K. Kurihara

A new shape reproduction method based on the Cauchy-condition surface for real-time tokamak reactor control

A new shape reproduction method is investigated on the basis of an applied mathematical approach. An analytically exact solution of Maxwells equations in a static current field yields an (boundary) integral equation. In application of this equation to tokamak plasma shape reproduction, it is made clear that a Cauchy condition (both Dirichlet and Neumann conditions) on a hypothetical surface is necessarily identified. To calculate the Cauchy condition using magnetic sensor signals, conversion to numerical formulation of this method is conducted. Then, reproduction errors by this method are evaluated through two numerical tests: The first test uses ideal signals produced from a full equilibrium code in the JT-60 geometry, and the second test uses actual sensor signals in JT-60 experiments. In addition, it is shown that positioning and shape of the Cauchy condition surface is insensitive to reproduction error. Finally, this method is clarified to have preferable features for real-time tokamak reactor control.

Improvement of Tokamak Plasma Shape Identification with a Legendre-Fourier Expansion of the Vacuum Poloidal Flux Function

In this paper, a method to identify the shape of tokamak plasmas with a Legendre-Fourier expansion of the vacuum poloidal flex function in toroidal coordinates is improved for the JT-60 Upgrade plasmas. These are pulse plasma discharges that have different sizes, positions, shapes, and internal quantities. The method is based on an analytical solution of the Grad-Shafranov equation in a vacuum region using toroidal coordinates. Although many identification methods previously proposed allow very small perturbations of certain parameters of the nominal plasma, the method presented in this paper can relax the identification restriction on plasmas. hence, it is applicable to accurate feedback control and real-time visualization of various plasma configurations.

The JT-60 central control system

This paper describes the central control system of the JT-60 tokamak. This control system not only has to organize plasma operation, but also has to utilize devices under isolated operation. Therefore the control system has been designed in a special way.

Feedback control of radiation region in radiative divertor plasma on JT-60U tokamak

The sustainment of radiative divertor plasma in JT-60U is performed by the feedback control of the gas puffing rate into the divertor region with an actuator of the divertor radiation power. The feedback control system switches the piezoelectric gas valve every 10 ms, so that the proper amount of gas flux is fed for divertor radiation power to follow the reference waveform. The divertor radiation of 6 MW, which is about 40% of the heating power, is sustained for about 3 s by the hydrogen gas puffing with feedback control. The present logic of the feedback system, however, is not sufficient to fully control the divertor radiation which is modified by the change in heatings and plasma parameters. In order to prevent the loss of control, multi actuators for the feedback control, for example, the involvement of the heating power, and the selection of observed bolometer channels may be required.

Eddy current effect study on the JT-60 plasma equilibrium control

Abstract The A-ф method (A is the magnetic vector potential and ф is the scalar potential) was applied to eddy current analysis in the JT-60 tokamak fusion device. This analysis agrees with the coil excitation experiments much better than the filament-current-coil approximation. The accuracy of the A-ф method is estimated by comparing it with the analytical solution of magnetic field penetration into an infinitely long cylindrical conductor. A JT-60 plasma equilibrium-control-dynamics (ECD) model including eddy currents induced in the vacuum vessel is proposed for control system design and analysis. Considering the plasma-vessel interaction, the one-point plasma model and its equilibrium force balance are assumed. Comparison of the simulation and experimental data identifies the essential points for reproduction of the plasma ECD: (a) The magnetic field induced by the plasma motion is proportional to its velocity. (b) Magnetic field penetration is regarded as a response of a first-order differential equation. Application limits of the employed model are also identified: (a) Plasma volume effects are not accurately involved. (b) The influence of eddy current on shape evolution is not taken into consideration.

Remodeling of JT-60 discharge control system

Remodeling of the JT-60 discharge control system (DCS) is initiated to minimize the cost and time for hardware maintenance and software development. Three principles have been adopted for remodeling. (1) The functions of the DCS must remain intact. (2) The shutdown period must be minimized to maximize JT-60 experiment time. To realize the new DCS, (3) a combination of the host workstation (WS) and target Versa Module Europe (VME)-bus system with real time OS was chosen as the fundamental structure. The prototype DCS was designed and tested to evaluate the performance of the converted discharge sequential control software modules and the message communication program. The new configuration of the DCS and procedure of smooth remodeling without shutdown is described in detail.

TOKAMAK PLASMA SHAPE IDENTIFICATION BASED ON BOUNDARY INTEGRAL EQUATIONS AND THE REAL-TIME SHAPE VISUALIZATION SYSTEM

A tokamak plasma shape identification method based on boundary integral equations (BIE method) (ref. 1) using magnetic sensors is discussed in application to plasmas with low K to high K. This BIE method, the filament-current-plasma (FCP) method (ref. 2) and the Legendre-Fourier eigenfunction expansion (LFE) method (ref. 3) are applied to realtime visualization of plasma shape. The flux function value at a point × in the three methods is calculated in the same-type formula using poloidal field coil currents, magnetic sensor signals and coefficients depending on x. The design of a realtime visualization system is presented and the performance test results are reported.

APPLICATION OF THE FAST ARRAY PROCESSOR FOR JT-60 PLASMA CONTROL

Improvements of faster and more accurate computation in the JT-60 feedback control system are required for control of high beta plasmas which are produced in the additional heating experiment. This paper reports how we design its upgrade control system. The feedback control system is modified to be a multi-step pipelined system which is composed of two minicomputers for control calculation and a fast array processor, which calculates the state variables such as plasma position and shape prior to the control calculations.

SYSTEM DESIGN OF ZENKEI, THE CENTRAL CONTROL SYSTEM OF JT-60

ZENKEI, the central control system of JT-60 is in the final design stage. Since the last report we have been extending the system to make it possible to control the plasma parameters such as density and temperature. This paper gives a brief description of the most recent design, especially those of the modification of the discharge control system. An additional computer and dual port memory with D-port were adopted. For event-oriented control of plasma discharges, the timing system was improved.

SAFETY DESIGN OF THE JT-60 CONTROL SYSTEM

The control system of JT-60 consists of several computer-CAMAC combinations, timing systems and hard-wired interlock systems. The functions of the control system are roughly represented by discharge control including real-time feedback control, plant operation support and safety interlock.