Syllabus #

The goals of this course are:

• To teach the function of diagnostic components, and the specific plasma mechanisms that are being measured. You will be come familiar with the analysis procedures required to get (often indirect) measurements.
• To appreciate uncertainty and error quantification in reporting experimental measurements.

This course will use I.H. Hutchinson, Principles of Plasma Diagnostics as the primary text, with the standard R.J. Goldston and P.H. Rutherford, Introduction to Plasma Physics for reference.

Introduction #

We’ll set the stage with a few quotes from giants in the field:

“When you can measure what you are speaking about and express it in numbers, you know something about it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science.” - Lord Kelvin, 1883

“An experiment is a question which science poses to nature, and a measurement is the recording of nature’s answer” - Max Planck, 1949

“Measurements are meaningless without quantifying the uncertainty” - Uri Shumlak, 2021

Many of the measurements we report are plasma parameters we extracted indirectly via other means, so proper propagation of error and uncertainty is particularly important.

What makes plasma diagnostics particularly difficult is that we are measuring the physical properties of a plasma that range in temperature from \( 1 eV \) to \( 10 keV \) and also in number density from \( 10^{13} cm^{-3} \) to \( 10^{19} cm^{-3} \). We’ll find that some diagnostics are only appropriate in a certain portion of this range. Further complicating the situation, spatial scales can vary from \( 10 \mu m \) to \( 1m \) and temporal scales can vary from \( 10ns \) to hundreds of seconds.

Instruments inserted into the plasma (Langmuir probes, Mach probes, RPA’s, etc.) can alter the plasma by creating perturbations which alter the local plasma properties. High-energy density plasmas can even destroy the instruments.

Plasma diagnostics extract plasma properties by measuring nuanced interactions between EM fields and detailed velocity distribution functions for the various relevant species. These species might be electrons, bulk ions (\( H^+, D^+, He^{++}, Ar^+ \)), and impurity ions (\( C^+, O^+ \)). The field of plasma diagnostics is vast, and new techniques are currently being developed. Specialized journals and conferences are dedicated to the topic. The goal of this course is to develop a foundation to understand common diagnostics and to understand more advanced diagnostic techniques.