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 Ibody from the body
voltage has been investigated. In the measurements it has been found:
1:
Ibody is dominated by the current Iins to the
insert, Ibody » Iins. The current to the outer wall is
practically negligible.
2:
Iins increases approximately linearly with the body voltage Ubody
which is applied as well to the anode, the gyrotron body and the coaxial
insert.
At operation without depressed potential (Ubody
= 0 kV) Iins is only about 2 mA (~ limit of accuracy)
and rises to about 35 mA at Ubody = 27 kV at a beam
current of ~50 A. Iins 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 (Ubody > 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 Iins under stationary conditions a significant contribution
of the trapped electrons to the ionization of the neutrals is needed. With
increasing value of Ubody 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 TE34,19 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.