The Collaborative International Dictionary of English v.0.48:

dee \dee\ (d[=e]), n. (Physics)
   an electrode with a large interior cavity, shaped like the
   letter "D", used in opposed pairs to accelerate particles in
   a cyclotron.
   [PJC]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

DEE
       DatenEndEinrichtung