Diffusion and drift losses and electron cooling in helium afterglows confined by a toroidal magnetic field
Olson, Orlando Duane
This thesis describes theoretical and experimental investigations of the decay of electron density and temperature in an afterglow confined by a toroidal magnetic field. Measurements were carried out in helium afterglows by means of double floating probes. The plasma density was high enough so that coulomb interactions were dominant at all times. An analysis of the rate of diffusion of a plasma across a magnetic field is presented and compared with experimental results. For ⃑B ≤ 0.0160 Wb/m2 diffusion is the dominant density decay mechanism and the observed rate of diffusion agrees with the calculated rate within a factor of 2 for a wide range of electron densities and energies. At higher magnetic fields drift in the inhomogeneous magnetic field becomes the dominant loss mechanism. An analysis of the drift effect is presented and comparisons with the experimental results are made. Theory and experiment agree within a factor of about 2 on the average but the results are less conclusive than in the case of diffusion. Observed electron energy decay rates are compared with calculated cooling rates due to elastic collisions with ions and neutral atoms. For sufficiently low discharge power and neutral gas pressure the agreement is reasonably good. However for higher pressures and larger discharge powers the cooling rate is slower than that expected from recoil cooling alone. There is some evidence that this slow cooling is due to heating or the electrons through interactions with metastable atoms during the afterglow.