Results of 10-Joule wire-burn test performed on 70 kV rail-gap crowbar protection system for high power klystrons and gyrotron

Pulsed and continuous high power microwave tubes need to be protected against arc faults within the tube. If the energy dumped in such arc fault is more than the critical crater energy of the tube, irreparable damage can occur. Modern switch mode powers supplies (SMPS) that are generally opted for the new systems store little energy and are capable of switching HV in few microseconds. So they do not need separate protection. However, when conventional power supplies feed high power microwave tubes a reliable crowbar protection system, tested separately to limit the energy, must be used to assure the tube safety. Initial testing and commissioning of two klystrons, TH2103D have been carried out at our Institute. Each of these klystrons is capable of delivering 500 kW at 3.7 GHz, to be used for non-inductive lower hybrid current drive (LHCD) In the Steady-State Superconducting Tokamak-1 (SST-1). Commissioning of the gyrotron VGA8000A19, for breakdown and electron cyclotron resonant heating (ECRH) experiments on Aditya tokamak, has also been carried out.© IEE

Electron cyclotron resonance frequency system on tokamak Aditya

A 28 GHz ECRH system has been successfully commissioned on the tokamak Aditya to carry out breakdown, start up and heating experiments. The microwave source gyrotron VGA8000A19 capable of delivering 200 kW CW is commissioned and tested with a water dummy load for pulsed operation. The output mode of the gyrotron (TE02) is externally converted to the HE11 mode with the help of a mode converter and Matching Optics Unit of the transmission line. The transmission line consists of a mode converter, MOU, DC breaks, mitre bend, polarizer unit and different sizes of corrugated waveguides. The total transmission loss of the transmission line including 10 m long waveguides is measured to be less than 1.1 dB. The burn patterns at different locations of the transmission line confirm the mode purity to be better than 93% in the TE02 mode. The transmission line terminates at a launcher box through a barrier window. The ECRH launcher consists of two mirrors to focus the microwave beam at the plasma center. The first mirror is convex while the second mirror is a concave focusing mirror. The mirrors are designed based on quasi optical analysis of the launcher system. The focal length of second mirror is 392.9 mm, which focuses the microwave beam to 35 mm (beam waist radius) at the plasma center. Beam steering in the plasma volume is restricted to ±2°. The gyrotron is tested up to ∼80 kW output power. A hard-wired interlock for various fault conditions, operates a rail-gap crowbar in less than 10 μS to protect the gyrotron. The gyrotron output is coupled to the tokamak Aditya (O-mode, perpendicular launch from low field side) and successful breakdown of the neutral gas is observed at different tokamak parameters. The paper describes the technical aspects of commissioning of the ECRH system and breakdown results on Aditya.© IEE