Status of the new ECRH system for ASDEX Upgrade

Abstract

The new ECRH system which is currently under construction at ASDEX Upgrade is aimed for a total power of 4 MW, generated by 4 gyrotrons, at a pulse length of 10 sec. A particular feature is the use of multifrequency gyrotrons, where the gyrotrons will work either at two frequencies (105/140 GHz) or will be step-tunable in the range 105-140 GHz. The system should allow for central heating at different magnetic fields. It will also give more flexibility for the stabilization of neoclassical tearing modes (NTM) through the possibility of current drive on the high field side without changing the magnetic field and very localized feedback controlled power deposition with a narrow beamwidth by using a fast steerable launcher. The transmission line components are especially designed to cope for the large bandwidth of the system. RequirementsThe power deposition in the plasma is primarily determined by the magnetic field. For a single frequency ECRH system this has the consequence that for central heating the magnetic field is no longer a free parameter. However, for plasmas with different plasma currents or different equilibria, the magnetic field should be a free parameter in order to operate at a reasonable edge safety factor q(a). Furthermore, in a plasma with given parameters, some experimental program points, like suppression of neoclassical tearing modes, require to drive current on the high field side without changing the magnetic field. These requests can be satisfied if the gyrotron frequency is variable. In the experiments performed up to now in ASDEX Upgrade the available power was only 2 MW, of which 1.6 MW is injected into the plasma. This imposed a real limit for current drive, NTM stabilization or generation of internal transport barriers [1]. The requirement for the new system is therefore an installed power of 4 MW.Since the current diffusion time in hot plasmas, like those with an internal transport barrier and Te > 10 keV, is several seconds, we require a pulse duration of 10 sec compatible with the limit of flat top discharges. A further requirement is the capability for very localized power deposition such that its center can be feedback controlled, for instance to keep it on a resonant q-surface. For this purpose we will install fast moveable mirrors. GyrotronsThe system will be constructed in several steps. The first gyrotron to be installed can work at 105 GHz and at 140 GHz. The corresponding operating modes are TE 17,6 and TE 22,8 (two-frequency gyrotron). Here we make use of the 3λ/2 and 4λ/2 resonances (λ is the wavelength) of the single disk synthetic diamond vacuum window at these frequencies. A second gyrotron will be steptunable, i.e. it can work at several frequencies within the same frequency range. For each frequency the gyrotron oscillates in a different cavity mode. Fig. 1 shows the possible modes and frequencies. Short pulse test gyrotrons have demonstrated efficient generation of power in excess of 1 MW at all these frequencies [2,3]. The frequency can be changed between two ASDEX...

Extension of the ECRH operational space with O2 and X3 heating schemes to control tungsten accumulation in ASDEX Upgrade

“…A new beam position detection system, based on Ohmic losses of the microwave on the mirror surface, was installed. With thin NiCr-Ni thermocouples (diameter 0.25 mm and only a few ms rise time) it is planned to react on beam movements, caused for example by density changes, in real time by adjusting the poloidal injection angle with the fast steerable launcher [1]. A reaction time of a few 10 ms is foreseen for this issue, which is smaller than the expected energy confinement time τ e ≈ 100 ms.…”

“…The new ECRH system will consist of four multi-frequency gyrotrons delivering 1 MW over a complete ASDEX Upgrade discharge (10 s) [1,2]. The typically used second harmonic extraordinary mode (X2 mode) at 2.5 T has the advantage of a complete and localized absorption.…”

ECRH on ASDEX Upgrade - System Status, Feed-Back Control, Plasma Physics Results -

“…The ECRH system of AUG consists of two parts: four units each delivering 0.5 MW at 140 GHz for 2 s, in the following called old system or ECRH-1 [2] and four new units (out of which three are operational) each delivering 1 MW at 140 GHz or 0.8 MW at 105 GHz, both for 10 s, in the following called new system or ECRH-2 [3,4]. Both systems operate with air-filled 87mm corrugated waveguides.…”

Real-time diagnostics at ASDEX Upgrade—Integration with MHD feedback control

“…Figure 1 shows, how the magnetic flux surfaces link island position, ECE observation channels and ECH beam deposition and illustrates the necessity of combined diagnostic evaluation. With this information the controller directs the beams of the electron cyclotron heating (ECH) to the resonant flux surface, where the mode is located, by moving the steerable mirrors [3]. There, by depositing power and driving current, the mode is stabilized.…”