Equilibrium evolution in oscillating-field current-drive experiments

Abstract

Oscillating-field current drive ͑OFCD͒ is a proposed method of steady-state toroidal plasma sustainment in which ac poloidal and toroidal loop voltages are applied to produce a dc plasma current. OFCD is added to standard, inductively sustained reversed-field pinch plasmas in the Madison Symmetric Torus ͓R. N. Dexter et al., Fusion Technol. 19, 131 ͑1991͔͒. Equilibrium profiles and fluctuations during a single cycle are measured and analyzed for different relative phases between the two OFCD voltages and for OFCD off. For OFCD phases leading to the most added plasma current, the measured energy confinement is slightly better than that for OFCD off. By contrast, the phase of the maximum OFCD helicity-injection rate also has the maximum decay rate, which is ascribed to transport losses during discrete magnetic-fluctuation events induced by OFCD. Resistive-magnetohydrodynamic simulations of the experiments reproduce the observed phase dependence of the added current.

What is a Reversed Field Pinch?

“…A path to escape the constraints of full self-organization of the RFP plasma is to drive the plasma either impulsively or by sustained current drive [17,18]. This line of research is being very active on the MST machine [19].…”

Validation of single-fluid and two-fluid magnetohydrodynamic models of the helicity injected torus spheromak experiment with the NIMROD code

“…2,3 Traditional ohmic drive used on tokamaks is one example of magnetic helicity injection. Other methods include (electrostatic) coaxial helicity injection (CHI) used on spheromaks [4][5][6][7] and spherical tori, [8][9][10] and inductive methods like the oscillating field current drive (OFCD) used on the Madison symmetric torus (MST) 11 reversed-field pinch (RFP 12 ), and steady inductive helicity injection (SIHI) on the Helicity Injected Torus experiment (HIT-SI). 13 It is not clear how these complex processes, which are often difficult to measure and quantify, drive the plasma to a relaxed state.…”

Simultaneous feedback control of toroidal magnetic field and plasma current on MST using advanced programmable power supplies

“…In the context of F control, although it is possible to tune a pre-programmed B T waveform shot-to-shot to approximately achieve the desired waveform, as in the black signal in Figure 9, shot-to-shot changes in the time evolution of B T still affect the ratio F , and better control can be achieved with feedback on the real-time signals, as discussed above. The superiority of closed-loop over open-loop control is expected to be especially important in the cases of OFCD [4], PPCD [31], and SSRD [7].…”

“…Programmable power supplies are used in many fusion experiments, but the RFP has the special challenge of large power flow between the poloidal and toroidal magnetic field circuits via nonlinear relaxation processes regulated by the plasma. This is particularly acute with oscillating field current drive (OFCD), where megawatts of reactive power oscillate between the circuits and regulated by the plasma [4]. Precise phase control of the AC toroidal and poloidal inductive loop voltages is essential for OFCD.…”