Pulsed Poloidal Current Drive Research Articles

Microturbulence studies of pulsed poloidal current drive discharges in the reversed field pinch

Experimental discharges with pulsed poloidal current drive (PPCD) in the Madison Symmetric Torus reversed field pinch are investigated using a semi-analytic equilibrium model in the gyrokinetic turbulence code Gene. PPCD cases, with plasma currents of 500 kA and 200 kA, exhibit a density-gradient-driven trapped electron mode (TEM) and an ion temperature gradient mode, respectively. Relative to expectations of tokamak core plasmas, the critical gradients for the onset of these instabilities are found to be greater by roughly a factor of the aspect ratio. A significant upshift in the nonlinear TEM transport threshold, previously found for tokamaks, is confirmed in nonlinear reversed field pinch simulations and is roughly three times the threshold for linear instability. The simulated heat fluxes can be brought in agreement with measured diffusivities by introducing a small, resonant magnetic perturbation, thus modeling the residual fluctuations from tearing modes. These fluctuations significantly enhance transport.

Maximum Entropy Analysis of the 2D Density Turbulence Measured by MIR in TPE-RX

Two-dimensional (2D) turbulence measurement by using microwave imaging reflectometry (MIR) has been performed in the reversed field pinch (RFP) plasma in TPE-RX. In this work, the maximum entropy method (MEM) is used to estimate the 2D k-spectrum. The turbulences between the pulsed poloidal current drive (PPCD) and the high Θ plasmas are studied. The m = 1 modes are dominant in PPCD plasma, while in high Θ plasma the k spectrum expands to the high k range and the turbulence propagates in the electron drift direction. The preliminary result shows that PPCD plasma switches off the turbulence, and the high Θ plasma has strong turbulence.

Comparison of particle transport properties in TPE-RX standard and PPCD plasmas

The global transport properties of a magnetic confinement device are strongly affected by the level of magnetic and electrostatic fluctuations. Transport in the plasma core of low current (Ip ⩽ 0.6 MA) reversed field pinch devices is driven mainly by magnetic chaos associated with the presence of several MHD modes. In order to reduce the magnetic chaos, the technique of the pulsed poloidal current drive (PPCD) (Sarff et al 1994 Phys. Rev. Lett. 72 3670–3) has been applied to the TPE-RX plasma. This paper presents the comparison of the confinement properties in discharges with and without PPCD.It turns out that PPCD pulses show longer particle confinement time than standard plasmas. Moreover, for the first time, particle transport analyses have been carried out at TPE-RX both in standard and PPCD discharges to investigate the local particle diffusion coefficient. The comparison of the results confirms the strong reduction in the transport when the magnetic fluctuations are damped, in quantitative accordance with the theoretical predictions.

High Beta and High Density Operation in TPE-RX

A high poloidal beta, βp, was achieved using pulsed poloidal current drive (PPCD) in a toroidal pinch experiment-RX (TPE-RX). The plasma electron density and temperature increased, hence improving βp from 5 to 30% during PPCD. βp is almost equal to the total beta in the reversed-field pinch (RFP). D-alpha emission from recycling deuterium by the plasma-wall interaction decreased during the PPCD. An improved particle confinement time was indicated by a ten fold increase in the ratio of the total number of particles to the D-alpha emission. Single pellet injection into the good particle confinement plasma, achieved by the PPCD, produced high density plasma with high beta values. The plasma electron density rapidly increased to triple that of standard plasma, and high density was maintained till the end of the PPCD period. The electron temperature was lower than that of PPCD without ice pellet injection, but βp remained almost the same because the plasma electron density was higher with pellet injection.

Microwave Imaging Reflectometry Experiment in TPE-RX

Microwave imaging reflectometry (MIR) was developed in TPE-RX, one of the world’s largest reversed field pinch (RFP) devices. The system optics are made of aluminum mirrors, Teflon lenses, and Plexiglas plates in order to reduce size. In this system, frequencies are stabilized so that noise can be reduced using narrow bandpass filters. A 4×4 2-D mixer array and phase detection system have also been developed. With this system, density fluctuations in the high-Θ RFP plasma, pulsed poloidal current drive (PPCD) plasma, and quasi-single helicity (QSH) plasma are observed in TPE-RX. This is the first MIR experiment in an RFP device.

Turbulence and particle confinement in a reversed-field pinch plasma

The confinement properties of a reversed-field pinch (RFP) device are limited by the high level of magnetic stochasticity produced by the interaction of tearing modes in the plasma core. A way to reduce the core magnetic stochasticity is the application of the pulsed poloidal current drive (PPCD) technique. By applying the PPCD, tearing modes are stabilized, magnetic stochasticity and turbulence are reduced and hence the confinement is enhanced. In the TPE-RX device, standard RFP plasmas are characterized by magnetic and electrostatic edge turbulence dominated by intermittent phenomena; on the contrary, the improved confinement regime generated by the PPCD is characterized by turbulence with a self-similar nature. We show that the increase in τp is well correlated with the change in the nature of both the magnetic and the electrostatic turbulence. While plasmas with low confinement are dominated by intermittent events and by magnetic and electrostatic activity with a non-self-similar nature, plasmas with high confinement show turbulence with a self-similar character.

High-Beta Plasma Confinement in TPE-RX During Pulsed Poloidal Current Drive Operation in Reversed-Field Pinch Plasma

A very high beta value was obtained during pulsed poloidal current drive (PPCD) operation in a reversed-field pinch (RFP) plasma during a TPE-RX experiment. Poloidal beta is almost equal to total beta in the RFP, and is as high as 30% near the end of PPCD operation. During the PPCD operation, the magnetic fluctuation associated with the dynamo effect is reduced, and improved confinement is realized, which results in this high beta value.

Role of locked mode in the effectiveness of pulsed poloidal current drive regime in the reversed-field pinch

The close relationship between the locked mode (LM) and pulsed poloidal current drive (PPCD) regime in a reversed-field pinch device is described. If the modes lock close to the shell gap then the LM is enhanced and there is a high probability that the PPCD will not improve plasma performance. The physical reason will be discussed. If the position of the locked mode is far from the shell gap then the PPCD is effective with a high probability. LM energy is reduced and the more the LM energy decreases, the better the plasma performance.

Magnetohydrodynamic Simulation of Pulsed Poloidal Current Drive in Reversed Field Pinch Plasmas

Numerical simulations using magnetohydrodynamic (MHD) equations are performed for the pulsed poloidal current drive (PPCD) in reversed field pinch (RFP) plasmas. The PPCD is simulated by reducing the toroidal magnetic field at the wall, B z ( a ), in the short period. The simulations take into account the values of reduced B z ( a ), the period of changing B z ( a ) and the start time as the characteristic parameters of the PPCD. The effects of the parameters on m =1 resistive modes, which play the main role in the sustainment of RFP configurations, are calculated. The PPCD has stabilizing effect for the m =1 modes over a wide range of the parameters. The dependence on the parameters for the formation of quasi-single helicity after the PPCD and for the growth of the m =0 mode is calculated. The stabilizing factor due to the enhanced magnetic shear for the resistive modes is also analyzed by the linear growth rate in the equilibrium model of the transient configuration during the period of the PPCD.

An empirical scaling law for improved confinement in reversed-field pinch plasmas

A database of the confinement properties of the toroidal pinch experiment (TPE) series reversed-field pinch (RFP) devices was established, and an empirical scaling law for the energy confinement time, τE, was deduced for a consistently selected set of the database (Yagi Y. et al 2003 Nucl. Fusion 43 1787). The scaling for τE [TPE-scaling; ], in particular, is a power law similar to that predicted on the basis of the tearing modes, as a function of the plasma minor radius (a), plasma current (Ip), and Ip/N, where N is the column density. On the other hand, the TPE-scaling has a strong dependence on the pinch parameter, Θ, namely τE ∼ Θ3. Recently, we have investigated the database of the improved confinement in the pulsed poloidal current drive (PPCD). We show that the TPE-scaling agrees well with the improved τE in the PPCD database, because of its strong Θ dependence. Namely, Θ3 is a factor of merit for RFP plasmas. We discuss why this agreement is obtained in spite of the general difference between the underlying transport mechanisms of the standard and PPCD discharges. We also show that this improvement, represented by Θ3, is related to the increase in magnetic shear with Θ.

Performance improvement conditions and their physical origin in the pulsed poloidal current drive regime of the reversed-field pinch device TPE-RX

The application of the pulsed poloidal current drive (PPCD) technique in reversed-field pinch (RFP) devices leads to the improvement of the confinement properties of the plasma. In this article the conditions necessary to achieve high PPCD performance in the TPE-RX [Y. Yagi, S. Sekine, H. Sakakita et al., Fus. Eng. Des. 45, 409 (1999)] RFP device will be shown. One of the key parameters is the component of the electric field parallel to the magnetic field E∥. The positive trend between E∥ and the PPCD performance can be explained in terms of reduction of magnetic fluctuations. It will also be shown that important roles are played by the triggering time of the PPCD pulses, the filling pressure of the deuterium gas, and the wall condition.

Modelling of PPCD in the reversed field pinch RFX by the transport code RITM

The modifications in plasma parameters and heating power under conditions of pulsed poloidal current drive (PPCD) in Reversed Field eXperiment (RFX) are modelled by the one-dimensional transport code RITM (Radiation from Impurities in Transport Model). Several channels for the PPCD effects on plasma confinement are taken into account: dissipation of externally driven poloidal currents, reduction in the electric field induced by the dynamo and decrease in magnetic fluctuations. It is demonstrated that the external currents are dissipated at the plasma edge and they cannot, alone, lead to any change in the temperature profile. The reduction in the dynamo electric field results in central peaking of the current and Ohmic power densities, accompanied, however, by a very modest rise in the central temperature. By taking into account the decrease in the level of magnetic fluctuations (predicted for PPCD conditions by non-linear MHD simulations) and the corresponding reduction of transport in the stochastic magnetic field, RITM reproduces, in agreement with experiments, a significant increase in the central temperature and a reduction in the loop voltage.

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

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.

Current profile modification experiments in EXTRAP T2R

Pulsed poloidal current drive (PPCD) experiments have been conducted in the resistive shell EXTRAP T2R reversed-field pinch experiment. During the current profile modification phase, the fluctuation level of the m = 1 internally resonant tearing modes decreases, and the velocity of these modes increases. The m = 0 modes are not affected during PPCD, although termination occurs with a burst in the m = 0 amplitude. The PPCD phase is characterized by an increase in the central electron temperature (up to 380 eV) and in the soft x-ray signal. Spectroscopic observations confirm an increase in the central electron temperature. During PPCD, the plasma poloidal beta increases to 14%, and the estimated energy confinement time doubles, reaching 380 µs. The reduction in the fluctuation level and the corresponding increase in the energy confinement time are qualitatively consistent with a reduction in parallel transport along stochastic magnetic field lines.

Role of the m=0 modes in the edge region of a reversed-field pinch during pulsed poloidal current drive

Changes in the edge profile of soft x-ray emission during pulsed poloidal current drive (PPCD) experiments are in agreement with the modifications of the plasma surface, as seen from magnetic measurements. In particular, the m=0 modes resonant at the reversal radius have been analyzed. A comparison with an m=0 island reconstruction model shows that PPCD reduces the bulging associated with the m=0 modes by shifting inward the reversal surface, where the m=0 island is located. Regarding the total m=0 amplitude, there are evidences that during PPCD, on average, it does not change significantly. Additional data coming from standard discharges support the idea that two competing phenomena act during PPCD: one is the inward shift of the reversal, which destabilizes the m=0 modes; the other one is the decrease of the m=1 fluctuations, which acts in the opposite direction.

Analysis and modelling of the magnetic and plasma profiles during PPCD experiments in RFX

In this paper, we analyse the main features of the pulsed poloidal current drive (PPCD) technique, used in the reversed field pinch configuration to achieve improved confinement conditions. In the RFX experiment, PPCD corresponds to a decrease of the magnetic fluctuations, to a peaking of the temperature profile, and to a reduced transport and plasma–wall interaction. A three-dimensional MHD nonlinear code and one-dimensional time-dependent transport models have been applied to study the effect of PPCD on the magnetic and plasma profiles. The three-dimensional MHD simulations show that the external inductive drive pinches and peaks the current profile driving the configuration through a transient phase, where the spontaneous turbulent dynamo action is quenched. The one-dimensional transport codes indicate that the experimental profile modifications associated with PPCD are consistent with a reduction of the stochastic transport.

Increased confinement improvement in a reversed-field pinch using double-pulsed poloidal current drive

The pulsed poloidal current drive (PPCD) [J. S. Sarff et al., Phys. Rev. Lett. 72, 3670 (1994)] experiment is conducted in a reversed-field pinch device, the toroidal pinch experiment RX (TPE-RX) after providing an auxiliary power supply system with increased energy in the main power supply system for the PPCD. The PPCD system thus provides double-pulsed operation with higher current in the toroidal coil than that in single-pulsed PPCD operation in TPE-RX [Y. Yagi et al., Plasma Phys. Controlled Fusion 44, 335 (2002)]. The central electron temperature, ion temperature, and electron density increase during PPCD, and there is, on average, a fivefold improvement in energy confinement, τE, relative to standard discharges. Double-pulsed PPCD yields better performance than that of single-pulsed PPCD operation where twofold improvement in τE was obtained. It is shown that the enhancement factor of τE in the double-pulsed PPCD experiment in TPE-RX is consistent with the trends, observed previously, versus magnetic fluctuation amplitude and versus Δγ, where Δγ is the difference in γ [=(1−F)/Θ] between the start and the end of the PPCD period.

Comparison of ion temperature diagnostics on the Madison symmetric torus reversed-field pinch

There have been three ion temperature diagnostics operating on the Madison symmetric torus (MST) for the past two years: (i) Charge-exchange recombination spectroscopy (CHERS), which measures the temperature of fully stripped impurity (C6+) ions, and has a spatial resolution of ±1 cm and a time resolution of 3 ms; (ii) Rutherford scattering (RS), which measures the temperature of the bulk majority (D+) ions, and has a spatial resolution of roughly ±7 cm and a time resolution of 30 μs; and (iii) the ion dynamics spectrometer (IDS), which measures a chordal average of the temperature of partially stripped impurity (C4+) ions, with a time resolution of 10 μs. The first two diagnostics use neutral beams, while the third is passive. The classical ion energy equilibration time τii<1 ms between all ion species, so we naively expect that all ion temperature measurements should agree in steady state. Here we present simultaneous measurements of: CHERS and RS profiles in high-current burst-free pulsed poloidal current drive (PPCD) discharges; CHERS and RS profiles in standard high-current discharges; and IDS and RS profiles in standard low-current discharges. Measurements in standard discharges are made a long time before and after magnetic reconnection events, during which there is believed to be a large, nonclassical input of energy to the ions. RS and CHERS measurements consistently agree; IDS measurements are consistently less than RS measurements, due to the effect of the C4+ emission profile on the IDS measurements.

A statistical analysis of pulsed poloidal current drive in the Reversed Field eXperiment

Pulsed poloidal current drive (PPCD) experiments have been performed in the reversed-field pinch device RFX [G. Rostagni, Fus. Eng. Design 25, 301 (1995)], with various control techniques of the edge toroidal field. A detailed statistical analysis has been performed over ∼430 RFX pulses and is presented in this paper. We have found that the temperature increase and m=1 fluctuations reduction scale with the applied electric field Eθ, which is a control parameter for plasma performances. Soft x-ray imaging shows an increase in the core, where often an m=1 helical coherent structure appears, similar to those observed during quasi-single helicity states in RFX. These phenomena are ultimately linked to the magnetic turbulence reduction and q profile modifications that take place during PPCD in the reversed-field pinch.

Improved Confinement Mode in Reversed-Field Pinch Plasmas

The improved confinement has been recently observed in reversed-field pinch (RFP) plasmas using the pulsed poloidal current drive (PPCD) technique. The record highest τE =10 ms in RFP, has been achieved in the Madison Symmetric Torus using PPCD. The article explains general characteristics of RFP configuration, present status of confinement properties in RFPs under the standard operating conditions, and the principle and the main results of the PPCD experiments in RFPs. The improved confinement observed in PPCD provides a hint to obtain confinement improvement in magnetic confined fusion plasmas.