M. Blaumoser

Engineering Design of a Flexible Heliac Device: The TJ-II Experiment

The flexible heliac TJ-II, is a medium size stellarator (R = 1.5 m; a =0.2 m), wiyh relatively low field (1T), a strongly helical magnetic axis and a high degree of flexibility that will allow to explore a wide range of magnetic configurations of the heliac type (rotational transform ranges from 0.9 to 2.5). It has obtained EURATOM preferential support and is now in construction. In this paper the main characteristics of the engineering design are discussed. The engineering has succeeded in maintaining all the physics objectives with very small sacrifices. It features a fully welded vacuum chamber that leaves all coils in the outside. High geometrical precision required in the chamber and ease of assembly are granted through modular construction in octants, which are the building blocks to be assembled together on site. All coils are made of directly cooled copper conductors. TF coils are made splittable in order to ease assembly of the machine. Accessibility to plasma is quite good through eighty eight ports of ample dimensions.

PLASMA FACING COMPONENTS FOR TJ-II: THERMAL SHIELDS, BEAM STOPS AND POLOIDAL LIMITERS

The scope of this paper is to identify the critical issues related to the development of all the protective elements for the TJ-II vacuum vessel. The design studies include an analysis of the heat deposition patterns in different plasma configurations, transient thermal analyses during the heating and cooling phases and finite element structural analyses. Results show that the shield can properly withstand the predicted loads even without active cooling. Original engineering solutions are presented for the shields locking system and for the graphite tiles bonding technique.

THE TJ-II HARD CORE DESIGN AND FEASIBILITY

The TJ-II is a flexible heliac to be mounted at the Euratom/Ciemat Association Laboratory in Madrid, Spain. The hard core is the set of central conductors around which the plasma twists helically. This component constitutes what might be called the most critical part of the machine, since its close proximity to plasma places on it the requirement of strict tolerances. This paper describes the engineering design features of the hard core. Included are the main characteristics and the design details. A description of the manufacturing methods is also presented.

VACUUM VESSEL DESIGN FOR THE TJ-II HELIAC DEVICE

One of the most critical components of the TJ-II heliac device is the vacuum vessel (VV). The considerable effort expended in optimizing its design resulted in a modular structure surrounding the central conductor coils (CC/HX). This solution allows a high grade of geometric accuracy and reliability and enables the best surface treatments for ultra high vacuum.

ELECTRIC POWER CIRCUIT FOR PLASMA STABILIZATION IN ASDEX UPGRADE

ASDEX Upgrade, a reactor-relevant divertor tokamak now under construction at Garching, requires a sophisticated plasma stabilization system for both the horizontal and vertical position control owing to the strongly elongated plasma. On the basis of nonlinear circuit simulation it is shown that 4-quadrant, phase-controlled thyristor converters operating in circulating-current control mode can meet these requirements. Two pairs of stabilizating coils for active plasma position control are installed. That pair of coils, located between the vacuum vessel and toroidal coils, are inductively coupled to the plasma in a way such that in the event of plasma disruption, a current would be induced in the coil which exceeds the maximum permissible coil current limit. To avoid this, two force-commutated DC thyristor circuit breakers connected in series with these coils are installed. Maximum current and voltage per circuit breaker: 30 kA/1.6 kV. In addition, a pyrobreaker protects the coils in case of malfunctioning of the thyristor switch or the current control system. A short overview of the fast digital controller will also be given, together with the present status of construction.