Page 4 - Suppressed ion-scale turbulence in a hot high-β plasma

ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13860
aa
0.24 ms
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0.82≤r/Rs≤0.87 FRC core 1.10≤r/Rs≤1.22 SOL
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ne (1019 m–3)
n˜/n n˜/n
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Figure 4 | The toroidal wavenumber spectrum of density fluctuations measured via Doppler Backscattering. (a) Normalized density fluctuation n˜/n level versus toroidal wavenumber normalized to the electron gyroradius kyre in the field reversed configuration (FRC) core inside the separatrix and in the scrape-off layer (SOL) (shots #29587–29610; #29750–29802). re is the electron Larmor radius. The Doppler Backscattering28–30 sensitivity limit is indicated in the figure (grey bar). (b) measured toroidal wavenumber spectra in the core plasma and in the scrape-off layer, plotted versus toroidal wavenumber normalized to the ion sound gyroradius kyrs. An inverted core wavenumber spectrum is observed for kyreo0.05 (kyrso7) indicating that long wavelength (ion) modes are not present. The SOL spectrum shows the highest fluctuation levels at low wavenumber, and the fluctuation level decays exponentially with increasing wavenumber. The error bars represent the typical standard deviation (s.d.) of the measurements. The fluctuations observed in the C-2 FRC core are qualitatively consistent with low frequency (oooci, where oci is the ion cyclotron frequency) electron drift/interchange modes (with a toroidal wavenumber range 0.05rkyrer0.45).
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of the fluctuation energy density, proportional to (n˜/n) , is
B0.33). Here, the local electron- to ion temperature ratio is Ti/TeB3.7. The observed fluctuation power density at the lowest measured wavenumbers is more than two orders of magnitude higher than in the FRC core. Exponential spectra have been observed in linear and toroidal confinement devices, for example in the core of tokamak low confinement mode (L-mode) plasmas32,33. These spectra are however dominated by much lower normalized (poloidal) wavenumbers (kyrsr2), consistent with dominant ion-temperature-gradient and/or trapped electron
Figure 3 | Radial profiles of the normalized density fluctuation level and plasma density. (a) Radial profile of the normalized density fluctuation level n˜/n at different times after compact toroid (CT) merging (t 1⁄4 0). The fluctuation levels measured via Doppler Backscattering28–30 have been calibrated via comparison to Far Infrared Scattering (FIR) data54. The error bars represent the standard variation (s.d.) of the measurements. (b) Plasma density profile measured via carbon dioxide (CO2) laser interferometry53 at three different times t. Typical Doppler Backscattering (DBS) probing radii (cutoff layer locations) are indicated by orange circles. The size of the thermal ion gyroradius, averaged along the field line length, is indicated at different locations in the closed fieldline region and in the scrape-off layer (SOL).
peak may be shifted with respect to the linearly most unstable modes; however, the spectrum clearly indicates that longer wavelength modes are subdominant or stable. This wavenumber spectral shape profoundly affects both ion and electron thermal transport, which is also predominantly carried by low-k (ion-scale) fluctuations. The measured spectral shape sharply differs from wavenumber spectra measured in the core of tokamak low confinement mode (L-mode) plasmas and linear confinement devices, where the fluctuation amplitude typically decreases monotonically with increasing normalized wavenumber32,33. We can conjecture that the large ion Larmor radius in the FRC core region stabilizes ion-scale modes9–11. This conjecture will be examined further below.
The SOL spectrum, on the other hand, peaks at low kyre and decays exponentially with increasing wavenumber, with a decay constant of akB0.16 (the corresponding exponent for the decay
i
4 NATURE COMMUNICATIONS | 7:13860 | DOI: 10.1038/ncomms13860 | www.nature.com/naturecommunications