Ken-ichi Hattori

High power neutral beam heating experiments on TFTR with balanced and unbalanced momemtum input

New long-pulse ion sources have been employed to extend the neutral beam pulse on TFTR from 0.5 sec to 2.0 sec. This made it possible to study the long-term evolution of supershots at constant current and to perform experiments in which the plasma current was ramped up during the heating pulse. Experiments were conducted with co and counter injection as well as with nearly balanced injection of deuterium beams up to a total power of 20 MW. The best results, i.e., central ion temperatures Tio > 25 keV and neo τE Tio values of 3 × 1020 keV sec m-3, were obtained with nearly balanced injection. The central toroidal plasma rotation velocity scales in a linear-offset fashion with beam power and density. The scaling of the inferred global momentum confinement time with plasma parameters is inconsistent with the predictions of the neoclassical theory of gyroviscous damping. An interesting plasma regime with properties similar to the H-mode has been observed for limiter plasmas with edge qa just above 3 and 2.5.

Initial results of RFP experiments in REPUTE-1

Experiments of a reversed field pinch device REPUTE-1 constructed in University of Tokyo started in 1984. The conductivity electron temperature and the line-averaged electron density increase approximately linearly with the increase of plasma current in the range up to 240 kA. The study of the effect of the bias toroidal field on the discharge characteristics shows that a higher bias field gives lower resistivity and a higher flat top plasma current. The toroidal flux at the current flat top tends to be a selective value independent of the initial bias field.

Remarkes on Relaxation Phenomena in Toroidal Discharge

MHD relaxation phenomena have been studied for a toroidal plasma in a safety factor q <1 regime. Ultra low- q (ULQ) equilibria with ( n +1) -1 < q < n -1 ( n ≧1) are setup through MHD relaxation, and are sustained quasistatically. A ULQ equilibrium has a q profile with d q / d r <0 ( r : minor radius) for a major region in the plasma, so that it is stable against global MHD modes. In the time-scale of the classical field-diffusion, the ULQ equilibrium deforms to take a peaked-current profile and destabilizes a global mode. Then a catastrophic transition to a higher q (lower n ) state is observed.

Magnetic field fluctuations in ripple reduction experiments on the REPUTE-1 RFP

The loop voltage of REPUTE-1 RFP is decreased by reducing the toroidal field ripple. The low frequency part of the toroidal field fluctuations, which is mainly composed of the m = 1 mode and has good coherence in space and time, is found to be reduced, especially outside the reversal surface, when a trimming field is applied. High frequency fluctuations, which have little coherence, are not influenced effectively.

M=1 Perturbation of Reversed Field Pinch Plasmas in the Presence of Field Errors Arising from Shell Gaps

Behaviors of m =1 kink modes of reversed field pinch plasmas are analyzed in the presence of field errors at shell gaps due to vertical field penetration. Perturbations in the vacuum and in the plasma are respectively expressed by using a class of modified Bessel functions and Bessel functions with m =1 modes and matched on the boundary. It has been shown that m =1 perturbation is amplified by the response of the plasma and its stability is deteriorated by the shell-gap field errors. Comparison with experiments is also discussed.

Asymmetric perturbations of toroidal flux in ramped-up discharges on REPUTE-1 reversed field pinch

Toroidally asymmetric perturbations of toroidal flux in a reversed field pinch (RFP) plasma have been observed with a set of 18 flux loops in ramping current discharges on the REPUTE-1 RFP. The toroidal flux perturbation is induced toroidally localized and spreads as it propagates in the electron-diamagnetic drift direction. The dominant toroidal mode number of the flux perturbation is usually 1, but becomes 2 when the toroidal flux is strongly generated, and the second peak merges with the first one as it propagates in the toroidal direction. Thus, a locally generated toroidal flux becomes toroidally symmetric.

Observation of m ≥ 2 modes in plasmas with 1/2 ≤ qa < 1

Internal modes with m = 1 and m ≥ 2, which localize around a pitch minimum, have been studied for ultra-low-q plasmas with 1/2 ≤ qa < 1 in REPUTE-1, where m is the poloidal mode number and qa is the safety factor at the plasma surface. One-dimensional stability analysis has shown that, for finite beta, m ≥ 2 modes have a relatively large growth rate compared with those of the m = 1 modes. The internal structure of the magnetic fluctuations with m = 1 and m ≥ 2 is found to be consistent with the numerically calculated eigenfunctions.

X-Ray Simulation of the Sawtooth Phenomena on TFTR

The m =1 island size or m =1 deformation of magnetic surfaces during sawtooth crashes is estimated on TFTR by comparing experimental observations of X-rays and ECE electron temperature with a simple simulation which calculates signals of line-integrated X-ray and local electron temperature assuming rigid rotation of the plasma. Sawteeth oscillations with a slow crash (τ crash ∼several ms) can be simulated by a Kadomtsev-type reconnection model in which an m =1 island grows to the size of the q =1 surface during the sawtooth crash. In the sawteeth which have a fast crash (50 µs∼τ crash ∼500 µs), the m =1 island size sometimes (not always) becomes ∼1/2 of the sawtooth inversion radius during the crash phase. The final structure, however, is hard to identify, because of the slow plasma rotation frequency. The “Hot Core” appears to move away from the plasma center in a Kadomtsev-type reconnection without causing very large transport or diffusion.

Application of multivariate data analysis method to plasma experiment.

Multivariate-analysis methods have been applied to plasma-data analyses. Many parameters obtained in plasma experiments are strongly coupled with each other, and are generally difficult to be resolved into simple physics laws. Multivariate-analysis methods are to find simple relations lying in data and to create scaling laws, which we may use, for example, to optimize operation parameters of devices. This paper introduces the factor-analysis method and describes an example of its applications to the REPUTE-1 RFP experiments.

Helically Assisted Low- q Tokamak with l =2 Helical Winding

An l=2, m=5 short-pitch helical field has been applied to low safety factor (q(a)~2) discharges in the TORIUT-5 tokamak. The externally applied helical field increases the safety factor effectively to avoid the current driven instabilities. An l=2 helical winding has the advantage, in comparison with an l3 system, of controlling the pitch near the magnetic axis and reducing the internal instability. The experimental results show a good agreement with theoretical estimates.