Tunnel probe

This page was last updated on 14 March 2019.

Introduction

The tunnel probe, more precisely called the double tunnel probe, has been used on tokamak GOLEM to measure the profile of the toroidal Mach number \(M=v_t/c_s\) where \(v_t\) is the toroidal plasma velocity and \(c_s\) is the ion sound speed. It may also be used for \(T_e\) measurement with high temporal resolution (see Literature section).

A tunnel probe consists of two electrodes: a tunnel and a back plate covering one of its opening. The GOLEM probe head includes two tunnel probes, placed back plate to back plate.

Mach number measurement

To measure the Mach number, all four electrodes of the double tunnel probe are biased to a large negative voltage (like \(-100\) V) so they measure the ion saturated current \(I_{sat}\). The probe tunnels must be facing in the toroidal direction (approximating the parallel direction, since on GOLEM \(q_{95} \gg 1\)). Then the Mach number may be calculated as \[ M = \frac{1}{4} \ln \left( \frac{I_{1,tun} + I_{1,back}} {I_{2,tun} + I_{2,back}} \right) \] where \(I_{1,tun}\) and \(I_{1,back}\) are the currents collected by one of the probes’ tunnel and back place, respectively. The same then applies to the second probe. If the \(M>0\), the plasma is flowing in the direction from probe 1 to probe 2.

Geometry

(More photos of the probe may be found in this folder.)

Probe connections

Reports

  • SOČ (Středoškolská odborná činnost, a report written by a high-school student) by Daniela Kropáčková, September 2018 - February 2019
  • Report by Jan Stöckel of tunnel probe measurements in November 2018

Literature