A Spinning Mirror for Fast Angular Scans of EBW Emission for Magnetic Pitch Profile Measurement

TL;DR

This paper presents a high-speed spinning mirror system for rapidly scanning EBW emission in a tokamak, enabling real-time magnetic pitch profile measurements and optimal heating conditions.

Contribution

It introduces a durable, high-speed spinning mirror design capable of fast angular scans for EBW diagnostics in fusion plasmas.

Findings

Successful implementation of a spinning mirror at 12,000 rpm

Rapid angular scans enable real-time mode-conversion efficiency mapping

Initial measurements demonstrate feasibility for magnetic profile diagnostics

Abstract

A tilted spinning mirror rapidly steers the line of sight of the electron Bernstein wave (EBW) emission radiometer at the Mega Amp Spherical Tokamak (MAST). In order to resist high mechanical stresses at rotation speeds of up to 12,000 rpm and to avoid eddy current induced magnetic braking, the mirror consists of a glass-reinforced nylon substrate of a special self-balanced design, coated with a reflecting layer. By completing an angular scan every 2.5-10ms, it allows one to characterize with good time resolution the Bernstein-extraordinary-ordinary mode-conversion efficiency as a function of the view angles. Angular maps of conversion efficiency are directly related to the magnetic pitch angle at the cutoff layer for the ordinary mode. Hence, measurements at various frequencies provide the safety factor profile at the plasma edge. Initial measurements and indications of the feasibility of the diagnostic are presented. Moreover, angular scans indicate the best launch conditions for EBW heating.