1 Chi Shung Yip Noah Hershkowitz JP Sheehan Umair Suddiqui University of Wisconsin Madison Greg Severn University 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 ID: 271869
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Slide1
The Maxwell Demon and its Instabilities
1
Chi-
Shung
Yip
Noah
Hershkowitz
JP Sheehan
Umair
Suddiqui
University of Wisconsin – Madison
Greg Severn
University of San DiegoSlide2
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…..Slide3
We implemented a Grid-type Demon to understand the instabilities.
3
Ceramic insulated stainless steel shaft
0.025mm Tungsten wires nicely lined up
We measure Te and Plasma potentials
In the usual way with Langmuir probes and
Emissive probes……..
(Dutifully made in sweat factories run by Chinese grad students.)Slide4
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 BIASESSlide5
The Demon efficiently raises Te, but also gives rise to global instabilitiesSlide6
*Gracefully photoshopped to show epic visual effects
Anode Spot
Maxwell demon creates anode spots, in some cases, they can be unstable:Slide7
A small plate gives similar effect, but makes stable anode spots.Slide8
[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 c
s
.Slide9
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 LapsedSlide10
Major change within a cycle: Plasma Potential and Density
Argon 1.7mTorr
Plasma current 0.65mA
Argon 1.7mTorr
Plasma current 0.75mASlide11
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)
(n
e
B)
(n
e
C)
e
n
e,2
n
e,1Slide12
The model gets the qualitative features of instability right, however, improvements are neededSlide13
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 extinguishedSlide14
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.