Laser Measurement of Anomalous Electron Diffusion in a Crossed-Field Plasma

The anomalous diffusion of particles and energy in magnetized plasma systems is a widespread phenomenon that can adversely impact their operation and preclude predictive models. In this Letter, this diffusion is characterized noninvasively in a low-temperature, Hall-type plasma. Laser-induced fluorescence and incoherent Thomson scattering measurements are combined with a 1D generalized Ohm’s law to infer the time-averaged inverse Hall parameter, a transport coefficient that governs cross-field diffusion. While the measured diffusion profile agrees with model-based estimates in magnitude, the measurements do not exhibit the steep “transport barrier” typically imposed in models. Instead, these results show that the electric field is primarily driven by a diamagnetic contribution due to the large peak electron temperature exceeding 75 eV. This finding motivates a reconsideration of nonclassical energy transport across field lines in low-temperature plasmas.

Figure 1

Illustration of a Hall accelerator with the employed laser scattering geometry. (a) Front view with azimuthal Thomson scattering wave vectors. (b) Cross section of channel showing magnetic shielding geometry and axial laser-induced fluorescence injection scheme.

Figure 2

(a),(b) Axial ion velocity distributions from laser-induced fluorescence with model fit, where $L_c$ is the H9 channel length. (c),(d) Azimuthal electron velocity distributions from incoherent Thomson scattering with Maxwellian fit, showing notch filter stop band (dashed lines).

Figure 3

Channel centerline plasma properties, with 95% credible intervals and magnetic field peak (dashed line). (a) Axial ion velocity and electric field. (b) Azimuthal electron temperature. (c) Electron density and axial electron velocity magnitude. (d) Azimuthal electron velocity compared to $u_{E\times B}$ and $u_{\text{drift}}$.

Figure 4

Inverse Hall parameter measurements with 95% credible intervals from methods “A” and “B.” The solid line corresponds to the inverse Hall parameter calibrated for simulation results from Ref. [55].