Optical emission measurements during beam plasma interactions
Mantjoukis, George A.
Haymes, Robert C.
Master of Science
3914 A light intensity profiles were measured with a geometrically scanning photometer during interations of an energetic electron beam with a weakly ionized plasma. Mass spectrometric measurements indicated N^ to be the dominant neutral constituent. The experiments were carried in the very large SESL vacuum facility at the Johnson Space Center which allowed an interaction length of ~ 2 m. Variable current (1-7 ma) and variable energy (5-16 V) beams were injected over a large pitch angle range ( to ~75°) for several applied magnetic field strengths (.89, 1.52, and 2.22 Gauss) over the pressure range .6-3. x 1“® Torr to allow study of the light intensity dependence on the experiment parameters. The photometer measurements were made at a fixed axial position (midway between the electron gun and the collector); overall measurements of the complete beam emission patterns (total light) were made with several low light level TV cameras. The measured 3914 Â intensity profile gives the radial distribution of the relative ionization rate while its integral gives the (relative) total ionization rate independent of geometry. The following important results were obtained 1) At low beam currents (1^ < Ic, the beam current required for BPD ignition) the relative light intensity and beam geometric configuration were consistent with single particle behavior. 2) For I], > Ic (BPD) the geometry of the illuminated region changes drastically and the 3914 total intensity increases by factors of 1-3 indicating the presence of new ionization sources (suprathermal electrons). (a) For beam injection parallel to the magnetic field (zero pitch angle), the radial width (FWHM) of the illuminated region is approximately twice the maximum (anti-node) width for preBPD conditions; it scales approximately as 1/B and E1/2. (b) For non-zero pitch angle injection, the full width of the illuminated region is approximately equal to the diameter of the single particle helix; at large pitch angle injection, the BPD shows significant limb brightening indicating a somewhat hollow configuation. Significant ionization outside the helical dimensions is not observed. (c) For all conditions the total 3914 light intensity during BPD can be fitted to the parabolic relationship QT ~ K Ic1/2 (Ib - Ic)1/2 Thus the total ionization rate increases nonlinearly with I (the nominal power supply return current) and remains proportional to the square root of the threshold current during BPD although I|j » Ic. The consistency of this scaling over the large injection pitch angle range implies that the important plasma processes are Independent of injection pitch angle. (d) The maximum efficiency of energy transfer from the beam to the plasma (as measured by the total ionization rate) occurs when I|j 2 Ic. Typical estimates of this maximum efficiency range from 3-12Z for the 2 meter path length.