Electronic implementation of a closed-loop gas-flow control system

The fast gas-injection system on the ISX Tokamak at Oak Ridge National Laboratory injects controlled amounts of impurity gases into a hydrogen plasma to study particle transport physics within the plasma. Since for a given discharge the plasma lasts only a fraction of a second at near vacuum pressures, fast, accurate control of the impurity injection rate is essential for discharge stability and meaningful quantitative analysis.

The gas-injection control system in place on this experiment exhibited excessive drift, poor maintainability, and limited versatility. This thesis investigates the design of an improved electronic control system featuring higher loop gain, automatic drift cancellation, high reliability and maintainability, fault protection, and increased versatility through computer control. The circuitry designed for this system exhibits negligible electronic noise and drift while providing repeatable response on a millisecond time scale. Unconditional stability and graceful recovery from saturation were observed. Problems with noise intrusion and thermal sensitivity remain. Further development will be required before an operational system can be fabricated.