Continuation Progress Report
Project title:Collaborative understanding presheaths and sheaths in plasmas
PI: Dr. Greg Severn, Dept. of Physics, University of San Diego
Time Period: 8/31/03-6/8/03
Recipient Institution:University of San Diego,
5998 AlcalaPark, San Diego, CA92110
Award \#: ER54728
Unexpended funds?
The initial amount of funds for equipment during the first year nominally $25,000, half ofwhich has been spent on the most important item, a diode laser system. The remaining fundsbudgeted for laser-induced fluorescence (LIF) optics, for a lockin-amplifier, plasma dischargepower supply, and laser power meter will be spent during the summer. An undergraduatestudent, Mr. Jason Wohlgemuth, a Junior at USD, has been appointed to work on the projectfor 10 weeks this summer. All personnel budgeted for this project have been retained andwill work on the project this summer. My normal teaching load is nominally 7 classes per year, I am the Physics Program Director, and I have secured a 1 class released time award to work on this project during this academic year and the next two. The lion's share of my time on the project will be spent during the three summers 2004, 2005, and 2006. It is not anticipatedthat any budgeted funds will be unspent during this first initial period.
Have the aims for the period changed?
Our plans have not changed. I will focus this summer onobtaining LIF measurements in Ar+He plasmas with the new diode laser system, and searchfor new LIF schemes in XeII, in Xe discharges (e.g. 5d4F9/2 -6p4D07/2, and 5d4F7/2 -6p4D07/2, which are excited at 699.3 nm and 680.8 nm, respectively) as per the original plan forthe initial first year period.
What were the principle scientific results for this period?
I have purchased the new extended cavity diode lasersystem[1] and have begun experiments to determine its suitability for plasmas specific to our experiments. Once the LIF signature has been detected with the new laser system, we will proceed with the planned experiment, that is, to check the generalized Bohm Criterion as neutral pressures are varied, between the weakly and fully collisional cases. I made thefirst of 5 scheduled research visits to Dr. Hershkowitz's research group at the Universityof Wisconsin-Madison, and I helped them begin their equipment purchases and lab set up forLIF measurements with the new diode laser system. I presented our results[2] showing thatthe Ar+ reach the sheath edge in a two ion species plasma traveling much faster thanthe Bohm speed for Ar+ ions at the APS GEC2003 meeting[3], discussing howthe advantages of using Xe+ instead of He+ would be. This substitution would in principle allow us to independently vary the concentrations of the two ions in the plasma since Penning ionization would be diminished. At the GEC2003 meeting, We were able tointerest another research group,led by Dr. Miles Turner of thePlasma Research Laboratory at DublinCityUniversity, in Dublin, Ireland, in our experimental results. Dr. Turner hasnearly completed a PIC (particle-in-cell) code simulation of our two-ion plasma results which match the velocity moments of the ion distribution functions for the Ar ions in thetwo ion species plasmas very well. We should submit the paper some time this summer. Referees commented on our previous work[2] that we should pursue corroboration via numerical simulation, and we have begun now to do this. This was an important thing to do. Further, Dr. Hershkowitz, my principal collaborator in this project has data that showsevidence of ion instabilities in the presheath. We hope to understand this better via simulation. One of Dr. Herhkowitz's students is doing experiments to understand this better,Mr. Xu Wang, one of the coauthors of our two-ion species paper. Understanding the limitsof the applicability of the generalized Bohm Criterion, and the physical processes by which the separate ion species attain their exit speeds upon leaving the plasma and enteringthe sheath would be an important contribution to basic plasma science and technology. Bothetching plasmas and fusion plasmas involve plasma-wall interactions that help determinehow the plasma behaves, and of course what happens at the wall.
Personnel
At USD, Department of Physics:Dr. Greg Severn, PI, andMr. Jason Wohlgemuth, undergraduate research student.My collaborators at University of Wisconsin-Madison, Department of Engineering Physics, include Dr. Noah Hershkowitz (PI) and 2 graduate students, Eunsuk Ko, and Xu Wang.
References
[1] A. M. Keesee, , R. Boivin, and E. Scime, accepted for publication in Rev. Sci. Instrum.(2004).
[2] G. D. Severn, X. Wang, E. Ko, and N. Hershkowitz, Phys. Rev. Lett. 90, 145001 (2003)
[3] ``Ar ions fall out `faster than Bohm' in a multiple ion plasma. Why?’’G. Severn, N. Hershkowitz, X. Wang, E. Ko, Poster [NWP.047], 56th Gaseous Electronics Conference October 21-24, 2003 San Francisco, CA.