G.L. Campbell

Design and operation of the multipulse Thomson scattering diagnostic on DIII-D (invited)

This paper describes the design and operation of a 40 spatial channel Thomson scattering system that uses multiple 20‐Hz Nd:YAG lasers to measure the electron temperature and density profiles periodically throughout an entire plasma discharge. As many as eight lasers may be fired alternately for an average measurement frequency of 160 Hz, or they may be fired in rapid succession (<10 kHz), producing a burst of pulses for measuring transient events. The high spatial resolution (1.3 cm) and wide dynamic range (10 eV–20 keV) enable this system to resolve large electron density and temperature gradients formed at the plasma edge and in the scrape‐off layer during H‐mode operation. These features provide a formidable tool for studying L–H transitions, edge localized modes (ELMs), beta limits, transport, and disruptions in an efficient manner suitable for large tokamak operation where shot‐to‐shot scans are impractical. The scattered light is dispersed by interference filter polychromators and detected by silicon avalanche photodiodes. Laser control and data acquisition are performed in real time by a VME‐based microcomputer. Data analysis is performed by a MicroVAX 3400. Additional features of this system include real‐time analysis capability, full statistical treatment of error bars based on the measured background light, and laser beam quality and alignment monitoring during plasma operation. Results of component testing, calibration, plasma operation, and error analysis are presented.

Real-time digital control, data acquisition, and analysis system for the DIII-D multipulse Thomson scattering diagnostic

A VME‐based real‐time computer system for laser control, data acquisition, and analysis for the DIII‐D multipulse Thomson scattering diagnostic is described. The laser control task requires precise timing of up to eight Nd:YAG lasers, each with an average firing rate of 20 Hz. A cpu module in a real‐time multiprocessing computer system will operate the lasers with evenly staggered laser pulses or in a ‘‘burst mode,’’ where all available (fully charged) lasers can be fired at 50–100 μs intervals upon receipt of an external event trigger signal. One or more cpu modules, along with a LeCroy FERA (fast encoding and readout ADC) system, will perform real‐time data acquisition and analysis. Partial electron temperature and density profiles will be available for plasma feedback control within 1 ms following each laser pulse. The VME‐based computer system consists of two or more target processor modules (25 MHz Motorola 68030) running the VMEexec real‐time operating system connected to a Unix‐based host system (a...

Installation and initial operation of the DIII-D advanced divertor cryocondensation pump

Phase two of a divertor cryocondensation pump, the Advanced Divertor Program, is now installed in the DIII-D tokamak at General Atomics and complements the phase one biasable ring electrode. The installation consists of a 10 m long cryocondensation pump located in the divertor baffle chamber to study plasma density control by pumping of the divertor. The design is a toroidally electrically continuous liquid helium-cooled panel with 1 m/sup 2/ of pumping surface. The helium panel is single point grounded to the nitrogen shield to minimize eddy currents. The nitrogen shield is toroidally continuous and grounded to the vacuum vessel in 24 locations to prevent voltage potentials from building up between the pump and vacuum vessel wall. A radiation/particle shield surrounds the nitrogen-cooled surface to minimize the heat load and prevent water molecules condensed on the nitrogen surface from being released by impact of energetic particles. Large currents (>5000 A) are driven in the helium and nitrogen panels during ohmic coil ramp up and during disruptions. The pump is designed to accommodate both the thermal and mechanical loads due to these currents. A feedthrough for the cryogens allows for both radial and vertical motion of the pump with respect to the vacuum vessel. Thermal performance measured on a prototype verified the analytical model and thermal design of the pump. Characterization tests of the installed pump show the pumping speed in deuterium is 42,000 l/sec for a pressure of 5 mTorr. Induction heating of the pump (at 300 W) resulted in no degradation of pumping speed. Plasma operations with the cryopump show a 60% lower density in H-mode.

Operation of a versatile multi-power supply system driving non-axisymmetric coil sets on the DIII-D tokamak

We describe the development, and implementation of the DIII-D tokamak power supply system driving non-axisymmetric coil sets. The coil system operates in a broad range of 0 to 7 kA and dc up to 40 kHz using different combinations of power supplies. Three types of power supplies drive the tokamaks non-axisymmetric coil sets; variable unipolar quasi-dc SCR supplies, bi-polar switching power amplifiers, and linear amplifiers. The coils are used primarily for error field correction, resistive wall mode studies, and edge localized mode suppression. A versatile computer program generates the power supply — coil interconnections in the patch panel area for a specific experiment. Control of individual coils, or combinations of coils, is accomplished using the plasma control system (PCS). An upgrade to a precision response of the high current unipolar SCR power supplies has been under development. We discuss routine operation, coil control using the PCS, and challenges in maintaining this complex system.

The Development of an in-Vessel Cryopump System for the DIII-D Tokamak

The design, testing and initial operation of the DIII-D advanced divertor cryocondensation pumping system is presented. The pump resides inside the tokamak plasma containment vessel where it provides particle exhaust pumping, and it is subjected to Joule heating and hot particle heat loads during each 10 second discharge. In addition, the pump must withstand plasma disruption induced electromagnetic forces and 400°C bake-out temperatures. Cooling is accomplished by forced flow liquid helium with the two-phase helium exhaust passing through a reliquefier for thermal efficiency. A prototype pump was constructed to study surface temperature rise as a function of flow geometry, applied heat load, helium mass flow rate, and pump outlet conditions. Prototype testing led to the development of a special geometry which was demonstrated to enhance two-phase flow stability and overall heat transfer. During initial operation, deuterium pumping speeds of 32,000 Lis at 2 mTorr pressure were achieved with a helium flow rate of 5 g/s. This speed was maintained during 300 W, 8 s long test heat pulses which meets operational goals.

Data acquisition and protection for new DIII-D in-vessel coils

The installation of new internal magnetic coils (I-Coils) in the DIII-D tokamak at General Atomics required extensive additions to the experiment data acquisition and protection capabilities. This set of 12 coils (up to 7 kA each) is designed to allow improved feedback stabilization of resistive wall modes which limit the plasma performance. The acquisition and signal conditioning needs of the I-Coil power system presented an opportunity to try a new data acquisition approach which increased both the sampling rate and sample size per channel compared to the standard DIII-D CAMAC acquisition equipment. A 96 channel compact-PCI (cPCI) digitizer system was purchased for the I-Coil project to acquire up to approximately 380 MB of power supply and coil current data per plasma discharge. Additional instrumentation and control was provided to protect personnel, the new coils, the tokamak, the facility and improve machine availability. This paper will present discussions of technical and programmatic requirements, bases for requirements, the design selection outcome, installation experience, integration issues, commissioning experience, and lessons learned. The data acquisition system is described in detail including a conservative signal isolation scheme, signal grounding standards, anti-aliasing filters, and synchronization of acquisition. Protection interlocks are described, including high voltage isolation, water flow measurement, and the coil grounding-shorting switches.

Two phase liquid helium flow testing to simulate the operation of a cryocondensation pump in the DIII-D tokamak

Abstract A liquid helium-cooled cryocondensation pump has been installed in the DIII=D tokamak fusion energy research experiment at General Atomics. The pump is located within the tokamak vacuum chamber beneath the divertor baffle plates and is utilized for plasma density and contamination control. Two-phase helium flows through the pump at 5 to 10 g/s utilizing the beat transfer and constant temperature characteristics of boiling liquid helium. The pump is designed for a pumping speed of 32,0001/s. Extensive testing was performed with a prototypical pump test fixture. Several pump geometries (simple tube, coaxial flow plug, and coaxial slotted insert) were tested, in an iterative process, to determine which was the most satisfactory for stable cryocondensation pumping. Results from the different tests illustrating the temperature distribution and flow characteristics for each configuration are presented.

Performance characteristics of the DIII-D advanced divertor cryopump

A cryocondensation pump, cooled by forced flow of two-phase helium, has been installed for particle exhaust from the divertor region of the DIII-D tokamak. The Inconel pumping surface is of coaxial geometry, 25.4 mm in outer diameter and 11.65 m in length. Because of the tokamak environment, the pump is designed to perform under relatively high pulsed heat loads (300 W m/sup -2/). Results of measurements made on the pumping characteristics for D/sub 2/, H/sub 2/, and Ar are discussed.

DIII-D neutral beam control system operator interface

A centralized graphical user interface has been added to the DIII-D neutral beam (NB) control systems for status monitoring and remote control applications. This user interface provides for automatic data acquisition, alarm detection and supervisory control of the four NB programmable logic controllers (PLC) as well as the mode control PLC. These PLCs are used for interlocking, control and status of the NB vacuum pumping, gas delivery, and water cooling systems as well as beam mode status and control. The system allows for both a friendly user interface as well as a safe and convenient method of communicating with remote hardware that formerly required interruption of machine operations to access. In the future, to enable high level of control of PLC subsystems, complete procedures can be written and executed at the touch of a screen control panel button. The system consists of an IBM compatible 486 computer running the FIX DMACS for Windows data acquisition and control operator interface software, a Texas Instruments/Siemens communication card and Phoenix Digital optical communications modules. Communication is achieved via the TIWAY (Texas Instruments) protocol link utilizing both fiber optic communications and a copper local area network (LAN). Hardware and software capabilities will be reviewed. Data and alarm reporting, extended monitoring and control capabilities will also be discussed.

The multipulse Thomson scattering diagnostic on the DIII-D tokamak

The authors describe the design and operation of a 40 spatial channel Thomson scattering system that uses multiple 20-Hz Nd:YAG lasers to measure the electron temperature and density profiles periodically throughout an entire plasma discharge. Interference filter polychromators disperse the scattered light, which is detected by silicon avalanche photodiodes. The measurable temperature range is from 10 eV to 20 keV, and the minimum detectable density is about 2*10/sup 18/ m/sup -3/. Laser control and data acquisition are performed in real time by a VME-based microcomputer. Data analysis is performed by a MicroVAX 3400. Unique features of this system include burst mode operation, where multiple lasers are fired in rapid succession (<10 kHz), real-time analysis capability, and laser beam quality and alignment monitoring during plasma operation. Results of component testing, calibration, and plasma operation are presented.<<ETX>>