T.E. O'Connor

TFTR neutral beam control and monitoring for DT operations

Record fusion power output has recently been obtained in TFTR with the injection of deuterium and tritium neutral beams. This significant achievement was due in part to the controls, software, and data processing capabilities added to the neutral beam system for DT operations. Chief among these improvements was the addition of SUN workstations and large dynamic data storage to the existing Central Instrumentation Control and Data Acquisition (CICADA) system. Essentially instantaneous lookback over the recent shot history has been provided for most beam waveforms and analysis results. Gas regulation controls allowing remote switchover between deuterium and tritium were also added. With these tools, comparison of the waveforms and data of deuterium and tritium for four test conditioning pulses quickly produced reliable tritium setpoints. Thereafter, all beam conditioning was performed with deuterium, thus saving the tritium supply for the important DT injection shots. The lookback capability also led to modifications of the gas system to improve reliability and to control ceramic valve leakage by backbiasing. Other features added to improve the reliability and availability of DT neutral beam operations included master beamline controls and displays, a beamline thermocouple interlock system, a peak thermocouple display, automatic gas inventory and cryo panel gas load monitoring, beam notching controls, a display of beam/plasma interlocks, and a feedback system to control beam power based on plasma conditions.

Operations with tritium neutral beams on the tokamak fusion test reactor

Abstract In November of 1993 the tokamak fusion test reactor began operating with a deuterium-tritium fuel mixture instead of the pure deuterium which it had used heretofore. The major portion of this tritium has been supplied as energetic neutral particles injected by the neutral beams. After an initial run in which some ion sources used a mixture of 2% T and 98% D to test tokamak systems, full tritium beam operations commenced, with some of the ion sources run on pure tritium and some on deuterium to optimize the fuel mixture in the core plasma. Hundreds of tritium source shots have now occurred, with reliability which is better than that typical of deuterium operation. The maximum power injected with deuterium and tritium beams was 39.6 MW. D-T fusion power levels of up to 10.7 MW have been produced. Energy confinement in D-T plasmas of the “supershot” variety appears to be better than in similar deuterium plasmas.

Neutral beam heating system for TFTR

The first two neutral injection beamlines have been installed on TFTR and are now operational. The operation of six ion sources producing either hydrogen or deuterium beams at energies up to 80 kilovolts has produced notable plasma heating results. Eventual neutral beam capability on TFTR should provide the energy necessary to demonstrate breakeven.

Tritium consumption and retention in TFTR neutral beams

750,000 Ci of tritium have passed through the TFTR neutral beamlines since December, 1993. During the course of 725 tritium heated discharges, 47,000 Ci have been extracted as ions from the ion sources and 27,000 Ci injected into TFTR as neutral atoms. Prior to the commencement of deuterium-tritium (DT) experiments, Los Alamos and Sandia National Laboratories made estimates of tritium retention in each beamline due to implantation in the 2 m/sup 2/ of copper beam absorbers and absorption on the 250 m/sup 2/ of internal surface area. Their estimates were: 500 Ci embedded per beamline in beam absorbers after 1000 DT Shots; and 100 Ci adsorbed per beamline after exposure to 1 torr of tritium for 1 hour. Both estimates were revised downward since the estimates assumed pure tritium operation, whereas the neutral beams used orders of magnitude more deuterium than tritium. Deuterium competes with tritium for implantation and adsorption sites, reducing the uptake of tritium relative to the use of pure tritium. Data from neutron detectors indicated that the quantity of tritium implanted is equivalent to the revised estimate. The amount of adsorbed tritium exceeds the prediction. While tritium operation was highly successful, there were problems with the failure of several ion sources and gas injectors. Ion source failures were not due to the use of tritium as the working gas. However, their removal yielded information regarding tritium retention. Ten tritium injectors failed during the three and a half years of tritium experiments; six were replaced. At the conclusion of TFTR operation, all working tritium injectors had throughput leaks. By comparison, the deuterium injectors, which used the same valves, had only two minor fill valve leaks, and no repairs were necessary. Tritium contaminated component removal required purging until the concentration of tritium in any released air was <20 /spl mu/ Ci/m/sup 3/. For ion source removal this necessitated 50 to 100 purges and the release of several Ci. Upon removal, source surfaces had to be further decontaminated, from several million dpm/100 cm/sup 2/ to levels at which repairs could be effected.

Progress in the neutral beam injection heating experiment on the Tokamak fusion test reactor

Abstract The principal heating system for the Tokamak Fusion Test Reactor (TFTR) consists of four beamlines to inject beams of neutral deuterium into the tokamak plasma. We have recently injected beams at total powers up to about 30 MW, and we have achieved ion temperatures in the plasma core of 30 keV and more.