Abstract
The electron cyclotron resonance heating system at ASDEX Upgrade (AUG) is currently being extended to eight similar Gyrotrons in total. Each Gyrotron operates at 105 and 140 GHz and is designed for up to 1 MW millimetre wave output power. A substantial part of the AUG program will focus on experimental conditions, where the plasma density may be above the X-2 cut-off density at 140 GHz. In order to cope with the high density, the heating system will operate in the O-2 mode scheme with potentially incomplete absorption in the first pass. Reflecting gratings installed into the heat shield on AUG’s inner column allow for a controlled second pass of the beam’s unabsorbed fraction. Thermocouple measurements serve to control the beam position on the grating. The beam geometry is being finalized for the launchers #1-4. Beam propagation is simulated with the TORBEAM code and previous high density experiments are used as a database. The geometry is optimized using three criteria: central deposition, high absorption and robustness of the beam dump after the second pass. The experimental conditions, and the plasma electron density in particular, may vary such that the Gaussian beam parameters of the incoming beam on the grating deviate from the design values. It is proposed to model the effect of the grating with an equivalent ellipsoidal mirror. Laboratory measurements are shown, which support this model.
Highlights
A substantial part of the ASDEX Upgrade (AUG) program will focus on experimental conditions, where the plasma density may be above the X-2 cut-off density at 140 GHz
In order to cope with the high density, the heating system will operate in the O-2 mode scheme with potentially incomplete absorption in the first pass
Beam propagation is simulated with the TORBEAM code and previous high density experiments are used as a database
Summary
Plasma electron densities up to 1 1020 m-3 are regularly achieved in ASDEX Upgrade (AUG) experiments. Examples are high heat flux experiments [1] and fuelling with pellets [2] In the latter case the pellet ablation temporary and locally increases the plasma electron density even further. Due to the finite absorption [3], there are several important constraints on the O-2 scheme: In a situation typical for an AUG experiment, the maximum optical thickness τ is around 1 (figure 1). This corresponds to approximately one third of the launched beam power leaving the plasma, the so-called ‘shine-through’, after the first pass. This is tested in a laboratory mock-up of the reflection grating with a 140 GHz source and network analyser
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