R. Hong

Increased power delivery from the DIII-D neutral beam injection system

The neutral beam system installed on the DIII-D tokamak uses eight 80-kV long pulse sources (LASs) mounted on four beamlines and was originally designed to deliver a nominal 12 MW of H/sup 0/ power to a plasma for pulses of up to 5-s duration. Essentially, all source components are of a common design, making the DIII-D version conservative in its rated parameters. A neutron shield has recently been constructed around the torus hall, allowing D/sup 0/ injection to become routine. Because deuterium beams have a better neutralization efficiency, the nominal power delivery per source has been measured to be approximately 2 MW (for a total of 16 MW) without any modifications. However, by reoptimizing the voltage gradients in the source, the perveance can be increased without degrading the optics. Key features of the neutral beam system are reviewed, and the techniques for raising the delivery power are summarized. The tentative costs of the techniques are included.<<ETX>>

Recent DIII-D neutral beam calibration results

The fundamental parameters by which injected DIII-D neutral beam power is calculated have all been recently measured using waterflow calorimetry and target tile thermocouples. The measured neutralization efficiency is 66%, close to the modeled equilibrium value of 70% for a 75 kV deuterium beam. Waterflow calorimetry has proven to be a useful tool to measure the neutralization efficiency, contrary to earlier results. The beamline transmission efficiency has been measured for each beam independently, and averages about 78% among the beams. Finally, the drift duct reionization survival rate is 95%. By making use of the independence of preshot target tile temperature and duct reionization, interpretation of thermocouple data has yielded more accurate results.<<ETX>>

Beam current regulation of DIII-D neutral beam long pulse ion sources

The slow increase of arc and beam currents during the beam pulse is an intrinsic characteristics of the neutral beam long pulse ion source installed on the DIII-D tokamak. This ramping is attributed to the heating of the filaments by energetic electrons backstreaming from the accelerator into the arc chamber. The corresponding change in beam perveance causes the beam optics to vary during a beam pulse, often resulting in an overdense condition. Stepping down the voltage applied to the filaments at beam turn-on was helpful, but its disadvantages called for a better scheme. A technique using a Langmuir probe signal for feedback regulation in the arc power supply is presented. Plasma density within the arc chamber is maintained at a constant value, as is beam current. The arc regulation method also features arc notching at beam turn-on to provide perveance matching during initial beam formation; this is crucial to obtaining smooth initial beam extraction, a high perveance beam, and thus higher-power beam operation. The beam power achieved with this arc notching and regulation technique is about 10% higher than that obtained with the filament voltage step-down method.<<ETX>>

Performance of a DIII-D Neutral Beam Ion Source with a New Accelerator Grid

The DIII-D tokamak utilizes seven neutral beam ion sources for plasma heating and current drive. These ion sources have performed with high availability and reliability since 1987. However, components of the ion sources, especially the grids of the accelerator, have since developed hardware failures either due to old age or fabrication defects. Due to lack of the plasma grid with diamond shape rails, we have built a plasma grid with circular cross section rails and installed it in one of the ion sources. Tests were performed on this ion source (we will call it the new ion source) and results were compared with one of the original ion sources. The new ion source runs just as reliably as the original ion sources, though the operation window is slightly smaller and the value of the optimum beam perveance is about 4% less (4% less beam current). This new ion source has been used to inject neutral beams into plasmas of the DIII-D tokamak to support physics experiments. No operational problems or difficulties were encountered during more than 5 weeks of operation. Details of the new plasma grid and the operational results will be presented

Schemes and optimization of gas flowing into the ion source and the neutralizer of the DIII-D neutral beam systems

Performance comparisons of a DIII-D neutral beam ion source operated with two different schemes of supplying neutral gas to the arc chamber were performed. Superior performance was achieved when gas was puffed into both the arc chamber and the neutralizer with the gas flows optimized as compared to supplying gas through the neutralizer alone. To form a neutral beam, ions extracted from the arc chamber and accelerated are passed through a neutralizing cell of gas. Neutral gas is commonly puffed into the neutralizing cell to supplement the residual neutral gas from the arc chamber to obtain maximum neutralization efficiency. However, maximizing neutralization efficiency does not necessarily provide the maximum available neutral beam power, since high levels of neutral gas can increase beam loss through collisions and cause larger beam divergence.

ENHANCEMENT OF DIII–D NEUTRAL BEAM SYSTEM FOR HIGHER PERFORMANCE

The DIII-D tokamak employs eight neutral beam systems for plasma heating and current drive experiments. These positive ion source neutral beam systems have gone through several improvements in operational technique and in system hardware since the start of conditioning of the first long pulse ion source in December 1986. These improvements have led to the routine operation in deuterium at beam power levels of 20 MW. The improvements in operational technique include filament power supply operating mode, accelerator grid voltage holding capability, mid changes in grid potential gradients. The hardware improvements include installation of arc notching, arc discharge density regulation, and control of neutralizer gas puffing. Each of these improvements are discussed in this paper. Successful testing and operation of the ion source at 93 kV deuterium beam energy, well above the design value of 80 kV, also led to the possibility of enhancing system capability to 28 MW power level, nearly twice the original design value. Upgrading of the beam system to 60 second pulse duration at the currently achieved power level is under consideration. Studies have shown that this pulse length extension can be achieved with improvements in beamline heat handling components and auxiliary systems, especially the powermorexa0» supply system. The drift duct (the section between neutral beam beamline and tokamak) upgrade and protection for the long pulse duration present the greatest challenges, and are crucial to achieving long pulse beam injection into the tokamak plasma.«xa0less

Design and implementation of a Macintosh-CAMAC based system for neutral beam diagnostics

An automated personal computer-based CAMAC data-acquisition system is being implemented on the DIII-D neutral beamlines for certain diagnostics. The waterflow calorimetry (WFC) diagnostic is the first system to be upgraded. It acquires data with a Macintosh II computer containing an IEEE-488 card, and uses LabView software. Macintosh communicates with CAMAC through an IEEE-488 crate controller. The Doppler-shift spectroscopy, residual gas analysis, and armor-tile infrared image diagnostics will be modified in similar ways. To reduce the demand for Macintosh CPU time, the extensive serial highway data activity is performed by means of a new Kinetic Systems 3982 list-sequencing crate controller dedicated to these operations. A simple local area network file server is used to store data from all diagnostics together, and in a format readable by a standard commercial database. This reduces the problem of redundant data storage and allows simpler interdiagnostic analysis.<<ETX>>

Analysis of DIII-D neutral beam power systems for 10 second operation

The neutral beam injector systems on the DIII-D tokamak are currently limited to 3 seconds of total beam on time with the existing beam line components. To achieve high performance discharges with a high fraction of non-inductive neutral beam current drive for pulse lengths longer than the resistive time, long pulse upgrades are planned for both the neutral beam line and the power supplies that drive it. The beam on time will be extended to 10 seconds to provide full drive throughout the tokamak shot.

Development for extending the beam pulse length of the DIII-D neutral besm System

The DIII-D fusion research tokamak utilizes seven neutral beam ion sources for plasma heating and current drive. These ion sources and the neutral beam system have performed with high availability and reliability since 1987. Experimental research has accomplished extensive understanding and insights of plasma physics, and requires more beam system capability to provide flexibility and enhancing scope of physics experiments. Extending the beam pulse length without lowering the beam power is one of the goals for the next 5 years. Currently, deuterium beam pulse length of ion source operated at 80 keV is limited to 3 s due to heat handling capability of some beamline internal components, which are used to collimate beam or to protect other beamline components from being damaged by residual (unneutralized) energetic ions. A systematic study based on actual heating of beamline internal components has been performed to develop a plan for extending the beam pulse length to more than twice the current operating limit. This study using temperature rise of thermocouples embedded in the beamline internal components and data extrapolation obtained the beam pulse limitation for each beamline internal component identified that the pole shield of the residual ion bending magnet is the component that limits the beam pulse length to 3 s for 80 keV beam operation. Beam pulse length at 80 keV can be doubled with the upgrade of the magnet pole shield alone. Temperature rise measurements also showed that a drift duct (the section connecting beamline and tokamak vessel) made of stainless steel will be able to handle the heat from the re-ionized neutral particles. In addition to beamline internal components, long pulse beam operation of ion sources and other beam subsystems need to be operationally confirmed. Tests have shown ion source, control system, pumping and cooling systems, and power supply system operated flawlessly at beam energy of 60 keV for 10 s. It gave us confidence that the current DIII-D neutral beam system could operate at least twice the current beam pulse length after upgrade of the magnet pole shield.

Automated calculation of DIII-D neutral beam availability

The neutral beam systems for the DIII-D tokamak are an extremely reliable source of auxiliary plasma heating, capable of supplying up to 20 MW of injected power, from eight separate beam sources into each tokamak discharge. The high availability of these systems for tokamak operations is sustained by careful monitoring of performance and following up on failures. One of the metrics for this performance is the requested injected power profile as compared to the power profile delivered for a particular pulse. Calculating this was a relatively straightforward task, however innovations such as the ability to modulate the beams and more recently the ability to substitute an idle beam for one which has failed during a plasma discharge, have made the task very complex.