excite Nitrogen oscillations (which play a role in the CO2 lasing effect). Importantly, the Cerenkov pump produces radiation amplitude that is dependent on the length of the medium and velocity of the moving particle, which means it can be easily scaled up to provide large quantities of pump photons.
   
6. We can have the Q-switched output at the negatively charged end of the cavity. Beyond the aperature we could put a high-surface area, metallic receptor (that could also double as the cathode to provide the chamber's E-field.)
   
7. When we want to create a pulse, we'll allow the laser to output a pulse via active Q-switching. Simultaneously, we'll increase the negative bias on the metallic receptor. Provided the beam is diffuse enough to cover the entire receptor, and the receptor is sufficiently biased to loose its electrons from infrared excitation, then we should observe a rapid (on the time scale of the pulse), depopulation of electrons from the cathode. Provided the transient depopulation is strong enough (coupled perhaps with a loss of voltage bias on the cathode), the recently released electrons should fly back towards the now very positive cathode. And so we've produced an oscillation of electrons, as desired.

Discussion:

• If you're familiar with photoelectric effect capacitors (see here, for example), then this idea isn't terribly novel. All I'm suggesting is that we see what would happen in the extreme case of exciting such a capacitor with an enormous impulse from a CO2 pulsed laser.
• I wasn't thinking of the photoelectric effect capacitor from the outset. My initial goal was to explore how to use Cerenkov radiation in a laser. Perhaps with my idea of using both photoelectric effect and standard lasing, you could increase the efficiency of output light energy to input energy.
• This device should be fairly "rugged", as rugged as CO2 lasers are, at any rate. It would likely require cooling, a high voltage power supply, a small vacuum near the cathode, and a flow-through gas tube -- along with the other laser components.

Some Brief Calculations:

• A commercial Q-switched CO2 laser GEM Q-400 has a pulse energy of 1 mJ and a FWHM pulse width of 150 ns at an output wavelength of 9.25 uM. Lets say that you tune the cathode to eject photons with zero KE at 9.25 uM, how many electrons would you eject in the 150 ns of the pulse? [Energy per electron = hv. v = 2*pi*f = 2*pi*(c/wavelength). Ee = 6.6E-34*2*pi*(1E8/1E-5) = 4E-20 J/electron. So 1 mJ would liberate about 2E16 electrons. That's much less than a mol of electrons so I think it's safe to say that a moderately sized cathode could donate this many electrons.