History, presence and future of gyrotrons

Gyrotron oscillators (gyromonotrons) are mainly used as high power millimeter wave sources for electron cyclotron resonance heating (ECRH), current drive (ECCD), stability control and diagnostics of magnetically confined fusion plasmas. The maximum pulse length of commercially available 140 GHz, megawatt-class gyrotrons employing synthetic diamond output windows is 30 min (CPI and European FZK-CRPP-CEA-TED Collaboration), at 44% efficiency, employing a single-stage depressed collector for energy recovery. The Japan 170 GHz ITER gyrotron holds the energy world record of 2.88 GJ (0.8 MW, 60 min.) and the efficiency record of 55% at 1 MW, 800 s for tubes with an output power of more than 0.5 MW. The Russian 170 GHz ITER gyrotron achieved 0.83 MW with a pulse duration of 203 s. Russian gyrotrons for plasma diagnostics or spectroscopy applications deliver P
_out
= 40 kW with tau = 40 mus at frequencies up to 650 GHz (eta > 4%), and P
_out
= 0.5 kW at 1.3 THz (eta = 0.7%). Gyrotron oscillators have also been successfully used in materials processing and ECR multi-charged heavy ion sources. Such technological applications require gyrotrons with the following parameters: int > 24 GHz , P
_out
= 4-50 kW, CW, eta > 30%. This paper gives a short review of the history of gyrotrons, an update of the present experimental achievements and an outlook into future developments of multi-megawatt coaxial-cavity gyrotrons, frequency-step tunable gyrotrons and gyrotrons for specific technological and spectroscopic applications.