The new closed-flux VTF configuration offers a unique opportunity for the study of the dissipation region in configurations where the guide magnetic field can be varied continuously. The plasmas in VTF are highly relevant to research on collisionless reconnection as the mean free path between electron ion collisions (up to 1 km) is larger than the dimension of the device. The detailed measurements of the diffusion region will allow the evaluation of nearly all terms in the generalized Ohm's law during reconnection. The length scales of interest can be varied over a large range by changing the ion mass and by changing the plasma density and electron temperature. Furthermore, the magnetic field configuration can be changed by scanning the relative strength of the guide magnetic field and the in-plane magnetic field. Also the drift speed of the electrons relative to the ions may be scanned from zero to several times their thermal speed. The latter is important for the possible observation of the nonlinear development of the Buneman instability and electron holes.

The electric and magnetic fluctuations that develop in the VTF current layers during magnetic reconnection will be measured with high frequency electrostatic and magnetic probes for the range of plasma parameters mentioned above. The importance of turbulence in facilitating enhanced reconnection rates will be studied by comparing its spatial and temporal variation with localized measurements of the resistivity. The turbulence will be characterized in terms of frequency resolved amplitude, correlation length and correlation times. This will facilitate a rigorous comparison with theoretical predictions regarding the type of turbulence observed and the level of turbulence needed in order to impact the local resistivity. The measurements will be made with a number of movable fast electrostatic and magnetic probes that facilitate the reconstruction of the time evolution of the charateristics of the turbulence during the reconnection process over the whole poloidal plane, including the entire dissipation region. The amplitudes and the various moments of the fluctuation spectra can be measured as a function of time and position.

The investigation of the self-generation of turbulence in the dissipation region during reconnection and associated anomalous resistivity and heating will be explored in a joint theory/computational/experimental effort. The project will be led by Mike Shay (MD). On the LAPD machine at UCLA, Carter and Gekelman will carry out controlled experiments of the development of streaming instabilities. Simulations of the experimental system will be carried at Maryland using the p3d code. Porkolab and Egedal will carry out parallel reconnection experiments on the VTF machine at MIT. Carter will work with the MIT group on measurements of fluctuations in VTF, in particular on the design of electrostatic fluctuation probes and associated electronics. Dedicated full particle simulations will again be tailored to the experimental parameters for benchmarking of theory and experiment. Cowley, Drake and Rogers will participate through analysis of the results and in the development of theoretical models of anomalous resistivity and particle heating. Shay will work with Kevrekidis and Gear to use p3d as the timestepping kernel for a projective integrator appropriate for the reconnection problem, i.e., without ordering out the physics associated with anomalous resistivity. The broadly important goal is to calculate the effects of the fast turbulence on the slower reconnection process.

Research Plan

• Year 1: Fabricate magnetic and electrostatic RF probes. Perform initial experiments characterizing the frequency spectrum of the turbulence in experimental scenarios with and without a guide magnetic field.
• Year 2: Based on the initial turbulence measurements, optimize the design of the magnetic and electrostatic probes and construct probe arrays. Extend the turbulence measurements to facilitate correlation length studies (with and without a guide magnetic field).
• Year 3: Assess and prepare scattering experiments, microwave (2 mm) and/or laser (CO2, PCI). Explore the role of turbulence and fluctuations on the reconnection process and compare with theoretical modeling.
• Years 4-5: Commence scattering experiments and compare with probe data and theoretical predictions.