Journal
JOURNAL OF INFRARED MILLIMETER AND TERAHERTZ WAVES
Volume 36, Issue 9, Pages 797-818Publisher
SPRINGER
DOI: 10.1007/s10762-015-0177-1
Keywords
Gyrotrons; Long pulse operation; Frequency tuning; Voltage depression; Space charge effect; Neutralization; Thermal cavity expansion
Funding
- Euratom research and training programme [633053]
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During the first hundred milliseconds, the frequency and RF output power of long pulse operating gyrotrons undergo deterministic variation. This well-known behavior is caused by the thermal expansion of the cavity and internal electrostatic processes related to the ionization of residual gas. A macroscopic analytical investigation of the gas conditions in modern gyrotrons indicates that ionization processes are unlikely to influence the overall internal gas pressure. In combination with electrostatic potential considerations, it was found that the beam depression voltage is not fully neutralized; in the case of W7-X gyrotrons, a maximum value of about 60 % neutralization is expected, in conflict with the common assumption of full neutralization in steady state. Using experimentally measured frequency shifts and the Evridiki gyrotron interaction simulation code, a fitting process was employed to further investigate these effects. The results are in very good agreement with the theoretical predictions and allow a separation of the time constants of the two processes causing the frequency tuning.
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