Page 2 - Suppressed ion-scale turbulence in a hot high-β plasma

ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13860
Magnetic confinement fusion offers the prospect of a carbon-neutral, environmentally responsible and inexaustible energy source. The present main-line approach to magnetic fusion energy is via the tokamak concept1 relying on a strong toroidal (doughnut-shaped) magnetic field to confine plasma at temperatures characteristic of the interior of stars (B100 million °K). The hydrogen isotopes deuterium (D) and tritium (T) fuse at these temperatures, releasing energy mainly in the form of neutrons. Fusion energy is converted to thermal energy in a blanket surrounding the plasma, and must be recovered via a thermal cycle. The most advanced tokamak to date, the International Thermonuclear Experimental Reactor, is presently under construction, to test and demonstrate the sustained production of fusion energy for the first time.
The field-reversed configuration2,3 (FRC) is characterized by a much higher ratio b of the plasma kinetic pressure to the external magnetic field energy density, with volume-averaged br0.9 and peak bmax 1⁄4 1. This magnetic configuration is of great interest as a fusion reactor concept due to its compact, axisymmetric geometry and the potential for aneutronic fusion based on advanced fuels, such as the proton-boron fusion reaction4 (p-B11). Other important advantages of the FRC concept include the presence of a natural divertor which provides for efficient exhaust of fusion products, and also allows for a large radial expansion of the magnetic flux. In turn the particle and heat outflux from the FRC core plasma is distributed over a larger surface area, and consequently the heat load to plasma-facing components is substantially reduced.
FRCs are also a powerful tool for fundamental plasma research, and allow investigating the properties and stability of high b, high temperature collisionless plasmas in a laboratory environment. Such conditions are ubiquitous in astrophysical plasmas, for example in accretion discs5,6, in the outer solar corona6, in solar mass ejections, and in stellar superflares7. In particular, the physics of magnetic reconnection, and the associated transfer of magnetic energy to kinetic (particle) energy is actively researched using FRC plasmas. 2,3
In virtually all magnetic confinement devices including FRCs as well as in astrophysical plasmas, anomalously large plasma resistivity, and plasma energy and particle transport in excess of classical collisional transport perpendicular to the magnetic field have been observed. Long wavelength turbulence, with wavelengths larger than the ion gyration (Larmor) radius ri, is typically most detrimental to plasma confinement. Due to the
high ion energy and the comparatively low magnetic field, FRC plasmas are characterized by large ion Larmor radii ri of the thermal ion population, and correspondingly high r* 1⁄4 ri/RB0.1, where R is the characteristic magnetic field reversal (null field) radius. Hence FRC turbulence properties can be substantially different8–12 from tokamak plasmas, where r*B10 3 10 2, and where ion-temperature-gradient-driven instabilities are often dominant, with perpendicular wavelengths substantially larger than the ion Larmor radius (k>rio1, where k>is the turbulence wavenumber perpendicular to the magnetic field).
Direct observation or systematic identification of the turbulent processes that influence or limit confinement has been lacking so far in FRC plasmas. Earlier work has focused on anomalous resistivity and transport due to the Lower Hybrid Drift Instability13 but an experimental investigation of short wavelength turbulence via microwave forward scattering did not show sufficiently large fluctuation levels14. Short wavelength turbulence driven by the radial electron temperature gradient15 (ETG modes) as well as other driftwave instabilities16 have also been postulated theoretically for specific ratios of density and electron/ion temperature gradient scale lengths, but not confirmed experimentally to date in FRC plasmas.
0.6 0.4 0.2
Divertor
Magnet coils Formation section
Rw
Rs R
SOL Divertor Formation section
Density (1019 m–3)
1.48 1.10 0.74 0.37
00 –10 Plasma Mirror –5 Separatrix 0 FRC 5 Mirror Plasma 10
gun plug
z (m) plug gun
Figure 1 | Schematic cross-section of the C-2 field-reversed configuration. Axial cross-section of the Tri Alpha Energy (TAE) field-reversed configuration (FRC) device C-2, showing the magnetic field configuration (indicated by magnetic field lines in black), and plasma density (overlayed colour contours; the colour scale bar indicates the plasma density in units of 1019 m 3) from a two-dimensional magneto-hydrodynamic (MHD) equilibrium calculation using the LamyRidge code24. r and z are the radial and axial coordinates. The null field radius (R), separatrix radius (Rs), and wall radius RW 1⁄4 0.7 m of the device are indicated. The separatrix location is indicated by a solid black line. The width of a flux tube at the thermal ion gyroradius in the scrape-off layer (SOL) is indicated by two dashed field lines. The mirror and mirror plug magnetic field coils are shown in blue; the solenoidal field coils in the confinement and formation sections, located at larger radii, are not shown here. C-2 has two formation sections and two divertors as indicated on opposite sides of the central confinement vessel. The locations of the plasma guns and mirror plug coils are also indicated.
2 NATURE COMMUNICATIONS | 7:13860 | DOI: 10.1038/ncomms13860 | www.nature.com/naturecommunications
In this paper we show that, in contrast to tokamaks, long
wavelength, ion-scale modes (k>rio1) are stable or substantially
reduced in the FRC core of the C-2 device17,18 (Fig. 1), resulting
in an inverted toroidal wavenumber spectrum. This observation is
consistent with essentially classical ion thermal confinement, with
the radial ion thermal diffusivity w ð1 2Þwcl evaluated from ii
1-D and 2-D transport analysis19, where wcl is the classical i
collisional ion thermal diffusivity. Finite ion Larmor radius (FLR) effects9–12 are found to contribute crucially to the observed stability of long wavelength modes, as confirmed by gyrokinetic stability calculations20,21. The scrape-off layer (SOL) plasma is found to be unstable to multiscale drift-interchange modes (0.5okyrsr40, where rs is the ion sound Larmor radius rs 1⁄4 1⁄2ðkTi þ kTeÞ=mi 1=2=oci, and oci is the ion cyclotron frequency, due to large radial density/temperature gradients in conjunction with the (moderate) field line curvature22. A critical density/electron pressure gradient has to be exceeded for instability near the separatrix and in the SOL. Finally, evidence for a radial transport barrier due to E B flow shear in the SOL is presented. This observation is very significant in that it affects the particle and thermal fluxes across the FRC separatrix, and hence
r (m)