These videos goes over various methods: How do we measure 15 million degrees? (from Tokomak Energy (4:34)) https://www.youtube.com/watch?v=p77af1ZivgA&t=57s ht

These videos goes over various methods: How do we measure 15 million degrees? (from Tokomak Energy (4:34)) https://www.youtube.com/watch?v=p77af1ZivgA&t=57s https://www.youtube.com/watch?v=p77af1ZivgA&t=57s And this longer one from UKAEA (55 minutes) Hotter than the sun and lighter than air; measuring the extreme conditions of fusion plasma https://www.youtube.com/watch?v=vms8JEky-Bo https://www.youtube.com/watch?v=vms8JEky-Bo Spectroscopy and laser measurements and other measures are used. The engineering and science in fusion research is amazing.