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Full wave simulation of lower hybrid waves in Alcator C-Mod experiment. Left figure is the first run and the right figure is after three iterations with the Fokker-Planck code. There are four 6cm high waveguides on the low field (right) side. Parameters of runs: (980N ψ x2048N θ ) D plasma, n =-1.55, f=4.6 GHz B 0 = 5.36 T, T e =2.33 keV, T i =1.0 keV n e (0) = 0.7 × 10 20 m −3 ,I=545 kA
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Lower hybrid (LH) waves have the attractive property of damping strongly via electron Landau resonance on relatively fast tail electrons at (2:5 3) vte, where vte (2Te=me)1=2 is the electron thermal speed. Consequently these waves are well-suited to driving current in the plasma periphery where the electron temperature is lower, making LH current d...
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... discharge 1060728011 at 1 s, the loop voltage was maintained at 0.2 V out of 1.0 V by lower hybrid current drive. The parameters of the simulation are given in Figure 2 and correspond to a scenario in which the lower hybrid waves are accessible to the center of the device. Full wave simulation of this shot shows that for damping on a Maxwellian distribution, the fields are space filling and are amplified by a factor of ≈ 200 from the applied field (1 V/m) because of the cavity effect in weak damping. ...
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... wave simulation of this shot shows that for damping on a Maxwellian distribution, the fields are space filling and are amplified by a factor of ≈ 200 from the applied field (1 V/m) because of the cavity effect in weak damping. The right plot in Figure 2 is of the contours of the parallel electric field after iterating to self-consistency with the Fokker-Planck code. ...
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... observing the evolution of non-Maxwellian features in the electron distribution we can see direct evidence of the spectral broadening. We have used the electric fields from the full wave solution shown in Figure 2(left plot) to formulate the RF quasilinear dif- fusion coefficient. This was then employed in the Fokker-Planck code CQL3D to evolve the electron distribution function. ...
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... was then employed in the Fokker-Planck code CQL3D to evolve the electron distribution function. In the right plot of Figure 2, the electric fields are less intense due to stronger absorption. In the left plot of Figure 4, we see the formation of a quasilinear plateau clearly at a pitch angle of θ = 0 at a flux surface of r/a ∼ 0.5 located in the region of power absorption. ...
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Over-dense plasma heating with electron Bernstein waves (EBWs) is of large importance for fusion, since ultra high density operation is a promising fusion scenario for stellarators and heliotrons as well as for spherical tokamaks. EBWs are generated by the OXB-mode conversion process, which necessitates a threshold density (O-mode cut-off density)...
The Hamiltonian character of the ray tracing equations describing the propagation of the Lower Hybrid Wave (LHW) in a magnetic confined plasma device (tokamak) is investigated in order to study the evolution of the parallel wave number along the propagation path. The chaotic diffusion of the “time-averaged” parallel wave number at higher values (wi...
Citations
... The disconnect between edge coupling, treated by coupling codes such as GRILL [26] or ALOHA [27], and core propagation/absorption, treated by traditional ray tracing/Fokker-Planck codes such as GENRAY/CQL3D, must be bridged and the two problems treated together. Recent advances in full wave simulations with codes such as TORIC-LH [28, 29] and COMSOL [30, 31] allow proper treatment of both edge coupling (including penetration through the pedestal) and core propagation/absorption in a seamless manner. It is imperative to consider not only the power leaving the antenna, but the power actually absorbed on closed flux surfaces when simulating tokamak plasmas in the low single pass absorption regime. ...
The goal of the Lower Hybrid Current Drive (LHCD) system on the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] is to investigate current profile control under plasma conditions relevant to future tokamak experiments. Experimental observations of a LHCD ``density limit'' for C-Mod are presented in this paper. Bremsstrahlung emission from relativistic fast electrons in the core plasma drops suddenly above line averaged densities of 1020 m-3 (omega/omegaLH~3-4), well below the density limit previously observed on other experiments (omega/omegaLH~2). Electric currents flowing through the scrape off layer (SOL) between the inner and outer divertors increase dramatically across the same density range that the core bremsstrahlung emission drops precipitously. These experimental x-ray data are compared to both conventional modeling, which gives poor agreement with experiment above the density limit and a model including collisional absorption in the SOL, which dramatically improves agreement with experiment above the observed density limit. These results show that strong absorption of LH waves in the SOL is possible on a high density tokamak and the SOL must be included in simulations of LHCD at high density.
... TORICLH (Wright 2004,2008) CQL3D (Harvey 1992) Dielectric for LH uses zero FLR electrons and unmagnetized ions. ...
Lower hybrid (LH) waves have the attractive property of damping strongly via electron Landau resonance on relatively fast tail electrons. These waves are well-suited to driving off-axis current profile control in reactor grade plasmas. The break down of WKB techniques at reflections from the plasma cutoff make traditional ray tracing techniques unreliable in the weak absorption limit. A massively parallel version of the TORIC full-wave electromagnetic field solver valid in the LH range of frequencies has been developed [Wright, et al. CCP, 4, 545 (2008)] and applied to scenarios at the density (10^14/cc) and magnetic field (5T) characteristic of devices such as Alcator C-Mod [Wright, et al. PoP, 16, TBP (2009)]. We find that retaining full wave effects due to diffraction and focusing at caustics and reflections has a strong effect on the location of wave absorption. Lower hybrid waves strongly modify the electron distribution and form a quasilinear plateau. In order to include this effect and field solutions consistent with the distribution function, we have coupled the CQL3D Fokker-Planck code to the lower hybrid solver, TORLH [Valeo, et al. 17th RF Topical Conf., n933, p. 297, New York, 2007]. Through iteration, a self-consistent solution is obtained. We discuss the effects off the non-Maxwellian distribution on power deposition and HXR spectra.
The spectrum change of the lower hybrid (LH) waves caused by a low-frequency density fluctuation in the scrape-off-layer is studied by applying the wave scattering model developed by Bonoli and Ott [Phys. Fluids 25, 359 (1982)] via a Monte Carlo method. Due to the influence of the density fluctuation, the perpendicular component of the LH wave-vector can be rotated in the 2D perpendicular space, which will further change the ray trajectory of the LH wave. A ray-tracing model specific to this purpose is developed to evaluate the probability distribution of both the poloidal refractive index (Nθ) and the parallel refractive index (N∥) of the LH wave at the last closed flux surface (LCFS). The scattering probability and the rotation scattering angle are determined by the combined effect from the geometric optics approximation term and the E×B drift term in the LH tensor elements. The probability distributions of N∥ and Nθ at the LCFS are studied using the EAST parameters as a function of wave frequency, the initial N∥, and the polar injection position, which may influence the lower hybrid current drive efficiency.
In this article, we present a first-principles electromagnetic–kinetic simulation of the slow-wave branch of the lower hybrid waves (LHWs) in electron–deuterium plasmas with real ion–electron mass ratios. Several models of two-dimensional slab configuration containing a grill antenna are constructed using different plasma parameters. Based on our recently developed fully kinetic charge-conservative electromagnetic non-canonical symplectic particle-in-cell method, we studied the coupling, propagation, absorption and current driving effect of LHWs in hot plasmas. The results for the coupling coefficient of the grill antenna, accessibility condition, and electron Landau damping rate show good agreement with theory and previous simulation. The long-term non-linear energy and current deposition of propagating LHWs are also presented, which show a decrement of heating and current-driving efficiency compared with the linear theoretical prediction. The collision effect between electrons and ions is shown to be important for actually driving the current. The relation between the density of the current generated and the amplitude of the LHW is given, which shows a qualitative agreement with the 1D theoretical prediction.
Plasma current start-up and ramp-up using the lower hybrid wave (LHW) were investigated on the TST-2 spherical tokamak. The LHW was launched by a dielectric-loaded waveguide array (grill) antenna. The antenna–plasma coupling of this antenna deteriorates as the input power exceeds several kW. This deterioration is believed to be caused by the density depletion due to the ponderomotive force. This conjecture was confirmed by the measurement of density reduction and the result of a non-linear full wave numerical calculation based on the finite element method (FEM).
The plasma current was started and ramped up to 10 kA using this antenna. The ability of this grill antenna to excite the LHW with different n∥ = ck∥/ω was used to identify the most favourable n∥ spectrum for plasma current ramp-up. It was found that effective current drive can be achieved by the LHW with n∥ less than 6. However, even in this case, the energetic electrons which account for a large fraction of the driven current, are lost rapidly because the poloidal field generated by this level of plasma current is not sufficient to confine high energy electrons.
Plasma current start‐up experiments using the lower hybrid
wave (LHW) excited by a new dielectric loaded waveguide array antenna are planned on TST‐2. The wave excitation efficiency of this antenna is investigated using a versatile FEM solver package, COMSOL. Propagation and absorption of the LHW are calculated by iterating between the TORLH full wave code and the CQL3D Fokker‐Planck code. Wave absorption is very weak for Maxwellian plasma, but becomes stronger as the quasilinear deformation of the distribution function develops.
