No abstract

In W 7-AS the H mode has been observed for the first time in a currentless stellarator plasma. H modes are achieved with 0.4 MW electron cyclotron resonance heating at 140 GHz at high density. The H phases display all characteristics known from tokamak H modes including edge localized modes (ELMs). The achievement of the H mode in a shear-free stellarator without toroidal current has consequences on //-mode transition and ELM theories.

It has been observed in the T-10 tokamak that immediately after off-axis electron cyclotron resonance heating (ECRH) switch-off, the core electron temperature stays constant for some time, which can be as long as several tens of milliseconds, i.e. several energy confinement times (τ E), before it starts to decrease. Whether or not the effect is observed depends critically on the local magnetic shear in the vicinity of the q = 1 rational surface, which should be close to zero. It is hypothesized that a small shear can induce the formation of an internal transport barrier. Measurements of density fluctuations in the transport barrier with a correlation reflectometer show immediately after the ECRH switch-off a clear reduction in the fluctuation level, corroborating the above results. The delayed temperature decrease has also been observed in similar discharges in the TEXTOR tokamak; however, the delay is restricted to ∼1×τ E .

Tests of the Russian ITER gyrotrons were continued after debugging of the test facility at Kurchatov Institute including the evacuated transmission line and power supplies capable to provide gyrotron CW operation. A new ITER gyrotron (V-10) with upgraded body insulation, liquid cooling system for the DC-break ceramic and improved design of the relief window has been manufactured and is currently under test. A maximal total efficiency near 55% at 1 MW power level was attained. The maximal pulse duration of 1000 s at 0.8 MW power was limited at that time by the test stand. At 1 MW power level the gyrotron V-10 up to now reached a pulse duration of 570 s. An advanced short-pulse (100 ms) gyrotron model operating in the TE 28.12 mode demonstrated a very robust operation at relatively high electron energies (up to 100 keV) necessary to achieve 1.5-2 MW output power. Three double frequency gyrotrons have been delivered to ASDEX Upgrade. The development of a multi-frequency gyrotron with a broadband or tuneable synthetic diamond window is in progress.

The present paper reports on the recent development of several oversized millimeter wave transmission line components for different applications. The studies include a circular TE 11 -to-Gaussian beam mode converting horn, a TM 01 -to-rotating TE 31 mode converter, a TE 11 -mode 90°bend, a series of different HE 11 -mode transmission line components, a notch filter and a fast laser controlled semiconductor microwave switch.