The Maxwell Demon and its Instabilities

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Chi-Shung YipNoah Hershkowitz

JP SheehanUmair Suddiqui

University of Wisconsin – Madison

Greg SevernUniversity of San Diego

What is a Maxwell demon?

Maxwell imagined a being that could measure atomic speeds and let the fast ones pass and block the slow ones…..

MacKenzie (‘71) implemented a plasma `demon’ using a positively biased grid immersed in a DC hot filament discharge…..Te rises of course, but instabilities limited this effect…..

We implemented a Grid-type Demon to understand the instabilities.

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Ceramic insulated stainless steel shaft

0.025mm Tungsten wires nicely lined up

We measure Te and Plasma potentialsIn the usual way with Langmuir probes and Emissive probes……..

(Dutifully made in sweat factories run by Chinese grad students.)

The Demon turns Bi-Maxwellians into single Maxwellian distributions by

absorbing cold electrons

Cold part is determined by subtracting the hot extrapolated into plasma potential from the Cold + Hot part.

Hot part

Cold + hot

NO GRID BIAS VARIOUS GRID BIASES

The Demon efficiently raises Te, but also gives rise to global

instabilities

*Gracefully photoshopped to show epic visual effects

Anode Spot

Maxwell demon creates anode spots, in some cases, they can be unstable:

A small plate gives similar effect, but makes stable anode spots.

[2] S. D. Baalrud, B. Longmier, and N. Hershkowitz, Plasma Sources Science & Technology 18 (2009).

Anode Spot:The Baalrud theory

The Baalrud theory of anode spots:• Anode spots occurs when number of ions born from ionizations

in a Debye cube in an electron sheath matches the number of total electrons within the Debye cube

• Ion density from ionizations would be given by

• For no better approximations, we assume electron density to be bulk density and ion exit velocity to be cs.

We measured plasma parameters in an unstable plasmas using the

`slow sweep’ method

• We fix the voltage, record current trace over time and then change voltage.

Probe fixed at this voltage

Time Lapsed

Major change within a cycle: Plasma Potential and Density

Argon 1.7mTorrPlasma current 0.65mA

Argon 1.7mTorrPlasma current 0.75mA

• A crude model can be formulated solving the production-loss equation:

Global loss-production balance as a mechanism of the relaxation time

• Solving the rate equation results in:

• Resolving the equation both when the anode spot turns on and off :

(A) (neB) (neC)e

ne,2

ne,1

The model gets the qualitative features of instability right, however, improvements

are needed

Conclusion

• The Maxwell demon has been revisited and found to be having identical functions as a small plate, except with a higher resistance to forming anode spots.

• Demon instability as MacKenzie et al. has discovered was investigated and (at least partially) its nature established

• We need to know more about how anode spots are formed and extinguished

Acknowledgements:

This work was supported by U.S. Department of Energy Grants No. DE-FG02-97ER54437 and No. DE FG02- 03ER54728, National Science Foundation Grants No. CBET-0903832, and No. CBET-0903783

The authors would like to thank Dr. Yevgeny Raitses and Dr. Igor Kanganovich of the Princeton Plasma Physics Laboratory for their advices on this work.