Operating Conditions to Achieve High Performance in PPCD in a Reversed-Field Pinch Plasma

Abstract

The behaviour of the pulsed poloidal current drive (PPCD) in a TPE-RX reversed-field pinch (RFP) device, under different operating conditions is presented. Three operating parameters to obtain high performance in PPCD are presented with statistical experimental evidence. RFP is a magnetic confinement system for thermonuclear fusion plasma. RFP has toroidal and poloidal magnetic field components of the same order of magnitude. While the toroidal current is sustained by an externally applied electric field, the poloidal current is sustained via dynamo activity through the fluctuations of velocity and magnetic field. These fluctuations are the consequence of magnetic tearing instabilities which create a stochastic magnetic field in the plasma core region, and enhance the energy transport. In order to suppress these tearing instabilities, PPCD was proposed and has been applied in four RFP devices, where the reduction of the magnetic fluctuations and the confinement improvement are observed. However, while the experimental methods for optimizing the performance in PPCD are stated in ref. 7, experimental results of the statistical tendencies have never been presented. This short note presents experimental results indicating that triggering time, filling pressure, and wall conditioning play key roles in achieving high performance in PPCD. We ran seven cases of the five-pulsed PPCD in TPE-RX. Each case contains 50 discharges. We varied the triggering time of the PPCD pulses, the filling pressure of deuterium gas (pD2), and the wall conditions. First, we changed the triggering time with pD2 1⁄4 67mPa after sufficient wall conditioning: CASE 1 (18.3, 22.0, 23.1, 24.2, and 26.7ms), CASE 2 (18.3, 22.0, 23.1, 24.2, and 27.7ms), and CASE 3 (18.3, 22.0, 23.7, 26.0, and 28.5ms). Second, we changed pD2 with a fixed triggering time the same as that of CASE 1 after sufficient wall conditioning: CASE 4 and CASE 5 (80 and 93mPa, respectively). Third, PPCD was operated with a fixed triggering time the same as that of CASE 1 and pD2 1⁄4 67mPa just after the vent of the vacuum vessel in order to observe the effect of wall conditioning: CASE 6 (after 72 discharges on the 5th day following repumping) and CASE 7 (after 10 discharges on the 4th day following repumping). The soft-X-ray (SXR) intensity is used to compare the performance of the PPCD. From previous works, it is evident that the SXR intensity increases as the confinement improves in PPCD. The histogram of the maximum intensity of SXR is shown in Fig. 1. Among all CASEs, CASE 1 is found to give the highest on-average SXR intensity. Upon changing the trigger time (CASEs 1, 2, and 3), the performance degrades as the pulses are delayed. Using the same triggering time (CASEs 1, 4, and 5), the best performance is obtained at the lowest pD2. Regarding the effect of the wall conditioning (CASEs 1, 6, and 7), sufficient wall conditioning, using more than 200 main discharge cleaning shots, provides the best performance. These tendencies are summarized in Figs. 2(a)–2(c). The effect of the triggering time is represented by the shotaveraged parallel electric field, Epar, in Fig. 2a), and the effect of wall conditioning is represented in terms of the shot-averaged line intensity of OV (278.1 nm), IOV, in Fig. 2(c). In Fig. 2, all of the values are calculated for shots with an SXR peak above 29ms (see below), and represent the average and standard deviation calculated in a time interval just before the first PPCD pulse (18ms) until 4ms after the last pulse. The positive trend in Fig. 2(a) indicates that higher intensities of SXR are obtained with higher Epar. Figure 2b) clearly shows a negative correlation between SXR intensity and pD2. A similar trend was also observed with the average value of D line intensity, ID . Finally, Fig. 2(c) shows a tendency that SXR intensity is improved as wall conditioning proceeds, i.e., as the oxygen content decreases. Looking at the temporal evolution of the SXR intensity, Fig. 1. Distribution of SXR in seven PPCD cases.