Particle flux diagnostics

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.

“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.”
- 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, the tunnel probe, the Mach probe 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 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, featuring a ball-pen probe and a Langmuir probe (perk: measurements of \(T_e\) with high temporal resolution)
  • double rake probe, featuring 16 Langmuir probes (perk: cross-correlation measurements of the poloidal plasma velocity \(v_p\))
  • Mach probe ring, featuring 8 Mach probes (perk: poloidally resolved measurement of the Mach number \(M\))
  • (single) rake probe, featuring 16 Langmuir probes (perk: radial profile measurements in a large range of \(r\) within a single discharge)
  • (double) tunnel probe, featuring two tunnel probes (perk: measurements of \(T_e\) with high temporal resolution, also measurements of the Mach number \(M\))
  • strip detector (a semiconductor detector developed by Peter Svihra)
  • Tsviatko’s probe, featuring bent wires
  • petiprobe, 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, featuring scintillators (it is currently in the design stage, responsible person being Pravesh Dhyani)
  • triple probe (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 or the experiment records.

#Library and references - two excerpts from IAEA reports - Robert L. Merlino, Understanding Langmuir probe current-voltage characteristics, American Journal of Physics 75/12 (December 2007)