Experimental Study of the Cathode Modulation for the TH2103A Klystrons on HL-2A Tokamak

The 3.7 GHz high power klystrons TH2103A are widely used in the lower hybrid current drive system for the tokamak experiment. In order to avoid the tetrode and simplify the power supply system, the cathode modulation for high power TH2103A klystron is experimentally studied. Both the independent Anode Power-Supply (APS) and a Passive Potential Divider (PPD) configuration are investigated. The beam current goes to zero in 10 ms in the APS mode directly. As a result, a stable beam current is achieved in the PPD mode when the resistor from the anode to the ground is less than 30 k Ω, the cathode voltage is below − 45 kV and its rise-time is between 100 and 150 μs. The threshold of the resistor and the rise-time may be caused by the inductance of the wire-wound resistors. Although the anode current exceeds 10 mA when the cathode voltage rises and falls, 5 years operation validates the safety of the klystron. A practical and simple method is developed to operate the anode modulation klystron by cathode modulation on HL-2A, without using a dedicated tetrode.