Browsing by Author "Bernstein, William"
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Item Optical emission measurements during beam plasma interactions(1985) Mantjoukis, George A.; Haymes, Robert C.; Bernstein, William; Wolf, Richard A.; Freeman, John W.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.Item RF wave observations in beam-plasma discharge(1983) Mesli, Rachid; Bernstein, William; Michel, F. Curtis;Stebbings, Ronald F.The Beam Plasma. Discharge (BPD) has been produced in the large vacuum chamber at Johnson Space Center (2 x 3 m) using an energetic electron beam of moderately high perveance (K ~ 1.4 x 1^-6 A V^-3/2). A more complete expression of the threshold current IC taking into account the pitch angle injection dependence is given. Ambient plasma density inferred from wave measurements under various beam conditions are reported. It is observed that the maximum frequency of the excited RF band behaves differently than the frequency of the peak amplitude. The latter shows signs of parabolic saturation consistent with the light data. It is found that the beam plasma state (pre-BPD or BPD) does not affect the pitch angle dependence. Unexpected strong modulation of the RF spectrum at half odd integer of the electron cyclotron frequency (n + 1/2)fce is reported (5 < n < 1). Another new feature, the presence of wave emission around 3/2 Fce for Ib = Ic is reported.Item The beam-plasma discharge revisited (a new theoretical model)(1983) Llobet Ribeiro, Xavier (b. 1963); Bernstein, William; Haymes, Robert C.; Michel, F. CurtisMany features related to the ignition of the Beam-Plasma Discharge can be explained with the requirement of a critical density, which in case of a self-made plasma translates into a critical current. However, other characteristics indicate that this requirement which arises from the geometry, is a necessary but not sufficient condition. In this thesis it is shown that a second requirement must exist, also taking the form of a minimum current. With these two constraints most of the ignition-related features can be explained. A preliminary analysis of the discharge showing how the second requirement arises is presented.