Electron Bernstein wave heating of over-dense H-mode plasmas in the TCV tokamak via O-X-B double mode conversion

Abstract

This paper reports on the first demonstration of electron Bernstein wave heating (EBWH) by double mode conversion from ordinary (O-) to Bernstein (B-) via the extraordinary (X-) mode in an over-dense tokamak plasma, using low field side launch, achieved in the TCV tokamak H-mode, making use of its naturally generated steep density gradient. This technique offers the possibility of overcoming the upper density limit of conventional EC microwave heating. The sensitive dependence of the O-X mode conversion on the microwave launching direction has been verified experimentally. Localized power deposition, consistent with theoretical predictions, has been observed at densities well above the conventional cut-off. Central heating has been achieved, at powers up to two megawatts. This demonstrates the potential of EBW in tokamak H-modes, the intended mode of operation for a reactor such as ITER.

Overview of TCV results*

“…To demonstrate EB heating over periods longer than the confinement time, long pulses and a more central power deposition scheme (ρ = 0.4) are used. A central temperature increase of up to 80 eV is measured consistently by Thomson scattering and the soft x-ray absorber method [10]. These results constitute the first demonstration of heating by O-X-B double mode conversion in an overdense, conventional aspect-ratio tokamak and motivate further investigations of the potential of using EB heating or EB current drive in H-modes of large tokamak devices.…”

“…Heating plasmas at densities above the second harmonic X-mode cut-off can lead to increasing both the plasma confinement and the pressure. The means to achieve high-density electron heating on TCV include quasi-vertically launched X3 EC waves, discussed in section 4 and low field side injected electron Bernstein waves [10,28]. EB waves are not subject to X2 cut-off, hence can be used to circumvent the limitation to the plasma density that can be accessed by O-and X-mode EC.…”

“…EB waves are not subject to X2 cut-off, hence can be used to circumvent the limitation to the plasma density that can be accessed by O-and X-mode EC. The O-X-B scheme is used on TCV, where the large densities and density gradients necessary for the O-X conversion are obtained in diverted H-modes with specific values of triangularity and edge safety factor, δ ∼ 0.55 and q 95 ∼ 2.5 [10]. The optimum injection angle for the O-X-B conversion is determined experimentally by varying the injection angle of EC waves modulated at a low duty cycle (6%) and measuring the microwave stray radiation.…”

“…This overview highlights the progress accomplished on TCV during the 2004-2006 campaigns, which were focused on five general themes, reflected in the structure of the paper: section 2 deals with particle [5], energy [6] and momentum transport in shaped plasmas, including recent findings on the spontaneous plasma rotation profile [7]; section 3 discusses our recent efforts in plasma edge physics, mainly devoted to understanding the origin of anomalous cross-field transport in the scrape-off layer (SOL) [8]; section 4 addresses the physics of H-modes under strong electron heating at reactor relevant β [9]; section 5 describes investigations on the physics of ECH and ECCD [9], including electron Bernstein waves [10]. These EC wave-plasma interactions can be used as tools to progress towards high performance, steady-state plasma scenarios, with electron internal transport barriers (eITBs) and large noninductive current fractions [11], discussed in section 6.…”

Feasibility study of O–X coupling for overdense plasma heating through O–X–B mode conversion in FTU

“…For this reason, in general, the O-X-B coupling scheme is interesting mostly for medium (B ≈ 1.5 T) and low (B ≈ 0.5 T) magnetic field devices, such as stellarators [9] and spherical tokamaks [10]. This scheme was demonstrated in the W7-AS stellarator [11] and has been tested for the first time in a standard aspect ratio tokamak in the TCV device [12,13]. Experiments with EBWs have also been carried out at electron densities higher than 2.43 × 10 20 m −3 in the W7-AS stellarator [14], by using the second, third and fourth electron cyclotron (EC) harmonics of the 140 GHz frequency.…”