---
format: markdown
title: Particle flux diagnostics
author: Ing. Kateřina Jiráková
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This page was last updated on 10 September 2020.

# Introduction

Particle flux diagnostics, more commonly called probes, are a type of edge diagnostic which finds great importance in today's plasma physics research. Probes offer fast sampling, good spatial resolution and a very low initial cost. On the other hand, interpretation of probe data is notoriously difficult.

<i>"It's a piece of wire that you stick inside the plasma. Out of it you get two numbers: a voltage and a current. But figuring out what those two mean, that's the real mastery."</i><br>
- Ing. Kateřina Jiráková, leader of the probe section on tokamak GOLEM, 5 March 2019

Contrary to this quote, not all probes are a piece of wire. They come in all shapes and sizes, such as the [ball-pen probe](/Diagnostics/ParticleFlux/BallPenProbe/index), the [tunnel probe](/Diagnostics/ParticleFlux/TunnelProbe/index), the [Mach probe](/Diagnostics/ParticleFlux/MachProbeRing/index) and many more. The probe shape and size greatly influence what the probe measures and how it can be interpreted. But, in essence, all probes are the same: a piece of conductor measuring in direct contact with the plasma.

# Probes on GOLEM

Tokamak GOLEM doesn't reach very high plasma parameters (central electron temperature $\sim 40$ eV and line-averaged density $\sim 0.5 \times 10^{10}$ m$^{-3}$), which are poor conditions for fusion, but excellent conditions for probe measurements. In other tokamaks, inserting a probe too deep into the plasma has detrimental effects both on the probe and on the plasma. At GOLEM, one can easily insert a double rake probe up to the centre of the plasma column and observe only a slight increase in the loop voltage, as evidenced by the [experiments](/TrainingCourses/FTTF/2015-2016/KatJira/session2016-3-24/report) conducted in the plasma practica course.

Note: Do not take this experiment as an example. It is prudent to measure only up to $r \approx 60$ mm.

There is a number of probe heads on tokamak GOLEM, featuring various probe types. These are:

- [combined ball-pen and Langmuir probe head](/Diagnostics/ParticleFlux/BallPenProbe/index), featuring a ball-pen probe and a Langmuir probe (perk: measurements of $T_e$ with high temporal resolution)
- [double rake probe](/Diagnostics/ParticleFlux/DoubleRakeProbe/index), featuring 16 Langmuir probes (perk: cross-correlation measurements of the poloidal plasma velocity $v_p$)
- [Mach probe ring](/Diagnostics/ParticleFlux/MachProbeRing/index), featuring 8 Mach probes (perk: poloidally resolved measurement of the Mach number $M$)
- [(single) rake probe](/Diagnostics/ParticleFlux/RakeProbe/index), featuring 16 Langmuir probes (perk: radial profile measurements in a large range of $r$ within a single discharge)
- [(double) tunnel probe](/Diagnostics/ParticleFlux/TunnelProbe/index), featuring two tunnel probes (perk: measurements of $T_e$ with high temporal resolution, also measurements of the Mach number $M$)
- [strip detector](/Diagnostics/ParticleFlux/StripDetector/library/SvihraFusendges.pdf) (a semiconductor detector developed by Peter Svihra)
- [Tsviatko's probe](/Diagnostics/ParticleFlux/TsviatkoProbe/index), featuring bent wires
- [petiprobe](/Diagnostics/ParticleFlux/PetiProbe/index), a combination of the ball-pen+Langmuir probe and the (double) tunnel probe

Aside from these built and functioning probes, this wiki mentions:

- [runaway electron probe](/Diagnostics/ParticleFlux/REprobe/), featuring scintillators (it is currently in the design stage, responsible person being Pravesh Dhyani)
- [triple probe](/Diagnostics/ParticleFlux/TripleProbe/Campaigns/1012MarcoM/Report) (a makeshift probe constructed by Marco Martinez in 2012 and later thrown out)


# Currently installed

The data here might be outdated. Always check the last update of this page.

- double rake probe
- petiprobe

# Past experiments

Check out the [probe history article](/Diagnostics/ParticleFlux/history) or [the experiment records](/Experiments/EdgePlasmaPhysics/).

# Library and references
- [two excerpts](/Diagnostics/ParticleFlux/library/IAEAbookExcerpt) from IAEA reports
- Robert L. Merlino, [Understanding Langmuir probe current-voltage characteristics](http://www2.physics.uiowa.edu/~rmerlino/Merlino_AJP_75_1078_2007.pdf), American Journal of Physics 75/12 (December 2007)