M.E. Oldaker

The TFTR 40 MW neutral beam injection system and DT operations

Since December 1993, TFTR has performed DT experiments using tritium fuel provided mainly by neutral beam injection. Significant alpha particle populations and reactor-like conditions have been achieved at the plasma core, and fusion output power has risen to a record 10.7 MW using a record 40 MW NB heating. Tritium neutral beams have injected into over 480 DT plasmas and greater than 500 kCi have been processed through the neutral beam gas, cryo, and vacuum systems. Beam tritium injections, as well as tritium feedstock delivery and disposal, have now become part of routine operations. Shot reliability with tritium is about 90% and is comparable to deuterium shot reliability. This paper describes the neutral beam DT experience including the preparations, modifications, and operating techniques that led to this high level of success, as well as the critical differences in beam operations encountered during DT operations. Also, the neutral beam maintenance and repair history during DT operations, the corrective actions taken, and procedures developed for handling tritium contaminated components are discussed in the context of supporting a continuous DT program.

Control of the D-T gas injection system for TFTR neutral beams

A gas injection system is currently being designed to supply the neutral beam (NB) long pulse ion sources (LPIS) in support of the tritium phase of TFTR (Tokamak Fusion Test Reactor) operation. Each NB source will be supplied by a dual gas injection system. Separate deuterium and tritium gas injection subsystems provide the greatest gas isolation as well as providing redundancy. A single programmable logic controller will provide central control of the D-T gas injection subsystems of all 12 NB sources. To maintain accurate T/sub 2/ inventory control, the PLC will accurately monitor the NB T/sub 2/ usage. The usage will be correlated with the Tritium Gas Delivery Systems (TGDSs) information on the quantity of T/sub 2/ delivered. Central control will enable an operator to quickly respond to any event affecting the NB tritium subsystems. Individual closed-loop controllers will maintain precise flow control for each LPIS during beam extraction. T/sub 2/ or D/sub 2/ will be delivered in small batches, to a local plenum on each individual LPIS. A piezoelectric valve, coupled with pressure feedback from the local LPIS gas storage plenum, will be used to establish a controlled bleeddown of the individual gas plenums during injection.<<ETX>>

D-T gas injection system for TFTR neutral beams

The authors describe the design of the TFTR (Tokamak Fusion Test Reactor) NB (neutral beam) D-T Gas Injection System and the results of tests conducted to confirm its functionality. Detailed design is progressing for a NB TGIS (tritium gas injection system) which includes the option of injecting either H/sub 2/, D/sub 2/, or T/sub 2/ into each of the 12 neutralizers at ground potential based on limited experiments to date. Most commercial components have been ordered and fabrication of special hardware is underway. Additional tests are planned to confirm LPIS (long pulse ion sources) operations compatibility with neutralizer gas injection, to inject the gas through shorter tubes which would simplify installation and interactions in LPIS parameters with alternating gases between pulses from H/sub 2/ to D/sub 2/ /sup 3/He. Tests of piezoelectric valves operated with the plenum pressure feedback controller will be conducted and a prototype NB TGIS will be installed and tested on TFTR during the September 91 to May 92 operational period. Installation of the complete system will take place during the pre-tritium shutdown presently scheduled for 1992-3.<<ETX>>

TFTR neutral beam D-T gas injection system operational experiences of the first two years

The TFTR Neutral Beam Tritium Gas Injection System (TGIS) has successfully performed tritium operations since December 1993. TGIS operation has been reliable, with no leaks to the secondary containment to date. Notable operational problems include throughput leaks on fill, exit and piezoelectric valves. Repair of a TGIS requires replacement of the assembly, involving TFTR downtime and extensive purging, since the TGIS assembly is highly contaminated with residual tritium, and is located within secondary containment. Modifications to improve reliability and operating range include adjustable reverse bias voltage to the piezoelectric valves, timing and error calculation changes to tune the PLC and hardwired timing control, and exercising piezoelectric valves without actually pulsing gas prior to use after extended inactivity. A pressure sensor failure required the development of an open loop piezoelectric valve drive control scheme, using a simple voltage ramp to partially compensate for declining plenum pressure.

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.

Conceptual design of the neutral beamline for TPX long pulse operation

The Tokamak Physics Experiment (TPX) will require a minimum of 8.0 megawatts of Neutral Beam heating power to be injected into the plasma for pulse lengths up to one thousand (1000) seconds to meet the experimental objectives. The Neutral Beam Injection System (NBIS) for initial operation on TPX will consist of one neutral beamline (NBL) with three ion sources. Provisions will be made for a total of three NBLs. The NBIS will provide 5.5 MW of 120 keV D/sup 0/ and 2.5 MW of partial-energy D/sup 0/ at 60 keV and 40 keV. The system also provides for measuring the neutral beam power, limits excess cold gas from entering the torus, and provides independent power, control, and protection for each individual ion source and accelerating structure. The Neutral Beam/Torus Connecting Duct (NB/TCD) includes a vacuum valve, an electrical insulating break, alignment bellows, vacuum seals, internal energy absorbing protective elements, beam diagnostics and bakeout capability. The NBL support structure will support the NBL, which will weigh approximately 80 tons at the proper elevation and withstand a seismic event. The NBIS currently operational on the Tokamak Fusion Test Reactor (TFTR) at the Princeton Plasma Physics Laboratory (PPPL) is restricted to injection pulse lengths of two (2) seconds by the limited capability of various energy absorbers. This paper describes the modifications and improvements which will be implemented for the TFTR Neutral Beamlines and the NB/TCD to satisfy the TPX requirements.

Operation of TFTR neutral beams and heavy ions

Moderate atomic number neutral atoms have been injected into TFTR (Tokamak Fusion Test Reactor) plasma in an attempt to enhance plasma confinement through modification of the edge electric field. TFTR ion sources have extracted 9 A of 62-keV Ne/sup +/ for up to 0.2 s during injection into deuterium plasmas, and for 0.5 s during conditioning pulses. Approximately 400 kW of Ne/sup 0/ have been injected from each of two ion sources. Operation under these conditions was at full bending magnet current, with the Ne/sup +/ just contained on the ion dump. Modifications to permit operation up to 120 keV with krypton and/or xenon are described. Such ions are too massive to be deflected up to the ion dump with the beamlines as designed. The plan is to procure a 3600-A power supply and pulse it into the magnet for initial Kr/Xe experiments. Information relevant to heavy ion operation was acquired during the course of ion source operation with small water leaks. Spectroscopic analysis of certain pathological pulses indicates that up to 6% of the extracted ions were water. After dissociation in the neutralizer, water produces oxygen ions which, as with Ne/sup +/, Kr/sup +/, and Xe/sup +/, are underdeflected by the magnet. Damage to a calorimeter scraper, due to the focal properties of the magnet, resulted. A power density of 6 kW/cm/sup 2/ for 2 s, from approximately 90 kW of O/sup +/, is the probable cause.<<ETX>>

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.

Experience with deuterium-tritium plasmas heated by high power neutral beams

The Tokamak Fusion Test Reactor has operated since November of 1993 with a deuterium-tritium fuel mixture for selected discharges. The majority of the tritium has been introduced as energetic neutral atoms of up to 120 keV injected by the neutral beam systems, with some of the twelve ion sources run on pure tritium and some on deuterium to optimize the fuel mixture in the core plasma. A maximum beam power of 39.6 megawatts has been injected, and deuterium-tritium fusion power production has reached 10.7 megawatts, achieving central fusion power densities comparable to or greater than those expected for the International Thermonuclear Reactor, and allowing the first studies of fusion-produced alpha particle behavior in reactor grade plasmas. Energy confinement in deuterium-tritium plasmas is better than in similar deuterium plasmas for most plasma regimes. Innovative techniques to manipulate the plasma current and pressure profiles are permitting studies of enhanced confinement regimes.