Edge turbulence in RFX-mod virtual-shell discharges

Edge turbulence scaling in RFX-mod as measured using GPI diagnostic

“…The emission of HeI line indicates that random 'bursts' emerge from the background turbulence. Since contiguous LoS's show a similar patterns, just shifted in time, there is evidence of a toroidal propagation of these bursts in the direction opposite to plasma current, the same as the E Â B drift flow [11,14]. The phase toroidal velocity v / of the fluctuations (from here on, for short, 'fluctuation velocity') is evaluated by one dimensional time-delay-estimation velocity technique, that infer the velocity of plasma fluctuations using the cross-correlation between spatially separated signals [15].…”

“…To count the number of emission bursts per unit of length we consider their number DN for each time scale s in a time window Dt, the linear density of emission structure is evaluated as N(s) = DN/(Dt Á v / ) at fixed s. In order to study the global effect on the particle losses, we need the total linear density of the structures N tot = P s N(s), by summing N(s) over all the turbulence characteristic time scales 1 ls < s < 10 ls. The lower limit is set by the diagnostic setup and by the turbulence' characteristic times as evaluated from its power spectrum [11]; the upper limit has been chosen empirically as well: no structures spanning times larger than this are observed and it corresponds at the characteristic frequency f * of power spectrum from which the power-law decay begins (f * % 100 kHz in RFX-mod). The behavior of N tot vs n/n G has been reported in Fig.…”

“…The data presented in this work are taken in discharges with plasma current I / = 0.3-1.1 MA, corresponding to 10-30 MW of ohmic input power, with an average electron density range of 0.5-6 Á 10 19 m À3 . Core electron temperature ranges between 200 and 500 eV (with records up and above 1 keV at higher plasma current and low n/n G ) and pulse lengths are up to 0.35 s. One of the parameters that affect the edge turbulence in RFP devices is the local distance k w (function of the angle /, h) between the last closed magnetic surface and the wall [11], which quantifies the local plasma deformation. Its value is negative when there is a gap between the well-confined plasma and the wall.…”

Invited Review Article: Gas puff imaging diagnostics of edge plasma turbulence in magnetic fusion devices

“…77,79,[95][96][97][98] The light fluctuations seen with GPI were compared with a thermal helium beam line ratio diagnostic of density and temperature fluctuations made using the same helium gas puff. On the TPE-RX field reversed pinch in Japan, the GPI system was similar to that in RFX-Mod, and there was a retractable Langmuir probe array at the same location as the GPI gas cloud for cross-comparison.…”

Feasibility of electron Bernstein wave coupling via O-X-B mode conversion in the RFX-mod reversed field pinch device

“…RFX-mod discharges are characterized mainly by two classes of density fluctuations: high-frequency fluctuations (≈ 100 kHz) at high amplitude (up to 100%) but with long correlation length (λ z = 10 ÷ 20 cm), and low-frequency fluctuations at lower amplitude (≈ 10%) and with shorter λ z (1.5 ÷ 4.5 cm) [24]. …”