Pulsed Poloidal Current Drive Research Articles

Improved confinement in the TPE-RX RFP by means of the PPCD

Pulsed poloidal current drive (PPCD) (Sarff J S et al 1994 Phys. Rev. Lett. 72 3670) is conducted in a reversed-field pinch (RFP) machine, TPE-RX. The PPCD yields a twofold improvement of poloidal beta and energy confinement time. A quiescent phase is observed in the magnetic fluctuations, δb, during the PPCD. The result is discussed in terms of the change of the equilibrium configuration along the F-Θ trajectory (F and Θ are the reversal and pinch parameters, respectively). Representative mode amplitude is numerically simulated. The result indicates that a transient nature of the PPCD, where τPPCD (characteristic time of the PPCD operation) <<τD(0) (resistive diffusion time of the core) holds, allows a trajectory with a deeper F which yields a less turbulent configuration than shot-by-shot F-Θ scans. It is shown that the improvement ratio of τE approximately scales as δb-2 for five cases of the PPCD experiments in three RFP machines, including the present work in TPE-RX.

Analysis and modelling of plasma response to pellet injection in RFX

Pellet injection experiments have been performed on the RFX reversed field pinch (RFP). The main aim of the experimental campaign was the study of the target plasma response to get information both on the possibility of an efficient pellet fuelling of the discharge and on the associated transport modification. In this paper, we present an analysis of different experimental scenarios. The tool employed is a one-dimensional transport code coupled with an ablation calculation that yields the pellet particle source and trajectory. It is seen that, in standard plasmas, the pellets that reach the plasma centre trigger a phase of rapid profile flattening and poorer particle and energy confinement, and an increase of the transport coefficients is required to reproduce the post-pellet behaviour of the profiles. On the other hand, the slow pellets that suffer large deflection and miss the plasma centre do not have such a deleterious effect and, in this case, the unperturbed transport coefficients are able to reproduce the post-pellet evolution of the profiles. Finally, when the pellets are launched in combination with some method that induces an improved confinement, the transport degradation is less evident. In our case, pellet injection has been combined with pulsed poloidal current drive (PPCD), a technique that reduces magnetic field stochasticization and transport in the RFP core. When the effect of the PPCD is strong enough, a synergy between pellet injection and PPCD is observed and plasma fuelling without dramatic confinement reduction is possible.

Issues in the plasma wall interactions in RFX

The standard intrinsic nature of transport in reversed field pinches entails that remarkable ohmic power input (20–80 MW) is involved in the all-graphite, MA level RFX device. MHD instabilities restrict the region of plasma-wall interaction (PWI), where power densities >100 MW m −2 may be locally reached. Means for handling such power have been successfully devised to avoid carbon bloom and allow reproducible experiments around 1 MA of plasma current. Improved confinement schemes (quasi-single helicity, pulsed poloidal current drive (PPCD)) corresponding to a more ordered magnetic configuration have opened new operational perspective in which PWI is a less severe problem. However, in general, plasma contamination is not a specific issue. Screening processes have been associated with low ionisation length to Larmor radius ratios and with the presence of an edge radial electric field. Impurities appear to have a role in the formation of the radial electric field by enhancing the ion prompt losses.

Effects of pulsed poloidal current drive on the edge region of areversed field pinch plasma

The plasma edge region of the RFX reversed field pinch experiment has beenstudied during pulsed poloidal current drive (PPCD) operation at low plasmacurrent. The edge region is largely affected during PPCD, and evidence ofimportant modifications of electric and magnetic field and ofE×B drift velocity are reported. A substantialdecrease of electrostatic transport has been observed during the PPCD phaseand this effect is ascribed to a combination of displacement and plasma columnshrinking.

Study of the scaling of magnetic fluctuations in the RFX reversed fieldpinch1

The scaling of magnetic fluctuations with the Lundquist number (S) isstudied in the RFX reversed field pinch (RFP) device for different operationalregimes. For the first time in a RFP experiment a complete set of diagnosticsto measure all the quantities needed to estimate S is available. Due to thehigh plasma currents (up to 1.1 MA) obtained in RFX, a broad range of Svalues is available for scaling. The results are compared with numericalMHD simulations (SPECYL code) providing a reasonable agreement(αexp = -0.18±0.02, αSPECYL = -0.22). Themodification to the expected scaling law due to viscous effects is alsodiscussed.We also perform a comparison of the amplitude of magnetic fluctuationsin discharges with different characteristics: standard, pulsedpoloidal current drive (PPCD), quasi single helicity and rotating lockedmode induced by an external rotating toroidal field modulation. Areduced level of fluctuations is obtained during PPCD operations, althoughthe dependence on S is the same. Concerning confinement time, at fixedI/N≈3×1014 A m, a favourable scaling with S is found. Itsinterpretation in terms of the simple Rechester-Rosenbluth model is brieflydiscussed.

Experimental measurements and modelling of the structure of the radial electric field in RFX

The experimental determination of the radial electric fieldEr and the associatedE ×B drift velocity at the edge of RFX is presented and possible mechanisms responsible for its generation are discussed.Er measurements by means of an array of Langmuir probes and those deduced from Doppler spectroscopy of impurity lines agree fairly well. In particular the rotation velocity of the plasma edge has been determined from the Doppler shift of a C III emission line. The observation of other ions characterised by different radial positions, such as B IV and C V, has allowed an estimate of the velocity shear. Typical values of plasma rotation at the edge are around 10 km/s while the velocity shear is of the order of (105−106)s−1 in the spontaneous layer, a few cm thick, usually observed in standard discharges. Plasma rotation has been artificially modified by both positive and negative edge biasing and the associated increase or decrease of the fluid velocity is well in agreement with the radial electric field change. The modification ofEr during edge biasing and Pulsed Poloidal Current Drive (PPCD) are also reported and interpreted within a momentum balance model. Analytical and self-consistent Monte Carlo models at the plasma edge suggest that impurities have a relevant role in the generation of the radial electric field, due to their relatively large Larmor radius.

Improved confinement and transport studies in the reversed field experiment (RFX)

This paper presents results obtained on the Reversed Field eXperiment, RFX [G. Rostagni, Fusion Engineering and Design 25, 301 (1995)], on the study of local transport properties of the RFP configuration both in the standard regime and during improved confinement phases obtained with poloidal current drive. The plasma core is dominated by magnetic turbulence, as indicated by both energy and particle transport analysis. Diffusivity decreases toward the edge and is reduced in the core at high Lundquist number. The edge region is apparently not affected by dynamo mode turbulence and determines global confinement in standard operation. Particle and heat diffusivity in this region show a favorable dependence with increasing density. The most interesting feature of the edge region of RFX consists of a sheared radial electric field with shearing rate comparable to that observed in tokamaks. Indications of turbulence reduction in the shear layer have been obtained. Enhanced energy confinement has been produced by pulsed poloidal current drive (PPCD). A clear reduction of the dynamo activity has been observed with a decrease of core thermal diffusivity to level comparable to the edge and global confinement improvement up to a factor of 3.

逆磁場ピンチプラズマにおける閉じ込め研究の現状と閉じ込め改善モード

Recent results of confinement studies and attempts to improve the confinement in reversed field pinch plasmas (RFP) are reviewed. Several scaling studies of RFP energy confinement time are summarized and the values of the confinement time of RFPs are compared with ITER−89P L-mode scaling. A new scaling law recently proposed is also described. Results of transport studies in RFPs are summarized and it is shown that the stochasticity of magnetic field lines caused by the magnetic fluctuations of dynamo action probably plays a dominant role in the core of RFP plasma. Recent Lundquist number scaling study for magnetic fluctuations is also described. Promising experimental and simulation results are introduced which are conducted to improve the RFP confinement. For examples, five-hold improvement of the energy confinement time is obtained in MST by driving the poloidal current in outer region with the pulsed poloidal current drive (PPCD). The increase of poloidal beta up to 20% is obtained in the Improved High Theta Mode in TPE−1RM20. Improved confinement by a flow shear is also observed in RFPs as in tokamaks and helical systems. Finally experimental and computational results are described which may indicate the bifurcation from the dynamo with multiple helical magnetic fluctuations to that with a single helical fluctuation through the increase of Prandtl number. In the case with a single helical fluctuation, the stochastic region caused by the over-lapping of magnetic islands can be reduced and the confinement degradation by the stochasticity of magnetic field lines may be avoided.