CIV interaction: laboratory experiments on the magnetic field and neutral density limits

Laboratory experiments are reported which determine the magnetic field and neutral density limits for critical ionization velocity (CIV) interaction in the impact configuration. A combination of microwave interferometry and spectroscopy has been used to measure how the electron energy distribution varies with the neutral density and the magnetic field strength. The efficiency of the CIV process is evaluated in terms of the efficiency factor eta of energy transfer to the electrons. This efficiency is studied as a function of the ratio VA/V0 between the Alfven velocity and the plasma stream velocity and the ratio nu i/ omega gi between the ionization frequency and the ion gyro frequency. With other parameters kept constant, VA/V0 is proportional to the square root of the magnetic field, while nu i/ omega gi is proportional to the neutral density. The authors have found that these two dimensionless parameters are coupled in such a fashion that a stronger magnetic field can compensate for a lower neutral density. For their strongest magnetic field, corresponding to VA/V0=4, CIV interaction is found to occur for a comparatively low value nu i/ omega gi approximately=0.1. For VA/V0=1, we found a clear absence of CIV interaction even for nu i/ omega gi approaching unity.