Page 5 - Robustness of waves with a high phase velocity

FIGURE 3. Normalized density fluctuation level ñ/n without plasma gun; (a) in the SOL; (b) inside the FRC separatrix; (c) toroidal E× B velocity for four probed densities, without plasma gun; (d) density fluctuation level ñ in the SOL; (e) ñ inside the separatrix; (f) E× B velocity, with plasma gun active.
least for t ≤ 0.7 ms [Fig.3(f)], and fluctuation levels remain low [Fig.3(d,e)]. During the shaded time periods, the n=1 wobble causes the beam incidence angle ζ to switch sign, preventing proper interpretation of the DBS data.
Figure 4 shows the normalized rms density fluctuation level vs plasma radius, at different times throughout the discharge. Fluctuation levels are clearly always highest in the SOL, but increase in amplitude substantially between 280 μs and 340 μs, both in the SOL and in the FRC core without plasma gun [Fig. 4(a)]. In contrast, it is remarkable that core fluctuations remain low throughout the discharge when the plasma gun is active [Fig. 4(b)]. SOL fluctuations increase after 300 μs but always remain lower by a factor of 2-3 compared to the case without plasma gun (note the reduced ordinate scale).
The toroidal turbulence wavenumber spectrum has been measured just inside and outside the FRC separatrix in the gun-assisted plasma (r~rs+5cm). Initial results have been reported [17] and a more detailed description will be provided in a separate publication [30]. In the SOL, the dependence on normalized toroidal wavenumber is best described by an exponential spectrum with a decay constant of αk~0.16 (the corresponding exponent for the decay of the fluctuation energy density [~(ñ/n)2] is ~0.33]). Exponential spectra have also been observed in the core of tokamak low confinement mode (L-mode) plasmas for lower normalized (poloidal) wavenumber, [31,32]. In both the FRC and the tokamak plasmas referenced here, (ñ/n)2 decays by roughly 3 orders of magnitude as increases by a factor of 5. This may indicate that the fluctuation energy cascades towards high wavenumbers, and that the viscous damping at small scales may be the dominant energy sink.
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