His mentor scattered particles in vacuum tubes? (9)

I believe the answer is:

thermions

I'm a little stuck... Click here to teach me more about this clue!

'particles in vacuum tubes?' is the definition.
'thermions' can be an answer for 'particles' (thermion is a kind of particle). I am unsure of the 'in vacuum tubes?' bit.

'his mentor scattered' is the wordplay.
'scattered' is an anagram indicator.
'his'+'mentor'='hismentor'
'hismentor' is an anagram of 'THERMIONS'.

Can you help me to learn more?

I've seen this clue in The Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.