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 - Presentation by Jamie Gunn on using the tunnel probe to measure \(T_e\) - J. P. Gunn et al, Measurement of electron temperature fluctuations with tunnel probe in the CASTOR tokamak, First Cairo Conference on Plasma Physics and Application (2003), Cairo, Egypt