M. A. Irzak

Analysis of the efficiency of lower hybrid current drive in the FT-2 tokamak

Results are presented from experimental studies of the efficiency of lower hybrid current drive (LHCD) in the FT-2 tokamak. The dependence of the LHCD efficiency on the grill phasing Δφ and RF oscillator power was determined experimentally in a wide range of plasma densities. It is shown that, at high plasma currents (i.e., at sufficiently high electron temperatures), current drive is suppressed when the plasma density reaches its resonance value nLH for the pumping wave frequency, rather than when parametric decay comes into play (as was observed in regimes with lower plasma currents and, accordingly, lower electron temperatures Te). In order to analyze the experimentally observed effect of LHCD and its dependence on the value and sign of the antenna phasing, the spectra of the excited LH waves, P(Nz), were calculated. Simulations using the FRTC code with allowance for the P(Nz) spectrum and the measured plasma parameters made it possible to calculate the value and direction of the LH-driven current, which are determined by the spectrum of the excited LH waves. It is shown that the synergetic effect caused by the interaction between different spectral components of the excited RF wave plays a decisive role in the bridging of the gap in the wave spectrum.

Impact of isotopic effect on density limit and LHCD efficiency in the FT-2 experiments

Current drive by lower hybrid waves (LHCD) is the most effective method to sustain the plasma current, but it is feasible only at the plasma density not exceeding some density limit nDL. In the present work the main attention is paid to the investigation of this effect on the FT-2 (R = 0.55 m, a = 0.08 m, BT ≤ 3 T, Ipl = 19–40 kA, f0 = 920 MHz) tokamak. The dependence of LHCD efficiency on isotopic plasma content (hydrogen/deuterium) is studied. Characteristic features of such an experiment are a strong influence of the isotope plasma composition on the LH resonance density nLH. For hydrogen plasma nLH ≈ 3.5 × 1019 m−3, while for deuterium plasma nLH ≈ 2 × 1020 m−3. The suppression of the LHCD and beginning of the interaction of LH waves with ions are determined by the hydrogen/deuterium plasma density rise. In the hot hydrogen plasma (Te(r = 0 cm) ≈ 700 eV) the density limit nDL of LHCD is approximately equal to the resonance value nLH ≈ nLC, where nLC is the point of linear conversion. In the hot deuterium plasma one could expect an increase of nDL because of a much higher value of nLH ≥ nLC ≈ 1020 m−3. However it appeared that the observed density limit for LHCD generation nDL ≈ (3.5–4) × 1019 m−3 is not determined by nLH. The role of parametric instabilities in CD switch-off is considered in both cases. The cooling of the plasma column and density rise could lead to a reduction of the threshold for the parametric decay of f0 and result in early suppression of LHCD. In both cases the LHCD was inversely proportional to the density, which corresponds to the theoretical predictions. In order to analyse the experimentally observed LHCD efficiency the GRILL3D and FRTC codes have been used.

Isotopic effect in experiments on lower hybrid current drive in the FT-2 tokamak

To analyze factors influencing the limiting value of the plasma density at which lower hybrid (LH) current drive terminates, the isotopic factor (the difference in the LH resonance densities in hydrogen and deuterium plasmas) was used for the first time in experiments carried out at the FT-2 tokamak. It is experimentally found that the efficiency of LH current drive in deuterium plasma is appreciably higher than that in hydrogen plasma. The significant role of the parametric decay of the LH pumping wave, which hampers the use of the LH range of RF waves for current drive at high plasma densities, is confirmed. It is demonstrated that the parameters characterizing LH current drive agree well with the earlier results obtained at large tokamaks.

Isotopic effect study in the LHCD and LHH experiments in hydrogen/deuterium plasmas of the FT-2 tokamak

Results of comparative experimental studies of the efficiency of lower hybrid current drive (LHCD) and lower hybrid heating (LHH) in the FT-2 tokamak in hydrogen and deuterium plasmas are presented. In the new comparative experimental runs in deuterium/hydrogen plasmas suppression of the LHCD and beginning of the interaction of LH waves with ions is controlled by the plasma density rise. Role of parametric instabilities in CD switch-off is considered. In order to analyze the experimentally observed effect of LHCD the GRILL3D and FRTC codes has been used.

Simulation of the conditions of wave excitation for the optimization of the lower hybrid current drive in the Globus-M spherical tokamak

At present, the method of current drive by means of lower hybrid waves is not applied to low-aspect-ratio tokamaks, because, in the traditional approach, it would be necessary to use waves with a very high slowing-down factor. However, studies of new transparency regions for waves in a nonuniform magnetized plasma, performed earlier at the Ioffe Physical Technical Institute, Russian Academy of Sciences, made it possible to develop an approach in which slow waves are excited in the poloidal (rather than toroidal) direction. In this approach, moderately slowed-down waves first propagate in the poloidal direction, but then turn in the toroidalal direction and get into the dense plasma. In this work, this approach is further developed using numerical methods. In particular, the influence of the density profile in the edge plasma on the efficiency of wave excitation for given antenna parameters is studied in detail.

Study of LHCD efficiency on the FT‐2 tokamak

The paper pays the main attention to the LHCD efficiency in experiments on the FT‐2 tokamak. LH wave spectra P(Nz) excited by the grill computed by GRILL3D code are presented. Physical processes responsible for termination of the CD with density rise are discussed for lower and higher plasma currents that are for relatively cold and relatively hot plasmas. A role of parametric processes and additional impurity influx for CD switching‐off in the plasma are considered for those conditions.