Page 2 - A combined mmwave and CO2 interferometer on the C-2W Jet plasma

Nucl. Fusion 58 (2018) 082011
R.M. Magee et al
primarily perpendicular to the magnetic field, and although they can acquire a significant parallel velocity, they retain their large orbits and are in no approximation tied to a field line. Early experiments of NBI into magnetic mirrors showed that beam driven modes near the ion cyclotron frequency could be excited which did not degrade plasma confinement [9].
While the injection geometry resembles that of a high power mirror machine, the high beta C-2U target plasma has more in common with spherical tokamaks (STs) than traditional tokamaks. In machines like NSTX/NSTX-U and MAST where the fast ion velocity can be several times larger than the Alfvén velocity (vA = B/√ε0mn, where B is the magnetic field, m is the ion mass, and n is the plasma density) the dominant fast ion driven modes are compressional and global Alfvén eigenmodes (CAEs and GAEs). [10] Although the precise identity of the modes described below has yet to be determined, the commonalities in both the plasma-fast ion environment (high beta, vfi/vA > 1) and mode characteristics (k ⊥ B) suggest a connection.
3. Observations
The C-2U FRC is a prolate toroid with typical plasma parameters: ne ∼ 3x1019 m−3, Te ∼ 120 eV, Ti ∼ 500 eV, Bext ∼ 700 G, r∆φ,0 ∼ 40 cm (initial excluded flux radius, approximately equal to the separatrix radius), and length L ∼ 2 m. The FRC is formed from the dynamic merger of 2 theta pinches [11] and sustained with 6 neutral beam injectors, each producing 120 A of neutral current at 15 kV injected tan- gentially with an impact parameter of 19 cm and at an angle of
20◦ (see figure 1(a)).
In non-sustained C-2U plasmas, the radius of the FRC
decays in time. When the radius reaches approximately 20 cm, high frequency magnetic fluctuations are observed at the plasma edge. These fluctuations typically have a frequency of ∼1 MHz, which is greater than the edge ion cyclotron fre- quency ( fci ≈ 500 kHz), precluding the shear Alfvén wave, but not the fast magnetosonic, as the responsible mode [12].
Measurements of the plasma density in the open field line plasma are unavailable in the bulk of the C-2U database, so it is difficult to calculate the edge Alfvén velocity in absolute terms. We can however use the measured FRC density, which is likely proportional to the edge density, and the measured edge magnetic field to investigate the scaling of the fluctua- tion frequency.
Plotted in figure 3 is the ratio of the edge magnetic field, Bext to the square root of the average FRC electron density, ne (which we assume to be equal to average core ion density via quasineutrality). This quantity is proportional to the edge VA ∼ 106 m s−1. Despite the uncertainties in the absolute value of the Alfvén velocity and the limited range of acces- sible Bext/√ni, a linear scaling is apparent, indicating that the
mode is indeed Alfvénic.
An additional striking feature is the two distinct groupings,
one with f > 1.3 MHz and one with f < 1.3 MHz. As we will show below, the high frequency modes tend to have a higher mode number.
Figure 1. (a) A CAD visualization of the C-2U vacuum vessel with FRC target in center and 6 neutral beam injectors. (b) The magnetic probe arrays in the vacuum vessel. Here we present data from the NB plane array. Reproduced from [7], with the permission of AIP Publishing.
40 30 20 10
0
0 2 4 6 8 10
t (ms)
f (MHz)
r ∆φ (cm)
log10 amplitude (δB/B)
1.4
1.2
1.0
0.8 -6 0.6
0.4 0.2
0 2 4 t (ms) 6 8 10
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Figure 2. Magnetic signature of EPMs. At least three distinct EPMs are observed in this magnetic spectrogram of a C-2U discharge:
an ion cyclotron mode (near f ∼ 0.6 MHz at t < 1ms), a low frequency chirping mode (bursty mode at f < 0.4 MHz throughout discharge), and an Alfvén mode (f > 1 MHz after t = 6 ms). By comparing the spectrogram to the top frame showing the excluded flux radius as a function of time, it can be seen that the Alfvén mode only appears after the FRC has shrunk in size.
Alfvén Eigenmodes, etc) [8]. To our knowledge, no Alfvén eigenmode has been shown to exist in the FRC.
The C-2U NBI geometry is more akin to a mirror device than a tokamak. The fast ions are injected with momentum
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