O. Vollmer

Rf ion sources for fusion applications: design, development and performance

Abstract The use of rf ion sources in neutral beam injection (NBI) systems offers reduced maintenance and cost due to the absence of filaments. For the first time high-power rf positive ion sources have been put in operation recently in a real NBI system on a tokamak. The performance of the new system is equivalent to that of a conventional one with arc discharge sources. Due to the relevance for ITER, rf sources are also being investigated in context with negative ions: 8.5 mA/cm2 H− have been achieved so far in a non-optimised version.

Progress in the development of a large RF negative ion source for fusion

Abstract A large area RF source for negative ion-based injection systems has been developed and for the first time H − -current densities have been extracted which are comparable to those produced by conventional arc sources. This paper describes the design of the source and gives the results of H − -extraction experiments. A feature of this RF source is an increase of extracted ion current by up to a factor of four if noble gases are added to the hydrogen discharge. By this effect, a current density of 9 mA/cm 2 at 0.65 Pa has been achieved in volume production. With caesium seeding up to 20 mA/cm 2 could be extracted at 1 Pa source pressure with the addition of argon and a hot plasma grid. Recently, the RF source has been modified for long pulse operation by the installation of a Faraday screen.

Recent developments of long pulse RF ion sources for NBI systems

Abstract For the new radial injector of the Wendelstein 7-AS stellerator, two medium size 40 kW RF sources (‘W7-AS RF source’) have been built and tested. A positive hydrogen ion beam of 28 A at 50 kV with a proton fraction of 60% has been extracted. For future injection systems, in particular those based on negative ions, large area beam extraction and long pulse capability up to cw operation are envisaged. At IPP two different design concepts of RF sources are being investigated in order to meet these requirements: in the first one, the plasma is generated in one or two small circular RF sources (‘drivers’), which are mounted onto the back plate of a 30 l expansion volume (‘bucket’) with magnetic multi-cusp confinement of the plasma (‘Type 6-1’ and ‘Type 6-2’ respectively). In the second concept the plasma is generated inside the bucket by an internal RF coil, which is insulated by a quartz tube (‘Type 5’). In both cases the applicability for negative as well as for positive ion production is being tested. The next application will be in the positive ion based injectors in the third injector of JET and in the W7X stellerator.

DESIGN OF THE NEUTRAL BEAM INJECTION SYSTEM FOR ASDEX-UPGRADE

For the ASDEX-Upgrade tokamak additional heating by long pulse neutral injection is forseen with a power of 6 MW when injecting hydrogen and 9 MW when injecting deuterium. This paper gives an overview of the injection system with the main emphasis on the mechanical parts. Additionally the basic concepts of the electrical systems and of the system control will be described.

Langmuir probe studies on a RF ion source for NBI

IPP Garching has been developing a RF ion source for H− production. In order to improve the data quality a new scanning probe system with passive RF compensation has been installed on the Type VI ion source on the BATMAN test stand. Using this probe, measurements have been carried out to study changes to the plasma parameters (electron density, electron temperature, and plasma potential) due to variation in the source operating conditions. The data were collected at a source pressure of 0.5 Pa and with 60±5 kW applied RF power. Presented are some of the results of these measurements, focusing on the effect of: argon seeding, addition of Cs to the source, and the newly added Faraday screen. The electron density behaves in a fashion that agrees with the theory of ambipolar diffusion. Typically there is little change to the average electron energy observed regardless of which effect is considered. The plasma potential shows the most significant changes with external source conditions, both in value for all cases and shape when the Faraday screen was added.

Initial Operation and Performance of the ASDEX Long-Pulse Injection System

The modified ASDEX neutral injection system started operation with long pulses (±2.5s) in spring 1987. The complete system has been operational since December 1987. Meanwhile more than 3000 ASDEX discharges have used neutral beam heating with a beam power of up to 80 % of the design value (3.2 MW). This paper reports the performance of the system in the first year of injection experiments with emphasis on the behaviour of the duct and the ion removal system.

PERFORMANCE OF THE FIRST ASDEX UPGRADE NEUTRAL BEAM INJECTOR

Plasmas of the ASDEX Upgrade tokamak have been heated with H 0 beams of up to 7 MW and D 0 beams of up to 10 MW. Beam modulation allows to inject at any power level between zero and full power. Measurements characterizing the NBI system performance, the power accountability, and the operational experience obtained so far are discussed.

MANUFACTURING AND ASSEMBLY OF THE NEUTRAL BEAM INJECTOR FOR ASDEX-UPGRADE

For ASDEX-Upgrade additional heating by long pulse neutral injection is foreseen with two injectors, each delivering a power of 6 MW with hydrogen and up to 9 MW with deuterium injection. This paper describes the mechanical design of the injectors and gives an overview of transportation and installation of the first injector in the torus hall.

Commissioning and first operation of the ASDEX Upgrade neutral beam system

The first injector of the ASDEX Upgrade neutral beam system has been commissioned and injection experiments started in June 1993. Within a few weeks an injected power of 5.2 MW (H/sup 0/), equivalent to 87% of the design power level, was achieved for pulse lengths of up to 1.5 seconds, and a variety of neutral beam heated discharges could be analyzed before the shutdown of ASDEX Upgrade in August.

Beam current feedback regulation of the RF neutral beam sources of ASDEX Upgrade

This paper reports on the technical implementation and the successful commissioning of the beam current feedback regulation system of the RF neutral beam sources for ASDEX Upgrade. The beam current is regulated by adjusting the RF input power. This is possible due to the fast response of the beam current to changes of the RF input power within a few milliseconds. With beam current feedback regulation, perveanced-matched operation of the sources can be maintained throughout the pulse, thereby increasing the beam quality and the source reliability.