Page 3 - 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
most power (around 60%) is damped inside the separatrix. Also, Figs. 4(b) and 4(c) indicate that the power partition is 72% on electrons and about 19% on ions. The remaining power (about 9%) is damped through collisions (Fig. 4(d)) due to a relatively low electron temperature at plasma edge. The “sawteeth”- like features seen in profiles in Figs. 4(a) and 4(c) represent ion cyclotron harmonic resonance damping.
The 2D power density profiles of damping on electrons, ions, and collisional damping are plotted in Fig. 5 with the marked area showing power damping region. As can be seen clearly in Fig. 5, the power damping regions for electrons and ions are overlapped, thus there is a competition of power partition between electrons and ions when HHFW propagates into this overlapped area.
4 (a) P
3 total
= 999 kW
750 500 250
(a)
2 1
0 4 3 2 1 0
(b) Pe = 720 kW
0.12 (b)
0.08
0.04
0.00 3i 0.8
4 (c) P = 194 kW
(c) 0.2
800
600
400
200
Fig. 6. Changes of (a) local |B(r, z)|, (b) imaginary part of perpendicular wave number ki, (c) ratio of |E///E|, and (d) parallel refractive index n//, with the distance along wave propagation.
2 1
0.6 0.4
0 4
(d)
P=85kW cl
Separatrix
1.5
3
2
1
0 0
0.5
1.0 Rho
2.0
(d)
0 20 40 60 80 100 120 Distance along ray (cm)
Fig. 4. Radial profiles of power density for (a) total absorption, (b) damping on electrons, (c) damping on ions, and (d) collisional damping.
0 -20 -40
-60 (a) electrons 0
-20
-40
-60 (b) ions 0
-20
-40
-60 (c) collisional
-150 -100 -50 0
Z (cm)
Damping region
50 100 150
Figure 6 can help to better understand the damping mechanisms of HHFW in FRC plasmas, which shows the changes of local magnetic field (|B(r, z)| = (Br2 +Bz2)1/2), imaginary part of perpendicular wave number ki, the normalized parallel component of wave electric field |E///E|, and parallel refractive index n//, with the distance along wave propagation. The value of ki in Fig. 6(b) describes power damping on electrons by both MP and LD; although we are presently unable to separate the contribution of MP and LD, we can use Fig. 6(c) to conclude where LD can have an effect of power damping on electrons because FLD = eE//. Clearly, if |E///E| is small, there can be no pronounced LD. Meanwhile, according to Ref. [5], MP is strongly related to the local magnetic field, and it is found to scale as
Fig. 5. The 2D profiles of power density for (a) damping on electrons, (b) damping on ions, and (c) collisional damping. Background contours show magnetic flux 2D profile.
B-3.As can be seen in Figs. 6(b) and 6(c), there is no collisionless damping on electrons at a distance of wave propagation from 0 to 40 cm because the equilibrium magnetic field remains large and at the same time the value of |E///E| is nearly zero. A significant power
3
R (cm) R (cm) R (cm)
Power density (W/cm3)
n|| |E|| / E| K (cm-1) |B| (G)