Final Report for DOE Grant DE-FG02-06ER54890

This investigation of shear Alfven waves focused on two aspects of Alfven wave physics that are of relevance and importance to space plasma physics. The two areas of study will be: (1) electron acceleration by inertial mode Alfven waves and; (2) experiments to test competing theoretical models for Alfven waves with data taken in the LAPD plasma. These models include the basic dispersion relation for inertial Alfven waves as well as model for field-aligned standing waves and their relation to field line resonance (FLR) theory in space plasmas. The measurements of electron acceleration by inertial mode shear Alfven waves have concentrated on the bulk acceleration of electrons and the process of resonant electron acceleration, topics of central importance for models of Alfven wave interactions in auroral plasmas. By measuring the parallel electron acceleration for a variety of wave parameters, we have tested theoretical predictions for this important acceleration process. These studies were done using the UI arbitrary spatial waveform antenna which allows for the very flexible generation of Alfven waves with good control of the perpendicular wave number. This antenna has been used successfully many times in the LAPD plasma. These measurements also utilized the amplified magnetic search coil antennas developed by UI for determination of the Alfven wave magnetic fields. These probes have excellent signal-to-noise ratios and allow for very sensitive detection of wave magnetic fields. For the electron distribution measurement we utilized the new whistler wave absorption technique being developed by our group. This technique, which uses the properties of Doppler-shifted whistler wave absorption at the electron cyclotron frequency, has shown good results and with further refinement should allow us to determining the distribution function of electrons along the direction parallel to the magnetic field with the needed resolution. Initial results of this technique have been written up and we expect to submit the manuscript by the end of the year. We have looked for two effects. The first is the 'sloshing' of electrons in the wave field which is the parallel current closure for the perpendicular current carried by ions. We have found evidence for this effect and have reported this at the recent IPELS 2011 meeting this past June in Whistler, Canada. The second effect is that of resonantly accelerated electrons, a process identical to that proposed as a mechanism for producing auroral electron fluxes. This is a much more subtle effect and we are continuing data processing in the post-grant time frame to determine if we have successfully measured this effect. Neither of these two effects has been directly measured in a laboratory plasma in the past, to our knowledge. The studies of theory split into dispersion relation experiments and FLR physics experiments. The dispersion studies have demonstrated that the basic theory of the inertial Alfven waves are correct and we have confirmed these in a PRL paper published last year and listed below. As part of this work, it was found that electron-ion collisions effects had to be included to get agreement with experimental results. This led to a second paper in the Physics of Plasmas which showed that collisional effects are important for the inertial Alfven wave, but not for the kinetic Alfven wave. These collisional results inspired work by a new theorist in our department to use these results for a comparison with a new collision operator in gyro-kinetic codes. These results were also published Physics of Plasmas. This illustrates the broad impact of this work - publication in strong journals that inspires and has applicability to other plasma studies. The work on FLRs has an ultimate goal of differentiating between competing models of how these field-aligned standing waves are structured along the magnetic field, but during the course of our experiments was more limited. We have demonstrated that we can see the standing wave pattern develop as expected and that it can be varied with frequency. However, the Q of the LAPD plasma is fairly low for the experiments that were run. This means lower amplitude variations and we are continuing to work on extracting good results from these data. As an offshoot of the FLR experiments, it became clear that it was important to understand reflections from the ends of the LAPD device. To measure these effects we developed the 'Elsasser' probe which simultaneously measures the perpendicular components of both the electric and magnetic field . This allows the direct computation of the Poynting flux and clearly distinguishes between the forward and reflected wave. This work has been published in Reviews of Scientific Instruments as an experimental technique.