Co-axial Helicity Injection on the STOR-M Tokamak

TL;DR

This paper explores the use of Co-axial Helicity Injection (CHI) to inject current into the STOR-M tokamak, detailing device modifications, experimental attempts, and diagnostic measurements, but finds no conclusive current drive evidence due to spurious signals.

Contribution

It reports on the development and testing of a CHI device on STOR-M, including circuit modifications and diagnostic methods, to evaluate its effectiveness for current drive in tokamaks.

Findings

Injection often caused tokamak disruptions.

Spurious signals mistaken for current drive were identified.

Diagnostic measurements provided estimates of CT density and temperature.

Abstract

Injection of relatively high density spheromaks with significant helicity-content into a tokamak has been proposed as a means for fueling and current drive. The CHI (Co-axial Helicity Injection) device was devised to inject current to the STOR-M tokamak. Various circuit modifications were made to the CHI controls, enabling testing of various injection configurations. The charge/discharge circuits for CT formation/acceleration and stuffing field were modified, and the power supplies and power converters were replaced. Various modifications were implemented to solve the original slow bank triggering problems. The CHI device was mounted on STOR-M for radial and vertical CT injection at various times. Spheromak injection into STOR-M usually resulted in disruption of the tokamak discharge. After modifying the CHI device to operate at increased power, it looked like tokamak current was increased by a few kiloamps just prior to disruption, but careful testing proved that the signals indicating a current drive were actually spurious, caused by inductive pickup. The CHI device was attached to a portable vacuum chamber that was constructed from spare parts, to characterise the CTs produced. Magnetic probes were constructed to measure poloidal and toroidal field near the CT edge. Langmuir probes were made and returned reasonable estimates for edge CT density and temperature.