Page 4 - Simulations of High Harmonic Fast Wave Heating on the C-2U Advanced Beam-Driven Field-Reversed Configuration Device

EPJ Web of Conferences 157, 03065 (2017) DOI: 10.1051/epjconf/201715703065 22 Topical Conference on Radio-Frequency Power in Plasmas
damping occurs in the area between 40 cm and 100 cm (also see the marked area in Fig. 3) due to a rapid drop of magnetic field (Fig. 6(a)), which leads to fast increase of ki (Fig. 6(b)). For the ray with blue color (n// = 5) in Fig. 6(c), the value of |E///E| is almost zero from 40 cm to 70 cm; this value is still less than 0.2 even when ki reaches a maximum value at the distance of 82 cm, where /k// 0.5VTe (this condition is different from either fast wave in tokamak plasmas or HHFW in ST plasmas). The value of ki drops to less than 1/3 of its maximum value when |E///E| reaches a maximum at 87 cm, where |B(r, z)| has a minimum and n// has a maximum. Therefore, conclusion can be made that the quick increase of ki is mainly attributed to high beta enhancement of MP, and power damping on electrons is almost completed before LD has a significant effect on power absorption.
4 Discussions
absorption (shorter distance along wave propagation). Calculations indicate that for the case plotted in Fig. 7, power absorption partition is around 15% on electrons and 75% on ions, and the remainder of 10% power is deposited through collisional damping.
In summary, our simulation results have demonstrated for the first time that the HHFW heating scenario can offer a promising path to efficient heating of electrons in the core of FRC plasmas. In a wide range of plasma parameters, single pass power absorption efficiency is 100%; power damping on electrons increases quickly with the fast drop of magnetic field strength in the core of FRC plasma, and it reaches a maximum value before LD has any significant effect on power absorption. It is obvious that MP is the dominated power absorption mechanism for HHFW electron heating in FRC plasmas.
Through extensive calculations, it is found that efficient HHFW electron heating in C-2U plasmas is more difficult to realize than the case of HHFW heating in ST plasmas. The unique magnetic configuration of FRC plasmas can cause the conflict between good wave penetration and favorable power partition between electrons and ions. In C-2U FRC plasmas, the parameters such as the ratio of / ci[D], the initial value of n//, the distance between antenna and plasma edge, and the launcher position along z axis, have significant impact not only on wave coupling and penetration, but also on power partition between electrons and ions. While in spherical tokamaks like NSTX, there is no issue of wave penetration into plasma core, thus one can only focus on how to avoid a parasitic ion heating or mitigate hot ion damping.
60 40 20
0 -20 -40 -60
-150 -100 -50 0
Z (cm)
50 100 150 1.0e13 1.6e13 2.0e13 (cm-3)
5e11 5.5e12
Fig. 7. The propagation trajectory of HHFW (5 MHz) in C-2U
In C-2U plasmas, on the one hand, in order to mitigate the competition from ion heating, one can maximize power damping on electrons by increasing (1) the ratio of / ci[D], or (2) the initial value of n//, or (3) the distance between antenna and plasma edge. However, the changes of those parameters may shift the area of power damping from inside the separatrix layer into the SOL. As a result, electrons at open field lines will be heated and therefore fast wave power will eventually be lost (at end divertors). Those results are somehow similar to the regime of Whistler wave heating.
plasmas. Background contours show electron density profile.
We would like to acknowledge the support and contributions from the rest of the TAE team and collaborators. A debt of gratitude is also due to our shareholders for their continuous support.
References
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On the other hand, in order to enhance HHFW penetration into the core of FRC plasmas, one can reduce (1) the ratio of / ci[D], or (2) the initial value of n//, or (3) the distance between antenna and plasma edge. However, although single pass absorption efficiency is still 100% in those cases, power absorption by electrons decreases substantially and most of the HHFW power is deposited to the ions through ion (D) cyclotron harmonic resonance absorption. By comparing with the case showninFig.2(f=7MHzandn// =5-7),onecanfind thatthecaseshowninFig.7(f=5MHzandn// =3-5) has a better wave penetration (getting more closely to the region of magnetic field null) and stronger power
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R (cm)