Impurity transport studies in the Madison Symmetric Torus reversed-field pinch during standard and pulsed poloidal current drive regimes

Abstract

The transport of intrinsic impurities is investigated during standard and improved confinement regimes of the Madison Symmetric Torus (MST) reversed-field pinch. The impurity diffusion coefficient (D) and pinch velocity (v) are obtained through comparing the time evolution of experimental impurity density profiles with the results of a one-dimensional impurity transport code. Experimental hollow fully stripped (C, O, B) ion populations in improved confinement discharges are reproduced with the transport code indicating outward convection of impurity ions. Estimated D and v are low and close to classical values. Standard MST discharges are characterized by a high level of stochasticity and nearly flat radial profiles of the fully stripped carbon. To reproduce this flat impurity profile a high outward convective velocity and high central D are assumed in the simulation.

Determination ofZeffby integrating measurements from x-ray tomography and charge exchange recombination spectroscopy

“…As shown in figure 6, = ± Z 2.3 0.1 eff in the core of these discharges, with a hollow radial profile peaking to = ± Z 3.3 0.2 eff near mid-radius. The core value of Z eff as determined from this method is in agreement with the estimate from CHERS only measurements in similar discharges [23]. As a result of using thick filters, 421 μm and 857 μm, the field of view of the SXR tomography system is limited to inside of ∼ r 0.3 m. Thus, Z eff cannot be determined outside of that radius from these data.…”

Measurements of Impurity Transport Due to Drift-Wave Turbulence in a Toroidal Plasma

“…The observed Doppler shift is therefore due to the radial velocity of the plasma flow. The C 2+ 464.7 nm emission shell, shown in light blue, is calculated using the charge-state profiles obtained from previous analysis using a time-dependent 1D collisional-radiative impurity transport code [26]. The location and extent of the predicted emission profile are consistent with radiance measurements taken across the poloidal cross section.…”

“…The measured fluctuation-induced carbon flux is directed inward, which is expected for an edge-peaked impurity profile. A practical concern for the RFP is that inward impurity transport opposes favorable temperature screening of impurity ions for the RFP configuration's native classical collisional transport [24][25][26]. For example, this could increase the plasma's electrical resistivity, η ∼ Z ef f , and adversely affect inductive control of MHD tearing instabilities, apart from the basic concerns related to impurity accumulation in a fusion plasma.…”

“…Similarly, the rms amplitude of v r is measured using groups of channels taken equally from either side of the peak to maximize sensitivity to blue and red-shift, which yields | v r | = 0.8 ± 0.2 km/s. Previous impurity particle balance analysis for MST plasmas yielded estimates for the fully-stripped C 6+ diffusion coefficient, D ex , and the (outward) convection velocity, v ex [26]. The coefficients for classical collisional transport, D cl and v cl , were also evaluated.…”

Using integrated data analysis to extend measurement capability (invited)

“…This approach takes advantage of all of the information available in absolute SXR brightness measurements as discussed earlier along with substantial information from previous impurity studies using the CHERS diagnostic. [24][25][26][27] Although the forward models have been described elsewhere, some time will be spent highlighting the information available that is incorporated into the analysis. The TS diagnostic is a multi-point, multi-pulse TS system measuring from the center of MST to the edge with 21 spatial points and a nominal temporal resolution of 2 kHz.…”