Vircators - (VIRtual CAthode OscillatOR) circa 1980

Keegan · Post by **Keegan** » Wed Feb 13, 2008 2:33 pm

We have been studying Plasmas for about 100 years now. After reading the POPS papers, it is easy to assume that we just recently married RF and Periodically Oscillating Plasma Spheres together. This is not the case.

Presenting the Vircator

"The present invention relates broadly to microwave generators, and in particular to a high power microwave generator apparatus
utilizing a relativistic electron beam to generate microwave energy as a consequence of induced virtual cathode oscillation."

* First patented in 1982
* Widely Tunable Frequency. (Tied to E & B field drive)
* GW powers (Pulsed)
* High efficiency (30-40%)
* TM Mode

So as usual they got things backwards. Instead of driving virtual cathodes with RF, they are driving RF with virtual cathodes.

The following vircator papers appear to be an excellent starting point for POPS / Virtual Cathode Physics

U.S. Patent 4,345,220 (1982)

High Power Microwave Generation From a Vircator (1983)

U.S. Patent 4,730,170 (1988)

One interesting thing to note is that the last patent was awarded to "Thomas J. T. Kwan, Charles M. Snell both from Los Alamos, New Mexico." Could this be where the POPS guys got their ideas ?

-k

Keegan · Post by **Keegan** » Wed Feb 13, 2008 2:44 pm

Patent 4345220 wrote: It is another object of the invention to provide an improved microwave generator apparatus
capable of generating high power microwaves in the order of 10^10 to 10^12 watts

Dear God .....

drmike · Post by **drmike** » Wed Feb 13, 2008 2:48 pm

If nothing else just being around the people who built the original ones might have given them ideas. That's the whole point of a scientific lab, so let's hope so!

Very nice find too, thanks!

(Edit: it is a weapons lab you know!!)

scareduck · Post by **scareduck** » Thu Feb 14, 2008 6:05 am

Missing where the ions fit into this device, i.e. how does plasma fit into the Vircator device. It looks to me like just another mechanism for generating microwaves, albeit some pretty serious ones.

It sort of looks like a cavity resonator, if I understand the physics correctly, except that the resonance of the cavity is tunable according to the size of the virtual electrode.

Keegan · Post by **Keegan** » Thu Feb 14, 2008 9:21 am

scareduck wrote:Missing where the ions fit into this device,

Well they dont.

It sort of looks like a cavity resonator, if I understand the physics correctly,

The cavity is there just to confine the electron beam and alow a virtual cathode to form. The virtual cathode forms, oscillates and generates RF according to plasma frequency.

So essentially there are two basic types of Periodically Oscillating Plasma Spheres (POPS). Virtual Cathodes and Virtual Anodes. The patents presented here are dealing just with virtual cathodes.

This is a very good as it once again validates the current work being done with POPS virtual anodes at LANL. If an electron is 1 / 1836 the mass of a proton then one would expect the frequency of a virtual cathode to roughly be 1836 times the frequency of a virtual anode. Which appears to be the case.

MSimon · Post by **MSimon** » Thu Feb 14, 2008 2:04 pm

Keegan,

It is an electron resonance and is dependent on density.

POPS is independent of density. Particle mass, charge, potential well, and size of reactor control POPS.

scareduck · Post by **scareduck** » Thu Feb 14, 2008 4:51 pm

No ions = no POPS (you're missing the second P = plasma).

Keegan · Post by **Keegan** » Mon Feb 18, 2008 2:44 am

Virtual Cathode Theory and Design of a Millimeter Wave Vircator.

The advantage of the vircator over a reflex klystron is that the virtual cathode moves opposite to the electron beam propagation direction during the bunching phase resulting in enhanced electron densities unobtainable otherwise. The periodic time-dependent nature of the bunching results in efficient microwave generation. (rrh)

Interesting.

Keegan · Post by **Keegan** » Mon Feb 18, 2008 3:46 am

Found this great paper on how to drive a virtual cathode. A source that matches the resonant frequency can drive it. Provided it is the correct distance away as energy is reflected back from the virtual cathode into the electron beam. This can cause distructive interference if one is not carefull.

We have simulated the operation of a vircator
klystron provided with an external feedback. It was
found that the oscillator allows one to efficiently handle
the formation of structures in the electron stream. If the
input resonator is tuned to the natural frequency of the
VC, three oscillation conditions are possible, depending
on the feedback delay: (1) almost sinusoidal oscillation,
(2) weak quasi-periodicity with two base frequencies
(one of which is dominant), and (3) chaos in
the form of fully developed two-periodic oscillation
with two incommensurable frequencies. The onset of
chaos is related to the formation of secondary structures
in the electron stream as a result of its modulation,
with the feedback delay being sufficiently large.

Keegan · Post by **Keegan** » Sun Mar 02, 2008 6:50 am

Hey all, i just finished reading a great Phd thesis by Yeong-Jer Chen.

Compact, repetitive Marx generator and HPM generation with the Vircator

Awesome details and construction of a portable marx generator, e field and power circuit design of a portable vircator. Also had a very interesting analysis of micro-grating (machining) the cathode surface to optimize emission.

I highly recommend you all download.

Keegan · Post by **Keegan** » Sun Mar 02, 2008 11:07 am

Quoted from ^Above^

drmike · Post by **drmike** » Sun Mar 02, 2008 3:32 pm

The photoetched cathode is pretty interesting, should definitly help with field emmission.
Thanks!