Page 4 - Robustness of waves with a high phase velocity

concomitantly with a very pronounced increase in FRC lifetime and more gradual radial separatrix contraction [Fig. 2(f)].
FIGURE 2. a) Time evolution of quadrature DBS spectrum in the SOL; plasma gun not active (contours are density fluctuation level ñ in a.u.; logarithmic scale); (b) Frequency-integrated density fluctuation level ñ; (c) excluded flux radius; (d) quadrature spectrum in discharge sustained via electrostatic biasing (plasma gun active); negative average frequency indicates E×B flow in the ion diamagnetic direction; (e) reduced density fluctuation level with biasing (note the difference in vertical scale); (f) time history of excluded flux radius with plasma gun active, indicating substantially increased FRC lifetime.
Figure 3 shows a comparison of rms density fluctuation levels in the near SOL (about 5 cm outside the excluded flux radius ), and several cm inside the FRC separatrix, for two plasma shots, without and with plasma gun activated. The time evolution of the FRC excluded flux radius, (in practice very close to, but outside the FRC separatrix) is indicated by grey lines.
The toroidal wavenumber range, determined by the beam launch angle ζ and the plasma geometry, is centered around ~ 2-3 cm-1 ( ~ 2), with ∼1.8−2.3 due to plasma curvature [27] for the data in Fig. 3; hence a relatively broad wavenumber range is included in the measurement.
In both shots ñ/n is relatively low for 150 μs < t < 250 μs after an initial high turbulence period likely related to FRC translation and merging. However, without plasma gun (# 19986), the fluctuation level increases substantially between 250 μs and 450 μs both outside and inside the separatrix [Fig.3(a,b)], typically concomitantly with the appearance of an n=2 rotational mode, producing a fast oscillation in the detected density fluctuation level due to the elliptical deformation of the FRC, and leading to radial FRC contraction (indicated by rapid reduction of rs). In contrast, with the plasma gun active [shot #20957, Fig.3(d,e)] the fluctuation level increases only slightly outside and inside the separatrix, the n=2 rotational mode is absent, and the separatrix radius contracts much less rapidly. There is a “benign” n=1 wobble of the FRC present that does not appear to cause FRC radial contraction (loss of diamagnetism), or substantial radial losses [17,20,28]. In later shots, where a second plasma gun has been employed in the north divertor, the n=1 and n=2 modes have been stabilized [17,29].
Fig. 3(c) shows the time evolution of the measured E×B velocity for shot #19896 (0.15 ms < t < 0.38 ms), at four density levels spanning the separatrix. The turbulence phase velocity has been neglected as the real frequency in the (generic) drift/interchange dispersion is expected to scale as due to finite Larmor radius (FLR) effects ( is the electron diamagnetic drift velocity), hence substantially reducing for . The radial E×B velocity shear is largest between 0.17-0.28 ms; this is the time interval where a relatively low fluctuation level is observed [Fig. 2(a,b)]. The E×B shear decreases substantially later during this shot. In contrast, with the plasma gun active (shot #20957), the shear between channels 2 and 3 [as indicated in Fig.1(b)] is maintained for at
030002-4