L. Pacios

Overview of TJ-II flexible heliac results

The TJ-II is a four period, low magnetic shear stellarator, with high degree of configuration flexibility (rotational transform from 0.9 to 2.5) which has been operating in Madrid since 1998 (R = 1.5 m, a 0.22 m, B0 = 1T , PECRH600 kW, PNBI3 MW under installation). This paper reviews the main technical aspects of the TJ-II heliac as well as the principal physics results obtained in the most recent TJ-II experimental campaign carried out in 2000.

Orbital partitioning errors in transition metal relativistic effective potentials

The large errors in atomic excitation energies of the first transition elements (seen in SCF calculations using Ar-core relativistic effective potentials) can be rationalized in terms of the partitioning of the 4s orbital as required to form nodeless pseudoorbitals. We are able to show that with the partitioning revised to better represent the 4s electron density (and therefore critical coulombic interactions) in the region dominated by the 3s subshell, errors can be dramatically reduced. Such potentials would make large transition metal cluster calculations simpler and might be appropriate for quantum Monte Carlo work.

A versatile Timing System based on OS9 for the Spanish stellarator TJ-II

We describe the Timing System for the TJ-II Stellarator, which is presently under construction in Madrid (Spain), and which is expected to start operation in 1996. The Timing System is an essential element of the TJ-II, that has been commissioned, designed, and built to provide synchronization for all the subsystems, diagnostics and auxiliary heating systems of TJ-II. Its structure is both centralized and distributed. The PSK (phase shift keying) modulation technique is used to distribute simultaneously both timing and event information via fiber optic link. The system provides absolute timing references with a variable time resolution ranging from 500 ns to 1 ms, depending on the span time selected, but in all cases, with a precision of 500 ns. The system allow a wide variety of programmable operating modes and control features to be configured easily in a user friendly environment. These are used to generate the sequence of signals required during each experimental pulse. In addition a new set of libraries and C programs named TEMPO have been developed for the OS9 real time operating system, and a set of VME cards has been designed. Remote access via ethernet LAN and multi-user capabilities are also provided. The Timing System has been fully developed by the TJ-II Monitoring and Control Team at CIEMAT. This paper outlines the detailed design of the hardware and software and gives results from the test phase.

MONITORING AND DIGITAL CONTROL SYSTEM FOR THE 130MVA PULSE GENERATOR SYSTEM OF THE SPANISH STELLARATOR TJ-II

The power supply of the Spanish Stellarator TJ-II will work in a wide range of the currents and voltages in order to provide different magnetic field configurations. The DC currents in the coils of the stellerator are variable between 5% and 100% of their maximum values. A digital control system will allow to drive the coil current with high precision at the required values. The control system must provide stability of entire power supply which is a very important aspect for the reliable operation. The paper describes the monitoring and control system of the entire pulse power supply of TJ-II and concentrates on the digital DC current control.

Magnetic configuration sweep control in heliac type stellarators

A novel magnetic configuration sweep control system for the TJ-II heliac device is described. The system is prepared to establish a reference for plasma, torus or vessel current while the coil configuration currents are swept during a single plasma discharge. It can also be run in a special simulation mode, which does not require powering the TJ-II device, intended for commissioning and experiment preparation purposes. Preliminary tests with the system have shown its ability to perform rotational transform scans while establishing a waveform for torus current independently, despite variable plasma conditions. This opens up new experimental possibilities to study the influence of rotational transform and shear independently.

Construction, commissioning and first results of the Spanish Stellarator TJ-II

The Spanish Stellarator TJ-II is a highly flexible medium-size fusion device of the heliac-type constructed at CIEMAT in Madrid, Spain from 1991 to 1997. The magnetic field mapping at low magnetic fields were conducted through in December 1996 and January 1997 with excellent results. From January 1997 up to now the commissioning of the device up to the design values is being done together with the assembly of the basic diagnostic equipment and the auxiliary heating systems. The first plasma is expected still for October 1997.

TECHNICAL ASPECTS AND STATUS OF THE SPANISH STELLARATOR TJ-II

Abstract TJ-II is a medium-size stellarator of the heliac-type being under construction in Madrid, Spain. The project was granted EURATOM preferential support phase II in November 89 and is expected to start operation in 95. This device allows the exploration of a wide range of magnetic configurations. The rotational transform can be varied from 0.9 to 2.5. Shear variation is possible up to 10%. Theory predicts maximum beta values as high as 6%. The average major plasma radius is 1.5 m. The minor dimensions of the bean-shaped plasma are approximately 0.4 m by 0.2 m. Toroidal field: 1T. Pulse length: 0.2 s to 1 s. Pulse break: 5 min. Auxiliary heating: 400 kW ECRH at 53.2 GHz in stage I; 2 MW NBI will be added in stage II. In stage III 4 MW NBI heating is planned to be applied. The paper gives an overview on the technical features of the experiment, describes the main aspects of the recent engineering work and presents the status of the device.

Transport Properties in the TJ‐II Flexible Heliac

TJ‐II flexibility is exploited to perform an investigation on the transport properties of this device. Rotational transform can be varied in a wide range, which allows one to introduce low order rationals and to study their effect on transport. On the other hand, confinement properties can be studied at very different rotational transform values and for different values of magnetic shear: Experiments on influence of the magnetic shear on confinement are reported. Plasma potential profiles have been recently measured in some configurations up to the plasma core with the Heavy Ion Beam Probe (HIBP) diagnostic and the electric field values measured in low‐density plasmas are consistent with neoclassical calculations near the plasma core. Plasma edge turbulent transport has been studied in configurations that are marginally stable due to decreased magnetic well. Results show a dynamical coupling between gradients and turbulent transport.