R.
Heidinger1a, I. Danilov1a, M. Rohde1a, J.
Burbach1a, A. Meier1a, M. Thumm1b,2,
G. Dammertz 1b, K. Takahashi3, K. Sakamoto3
1 Forschungszentrum Karlsruhe, Association FZK-Euratom,
(a) Institute for Materials Research I,(b) Inst. for Pulsed Power and Microwave
Technology, D-76021 Karlsruhe, Germany
2 University Karlsruhe, Institute of
High-Frequency Techniques and Electronics,
D- 76128 Karlsruhe, Germany
3 JAERI, Naka, Ibaraki, 311-0193, Japan
Since the demonstration of long pulse EC
wave power transmission through diamond windows at the Megawatt level, large
area CVD diamond disks are the reference window material for high power EC wave
systems. As a demonstrator for an ITER torus window, a large area disk (106mm
dia x 1.87mm) was neutron irradiated to 1021 n/m2
(E>0.1 MeV). Radiation induced reduction of thermal conductivity was
observed, thus only 840 W/m·K was measured in the as-irradiated condition. The disk
was joint to Inconel cuffs by an Al braze at 600-650°C. In a JA/EU cooperation,
a window unit was manufactured and pressure tested up to 0.4 MPa. The stiffness
of the window fixture could be parameterised with the same boundary factor
(G=3) that described pressure tests of an earlier Al-brazed window unit at
JAERI. Successful transmission of 0.48 MW/30 s at 170 GHz was
achieved. Photoacoustic measurements on the disassembled unit proved a partial
recovery of thermal conductivity to 1190 W/m·K which hints at point defect
annealing during the brazing processes.
With open resonator mm-wave loss measurements, surface absorption were distinguished from bulk absorption. In window units fabricated with high temperature (800-850°C) reactive braze ("Ag/Cu"), the occurrence of additional surface loss was found to be impeded. Consequently in a long pulse gyrotron prototype operated at 140 GHz at 540 kW/940 s and 890 kW/180 s, a corresponding CVD diamond window showed dissipated power close to bulk absorption, i.e. as low as 200W and 400W respectively. The application of this brazing process to join Cu cuffs to the diamond window faces allows very compact window units for which an advanced design of a double disk structure with an evacuated disk interspace of typically 5 mm is presented. Diamond disks joined to metallic Inconel or copper cuffs by the two alternative brazing concepts are analysed for hydrogen isotope permeation by measuring deuterium flow rates with a mass spectrometer in the downstream section of a window exposed to deuterium pressures of up to 0.2 MPa on the upstream section.