Page 3 - Suppressed ion-scale turbulence in a hot high-β plasma

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13860
ARTICLE
plays a decisive role in overall FRC particle and thermal confinement.
Results
Experimental set-up and diagnostics. The C-2 FRC plasma is created via injecting and merging of two preformed, compact high-b plasmoids into a central confinement chamber with wall radius RW 1⁄4 0.7 m and length L 1⁄4 4.5 m, with an external sole- noidal magnetic field Be 1⁄4 0.05–0.14 T. The magnetic configura- tion is shown in Fig. 1 and described in detail in the Methods section below and elsewhere18,23,24. Typical line-averaged plasma densities in the experiments described here are hnei1⁄42– 4 1019 m 3, and the ion and electron temperatures in the FRC core are TiB400–600 eV and TeB80–130 eV. The FRC null field radius is RB0.25–0.35 m in the first shot phase (tr2 ms). The FRC separatrix radius is approximated via the measured excluded flux radius Rs (RsB0.35–0.45m initially). In the C-2 FRC, neutral beam injection (NBI, beam voltage and power VB 1⁄4 20 kV, PBB4 MW) is used to extend the FRC pulse length toB5ms. The (closed flux surface) FRC plasma in C-2 is surrounded by an (open fieldline) mirror-confined SOL plasma. Annular washer plasma guns25 with inner diameter 0.11 m and outer diameter 0.13 m, located in the divertor chambers, are used for active plasma boundary control. The plasma guns produce an electrically biased plasma on open field lines in the divertor, resulting in a negative (inwards pointing) radial electric field mapping to the SOL region near the machine midplane and just outside the FRC separatrix26,27 (at radius rZRs) as discussed in more detail below in the Methods section.
The fluctuation data reported here is acquired via a multichannel microwave scattering diagnostic described in more detail below. Doppler Backscattering28–30 (DBS) measures the density fluctuation level at different plasma radii in the FRC core and in the SOL. Microwave radiation at six separate tunable frequencies 26GHzofo65GHz is launched via two lensed horn antennas and quasi-optically transmitted to the plasma (Fig. 2). Backscattering by plasma density fluctuations occurs preferentially near the plasma cutoff layer for each launched frequency29 according to the selection rules kS 1⁄4 kI 1⁄4 ky/2 and oS 1⁄4 oI ntky where the indices I and S denote the incident and
backscattered wave (Fig. 2b). nt 1⁄4 nE B þ nph is the toroidal fluctuation advection velocity (vph is the fluctuation phase velocity in the plasma frame and can be neglected here). ky is the probed resonant density fluctuation toroidal wavenumber. Variation of the toroidal launch angle allows acquiring the density fluctuation level versus toroidal wavenumber.
Fluctuation properties and wavenumber spectra. Figure 3 shows radial profiles of the normalized density fluctuation level n˜/n measured via DBS and the radial plasma density profile at different times during the discharge. Typical DBS probing positions are indicated via orange markers in Fig. 3b. The measured fluctuation toroidal wavenumber range (determined by the beam launch angle z and the plasma geometry) is centred here around kyB2–3 cm 1 (kyrsB6–9) with a spread Dky/kyB0.9–1.2 mainly due to toroidal plasma curvature30,31. Fluctuation levels are initially low in the core and also relatively low in the SOL. SOL fluctuations increase substantially in amplitude concomitantly with the density gradient (t 1⁄4 0.4 ms), as examined in detail below. FRC core fluctuations remain low throughout the discharge.
We now present evidence that the nature of the saturated fluctuation spectrum is entirely different in the FRC core and SOL, and also that FRC core fluctuations are fundamentally different from tokamak core plasma turbulence. Figure 4a shows the toroidal wavenumber spectra, measured just inside the FRC separatrix via DBS, and in the SOL. Fluctuation levels are plotted here versus the toroidal wavenumber normalized by the electron gyro-radius.
An inverted toroidal wavenumber spectrum is clearly observed in the FRC core, with fluctuation levels peaking at wavenumbers in the electron mode range (0.05rkyrer0.2, where re 1⁄4 ðkTe =me Þ1=2 =oce is the electron gyroradius), and kyrsZ5. The local ion-to electron temperature ratio is Ti/TeB6.8 as determined from Doppler spectroscopy and Thomson scattering. The core spectrum extends to kyreZ0.4 and falls off steeply at low kyre. It is clear that the integrated fluctuation level is substantially lower in the core, compared to the SOL. This is a strong experimental indication that ion-scale modes are absent in the FRC core, and electron modes are dominant. The spectral
ab FRC plasma
kI
kS
k
Confinement vessel
eζ Magnet coils
Be,ez = I–k v
Focussing mirror
40–90 GHz horn antenna 26–40 GHz horn antenna
I,kI
S E×B kS=kI–k
Figure 2 | Schematic of the Doppler backscattering diagnostic. (a) A view of the Doppler backscattering28–30 (DBS) microwave diagnostic beam optics components and beam path. Monostatic beam optics (collocated launch and receive beam optics) is used to launch six separate microwave frequencies via a beam combiner (polarizer) and an adjustable parabolic focusing mirror. The resulting common beam trajectory is indicated by yellow shading. Frequencies in the range 26–40 GHz are launched in X-mode polarization, while frequencies above 40 GHz are launched in O-mode polarization. Gaussian beams are launched into the plasma at an oblique angle z in the toroidal plane. (b) toroidal plane cross-section illustrating DBS launched/backscattered beam trajectory and the probed toroidal wavenumber (for one frequency channel); the relations between the launched and backscattered frequency and wavenumber, and the probed density fluctuation wavenumber are shown. The toroidal direction and the direction of the external magnetic field Be are also indicated.
NATURE COMMUNICATIONS | 7:13860 | DOI: 10.1038/ncomms13860 | www.nature.com/naturecommunications 3