A Coaxial Cavity gyrotron

- recent results and future plans -

B. Piosczyk, H. Budig, G. Dammertz, O. Dumbrajs², O. Drumm, S. Illy, J. Jin, M.V. Kartikeyan, W. Leonhardt, V. Manuilov³, A. Pavelyev³, M. Schmid, M. Thumm,

D. Wagner⁴, X. Yang

Forschungszentrum Karlsruhe, Association EURATOM-FZK,

Institut für Hochleistungsimpuls- und Mikrowellentechnik, D-76021 Karlsruhe, Germany

² Department of Engineering Physics and Mathematics, Helsinki University of Technology (HUT),

Association EURATOM TEKES, FIN-02150 Espoo, Finland

³ Institute of Applied Physics, Nizhny Novgorod, Russia

⁴ IPP, Garching, Germany

A built up of a Penning discharge inside the gun caused a limitation of the pulse length around 10 to 15 ms ( about 30 ms at a very low (< 1A) beam current) due to a strong increase of the current to the insert. Recently it has been experimentally verified that the occurrence of the Penning discharge can be suppressed by a suitable gun geometry which avoids trapping regions for electrons.

The dependence of the body current I_body from the body voltage has been investigated. In the measurements it has been found:

1: I_body is dominated by the current I_ins to the insert, I_body » I_ins. The current to the outer wall is practically negligible.

2: I_ins increases approximately linearly with the body voltage U_body which is applied as well to the anode, the gyrotron body and the coaxial insert.

At operation without depressed potential (U_body = 0 kV) I_ins is only about 2 mA (~ limit of accuracy) and rises to about 35 mA at U_body = 27 kV at a beam current of ~50 A. I_ins increases with time and becomes stationary after about 5 to 20 ms depending on beam current. The background pressure may vary during a pulse from about 10^-8 mbar up to about 10^-6 mbar.

Suggested explanation: In operation with depressed collector (U_body > 0) a negative potential barrier exists in front of the collector. Thus electrons created by ionization of the background gas become trapped axially between the negative potential wells of the cathode and collector and radially by the strong axially symmetric magnetic field. The current to the insert is due to diffusion of the trapped electrons across the magnetic field. To explain the measured value of I_ins under stationary conditions a significant contribution of the trapped electrons to the ionization of the neutrals is needed. With increasing value of U_body the density of the background plasma is rising. It may significantly exceed the charge density of the primary beam.

The work on a prototype of a 2 MW, CW coaxial gyrotron at 170 GHz started in cooperation between European laboratories (FZK Karlsruhe, CRPP Lausanne and HUT Helsinki) together with Thales as industrial partner. The TE_34,19 mode has been selected as operating cavity mode. In order to reduce the amount of microwave stray radiation an improved RF output coupler with an advanced launcher and three mirrors has been designed. To verify the design of some components as electron gun, cavity and the RF output system the 165 GHz coaxial gyrotron is under redesign for operation at 170 GHz. This will allow and to measure the amount of the stray radiation under relevant conditions. The new design of the electron gun avoids regions in which electrons can be trapped. Due to a maximum magnetic field of only 6.667 T a reduction of the operating voltage from 90 kV to 80 kV.

The experimental results will be reported and the design will be discussed.