Submitted Research Proposal
Plasmas exhibit a rich variety of oscillatory and wave-like phenomena in which charged particle motion gives rise to fields and fields result in particle motion. These wave-particle effects are employed in many particle acceleration and plasma heating methods as well as in wave generation mechanisms. Wave-particle and field-particle interactions also play a key role in particle acceleration mechanisms believed to be operating in the auroral acceleration region of the earth's magnetosphere and more generally, in the domain of space plasmas. Yet even though plasmas are often manipulated with EM waves, deriving a self-consistent model of plasma wave-particle interaction is still an outstanding fundamental problem in plasma physics, particularly when large amplitude fields are involved.
The experimental description of plasma properties under varying circumstances is a major part of the effort to develop such a treatment. This description may proceed on two different levels. On the macroscopic level, one may sometimes treat the plasma as a fluid described by the parameters of temperature, pressure, density, and flow velocity. A microscopic (or kinetic) description employs a more fundamental quantity in a plasma, the one-particle velocity distribution function f(v;x,t) of each particle species. Velocity moment integrals of the distributions connect the microscopic properties with the fluid parameters.
One approach seeing increasing application today in plasma diagnostics is laser-induced fluorescence (LIF) spectroscopy, which is based on laser excitation of transitions between quantum states of particles in plasmas. LIF has been used to measure quantities as diverse as atomic and ionic densities, ion velocities and temperatures, electron temperatures, as well as electric and magnetic fields, in both fusion and basic research plasmas. Scanning the frequency of a narrowband laser across the Doppler broadened absorption line of an ionic transition yields a direct measure of the ion velocity distribution function. The power of LIF lies in the non-perturbative nature of this diagnostic and also in the superb velocity (spectral), spatial, and temporal resolution (Dv/vthermal < 1%, Dx3 @2 mm3, Dt @ 1 msec).
Of particular relevance to this proposal is the application of LIF to the study of plasma wave-particle interactions, with significant participation and contributions by T.N. Good. A large body of research has been conducted in quiescent magnetized plasmas (in Q -machines). Researchers employing LIF have concentrated their efforts on discovering the properties of electrostatic ion cyclotron, ion acoustic, lower hybrid, and drift waves. Ion waves constitute a subset of the assortment of electrostatic and electromagnetic waves that may be supported by the plasma medium. Because ions contribute in a significant way to the dynamics of these waves, they are good candidates for an experimental inquiry of wave-particle interaction that utilizes the LIF diagnostic technique. Time scales for ion processes are slower than for those involving primarily electrons, adding advantage by making them easier to follow.
As ions respond to the wave fields via modifications of their trajectories, the ion distribution is perturbed. The ion response takes the form of a coherent oscillation and, in cases of significant absorption of wave energy, an incoherent (non-adiabatic) alteration of the ion distribution. Through the use of phase-lock techniques, or spatially localized measurements of the ion distribution synchronous with the wave field, a component of the perturbed distribution can be observed oscillating coherently with the wave. Single point detection of the phase-coherent, perturbed distribution, or spatial scanning of the velocity moments of this distribution yield measurements of wave vector and fluctuation amplitude1. Ion heating and acceleration, typical forms of the incoherent response to ion waves, are readily detected by measurements of the ion distribution function2. Test-particle diagnostics, utilizing ions selectively placed into metastable or spin polarized states by optical pumping, provide a means of investigating the behavior of particle orbits3.
The goal of the current proposal is to conduct experimental research of plasma wave-particle interaction utilizing LIF spectroscopy. I will construct an experimental apparatus for this purpose, install an LIF diagnostic system on the apparatus, and test the diagnostic in experiments designed to evaluate performance and the feasibility of the plan. It is my ultimate goal to advance our understanding of wave-particle interactions by applying LIF to the study of nonlinear ion acoustic waves, ion acoustic shocks, and solitons.
A plasma source, called a double plasma (DP) device, that possesses the desirable characteristics of quiescence and uniformity necessary for this task, has been developed at the UCLA Plasma Physics Laboratory by Taylor, Ikezi, and MacKenzie4 in 1969. The DP is now being employed in many parts of the world for basic plasma research. Because this source is economical to build and simple to operate, it is ideally suited to the undergraduate or graduate plasma laboratory. The DP device has been used to study a variety of plasma phenomena such as excitation and detection of linear plasma waves, propagation of plasma shocks and ion acoustic solitons, plasma wave-particle interactions, beam-driven plasma instabilities, and formation of double layer space potential structures.
The innovation in the proposed research is the introduction of LIF spectroscopic techniques for the first time to the study of ion wave-particle interactions in a double plasma device. I propose to construct such a DP device in a 135 liter vacuum chamber (length =120 cm, diameter = 38 cm) recently obtained through the DOE ERLE program. The chamber accommodates this task well, being equipped with rectangular port windows (28 cm x 100 cm) on the top and side that permit optical access to a large portion of the plasma.
Recently, plasma researchers at the University of Iowa have demonstrated the feasibility of applying LIF to multidipole-confined filament discharge plasmas of argon5. This result, coupled with personal experience gained employing LIF in hot cathode argon discharges at the CRPP-EPFL and at the University of Maryland has led to a great deal of enthusiasm on my part to see these efforts extended to the domain of plasma phenomena in DP devices. Furthermore, I am encouraged by private communications with several members of the plasma research community who are presently conducting projects in DP devices. Their statements underscore the promise of important contributions to the comprehension of ion acoustic soliton6 and double layer7 phenomena that this sensitive and powerful diagnostic can provide.
The development plan for the Picketts Charge Plasma Device (PCPD) will
proceed in measured steps. The first major task will be to construct
the double plasma device. Students, aided by a large body of literature
on the subject, will participate in the design and final assembly of the
apparatus. Precision machining of vacuum hardware will be contracted
out to the Department of Physics at West Virginia University, where
collaborators are presently building a similar device. Following the
construction phase, we will strike our first plasma discharge and examine it
with the aid of a Langmuir probe.
A Coherent model #699-05 scanning narrowband ring dye laser system is already in place and is currently being applied to a related educational and research program in laser optogalvanic spectroscopy in hollow cathode discharges. A Gettysburg College graduate (May 1992) who completed a senior project in this area is now pursuing a Ph.D. in plasma physics at West Virginia University. Employing a Fabry Perot for reference, the laser upgrade requested in this proposal will enhance the spectral resolution by a factor of 60 and, more importantly, will improve the operational stability of the LIF system through active feedback stabilization of the laser mode frequency.
In step two of my plan, a redirection of the laser beam into the double plasma device and the collection of the LIF signal with imaging telescopes will yield a characterization of argon plasma under diverse steady-state operating conditions. This effort will serve to bench-mark the device, using for corresponding measurements performed previously with intrusive instruments such as gridded electrostatic energy analyzers.
Stage three of the program will proceed with experiments that explore excitation and propagation of linear ion acoustic waves. We will measure wave dispersion (w vs. k) using LIF methods discussed above and will compare our results with predictions from linearized Vlasov models. The linear wave-particle interaction will be revealed by measurements of ion Landau damping. This part of the plan will be executed in order to verify the applicability of this diagnostic scheme to problems of conceptual simplicity appropriate for uninitiated student assistants. These topics will be incorporated into the junior curriculum as lab exercises that provide training for new students who may ultimately choose a plasma research project as seniors. Senior projects are required for the Bachelor of Science degree in physics.
Through this endeavor, we will develop scanning telescope and laser beam injection apparatus, data collection and analysis instrumentation, and the hands-on expertise of operating the experimental apparatus. These developments will extend the project into actual research in nonlinear wave-particle interactions. Specifically, I propose to investigate propagation and dissipation mechanisms of ion acoustic solitons. The fine resolution of the LIF technique, coupled with the fact that the laser beam does not perturb the plasma dynamics, will allow new, detailed studies of the ion distribution that will yield: soliton Mach numbers; transmission, absorption and reflection properties of solitons impinging on space charge sheaths; and the dissipation of ion-acoustic solitons by reflected and transmitting ions.
In addition, the proposed device will be employed in part of a collaboration between Gettysburg College and West Virginia University (contact: Professor Mark Koepke) on ion acceleration in plasma double layers. Certain aspects of the research will be performed at each institution. This collaboration emphasizes student participation and interaction at both schools and is described in a Space Physics Educational Outreach proposal to NASA.
In summary, with the funds requested from the Research Corporation, the Physics Department at Gettysburg College will provide its students with a state of the art laser diagnostic system that will see application in conceptually accessible phenomena involving linear and nonlinear plasma wave-particle interactions. Students will gain experience in laser spectroscopy, vacuum technology, and methods of plasma production. This program will complement ongoing studies of atomic and molecular physics that employ laser spectroscopic techniques.
Skiff and F. Anderegg, Phys. Rev. Lett., 59, 896 (1987).