Transport and fluctuations in high temperature spheromak plasmas

Abstract

Globally coherent magnetic fluctuations often observed during the driven phase after spheromak formation in the Sustained Spheromak Physics Experiment 1 (SSPX) can be reduced to small amplitude by programming the magnetic flux=ψ gun and the discharge current=I gun in the formation gun. Scanning the edge normalized current=λ edge =λ gun =µ 0 I gun /ψ gun above and below the minimum energy eigenvalue 2 =λ FC of the flux conserver provides a variation in the internal q=safety factor profile producing the expected q=m/n=poloidal/toroidal mode spectrum. By driving the edge with the proper λ gun , the system can be operated with the poloidal/toroidal mode spectrum between the m/n=1/2 and 2/3 modes producing low magnetic fluctuation amplitudes and high electron temperature=T e > 350 eV. Transport and confinement parameters calculated using Thomson scattering-measured T e and n e profiles coupled with the equilibrium code internal current profiles show a reduction in electron thermal diffusivity as T e increases. This scaling behavior is more classical-like than Bohm or open field line transport models 3 where thermal diffusivity increases with T e . Electron diffusivity is calculated to be less than 10 m 2 /s, approaching levels seen in tokamaks.

Magnetic Reconnection in the Spheromak: Physics and Consequences

“…The spectrum of magnetic fluctuations in the core of the spheromak correlates well with the qprofile, with good energy confinement obtained when low-order rational surfaces are absent in the spheromak, 3 as discussed further by McLean in this issue.…”

Energy confinement and magnetic field generation in the SSPX spheromak

“…This configuration is characterized by a flat or nearly flat parallel current profile, λ = µ 0 J || / B = constant. Best energy confinement is found for a profile which is slightly peaked on the magnetic axis, resulting in a qprofile which lies in the range 1/2 < q(ψ) < 2/3 except near the separatrix, thereby eliminating low-order rational surfaces in the plasma [9]. An additional consequence of λ ≈ constant is that it limits the drive for resistive tearing modes, which both take energy from the field itself and leads to stochastic field lines and parallel energy transport.…”

A Development Path for the Stabilized Spheromak

“…The SSPX has achieved good confinement in the core corresponding to χ ≈ 1 m 2 /s [15] without profile control. Building on these results, in Table 1 we give parameters for a Proof of Principle (POP) experiment with profile control satisfying Eqs.…”