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title: Tunnel probe
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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

[<img src="http://golem.fjfi.cvut.cz/wiki/Diagnostics/ParticleFlux/TunnelProbe/Slides/BasicPhoto/2018-11-07_12-42-21.jpg" width="300" />](/Diagnostics/ParticleFlux/TunnelProbe/Slides/BasicPhoto/2018-11-07_12-42-21.jpg)
[<img src="http://golem.fjfi.cvut.cz/wiki/Diagnostics/ParticleFlux/TunnelProbe/PG/0219DaniKrop/51034505_988025594728450_4862615161959088128_n.jpg" width="300" />](/Diagnostics/ParticleFlux/TunnelProbe/PG/0219DaniKrop/51034505_988025594728450_4862615161959088128_n.jpg)
[<img src="http://golem.fjfi.cvut.cz/wiki
/Diagnostics/ParticleFlux/TunnelProbe/PG/0219DaniKrop/51265886_2288199121416776_9189658145603977216_n.jpg" width="200" />](
/Diagnostics/ParticleFlux/TunnelProbe/PG/0219DaniKrop/51265886_2288199121416776_9189658145603977216_n.jpg)
[<img src="http://golem.fjfi.cvut.cz/wiki
/Diagnostics/ParticleFlux/TunnelProbe/PG/0219DaniKrop/50507942_757335287998946_4769717927039467520_n.jpg" width="200" />](
/Diagnostics/ParticleFlux/TunnelProbe/PG/0219DaniKrop/50507942_757335287998946_4769717927039467520_n.jpg)

(More photos of the probe may be found in [this folder](/Diagnostics/ParticleFlux/TunnelProbe/PG/).)

# Probe connections

[<img src="http://golem.fjfi.cvut.cz/wiki
/Diagnostics/ParticleFlux/TunnelProbe/HW/Adapter/1218HS_SCHEMA krabicky.png" width="300" />](
/Diagnostics/ParticleFlux/TunnelProbe/HW/Adapter/1218HS_SCHEMA krabicky.png)
[<img src="http://golem.fjfi.cvut.cz/wiki/Diagnostics/ParticleFlux/TunnelProbe/HW/Adapter/20181211_120642.jpg" width="300" />](/Diagnostics/ParticleFlux/TunnelProbe/HW/Adapter/20181211_120642.jpg)


# Reports

- [SOČ](/Diagnostics/ParticleFlux/TunnelProbe/soc_kropackova.pdf) (Středoškolská odborná činnost, a report written by a high-school student) by Daniela Kropáčková, September 2018 - February 2019
- [Report by Jan Stöckel](/Diagnostics/ParticleFlux/TunnelProbe/report_8_11_18.docx) of tunnel probe measurements in November 2018


# Literature 
- [Presentation by Jamie Gunn](/Diagnostics/ParticleFlux/TunnelProbe/tunnel_probe_for_Te_measurement.ppt) 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](/Diagnostics/ParticleFlux/TunnelProbe/tp_Te_article.pdf), First Cairo Conference on Plasma Physics and Application (2003), Cairo, Egypt