results for au:Spong_D in:physics

- Aug 08 2017 physics.plasm-ph arXiv:1708.02109v1Alfven Eigenmodes (AE) can be destabilized by energetic particles in neutral beam injection (NBI) heated plasmas through inverse Landau damping and couplings with gap modes in the shear Alfven continua. We describe the linear evolution of the poloidal flux and the toroidal component of the vorticity in a full 3D system using the reduced MHD equations, density and parallel velocity moments for the energetic particles as well as the geodesic acoustic wave dynamics. A closure relation adds the Landau damping and resonant destabilization effects in the model. We apply the model to study the Alfven modes stability in TJ-II, performing a parametric analysis in a range of realistic values of energetic particle $\beta$ (beta_f), ratios of thermal/Alfven velocities (Vth/VA0), energetic particle density profiles and toroidal modes (n) including toroidal and helical couplings. The study predicts a large helical coupling between different toroidal modes and the destabilization of helical Alfven Eigenmodes (HAE) with frequencies similar to the AE activity measured in TJ-II, between 50 - 400 kHz. The analysis has also revealed the destabilization of GAE (Global Alfven Eigenmodes), TAE (Toroidal Alfven Eigenmodes) and EPM (Energetic Particle Modes). For the modes considered here, optimized TJ-II operations require a iota profile in the range of [0.845, 0.979] to stabilize AEs in the inner and middle plasma. AEs in the plasma periphery cannot be fully stabilized, although for a configuration with iota = [0.945, 1.079], only n=7,11,15 AE are unstable with a growth rate 4 times smaller compared to the standard iota = [1.54,1.68] case and a frequency of 100 kHz. We reproduce the frequency sweeping evolution of the AE frequency observed in TJ-II as the iota profile is varied. The AE frequency sweeping is caused by consecutive changes of the instability dominant modes between different helical families.
- Apr 07 2017 physics.plasm-ph arXiv:1704.01632v1Energetic particle populations in nuclear fusion experiments can destabilize Alfv\'en Eigenmodes through inverse Landau damping and couplings with gap modes in the shear Alfv\'en continua. We use the reduced MHD equations to describe the linear evolution of the poloidal flux and the toroidal component of the vorticity in a full 3D system, coupled with equations of density and parallel velocity moments for the energetic particles. We add the Landau damping and resonant destabilization effects by a closure relation. We apply the model to study the Alfv\'en modes stability in Large Helical Device (LHD) inward-shifted configurations, performing a parametric analysis in a range of realistic values of energetic particle $\beta$ ($\beta_{f}$), ratios of the energetic particle thermal/Alfv\'en velocities ($V_{th}/V_{A0}$), magnetic Lundquist numbers ($S$) and toroidal modes ($n$). The $n = 1$ and $n = 2$ TAE are destabilized although $n = 3$ and $n = 4$ TAE are weakly perturbed. The most unstable configurations are associated with density gradients of energetic particles in the plasma core: TAE are destabilized even for small energetic particle populations if their thermal velocity is lower than $0.4$ times the Alfv\'en velocity. The frequency range of MHD bursts measured in LHD are $50-70$ kHz for $n=1$ and $60-80$ kHz for $n=2$ TAE, consistent with the model predictions.
- Mar 20 2017 physics.plasm-ph arXiv:1703.06129v2Neoclassical transport in the presence of non-axisymmetric magnetic fields causes a toroidal torque known as neoclassical toroidal viscosity (NTV). The toroidal symmetry of ITER will be broken by the finite number of toroidal field coils and by test blanket modules (TBMs). The addition of ferritic inserts (FIs) will decrease the magnitude of the toroidal field ripple. 3D magnetic equilibria with toroidal field ripple and ferromagnetic structures are calculated for an ITER steady-state scenario using the Variational Moments Equilibrium Code (VMEC). Neoclassical transport quantities in the presence of these error fields are calculated using the Stellarator Fokker-Planck Iterative Neoclassical Conservative Solver (SFINCS). These calculations fully account for $E_r$, flux surface shaping, multiple species, magnitude of ripple, and collisionality rather than applying approximate analytic NTV formulae. As NTV is a complicated nonlinear function of $E_r$, we study its behavior over a plausible range of $E_r$. We estimate the toroidal flow, and hence $E_r$, using a semi-analytic turbulent intrinsic rotation model and NUBEAM calculations of neutral beam torque. The NTV from the $\rvert n \rvert = 18$ ripple dominates that from lower $n$ perturbations of the TBMs. With the inclusion of FIs, the magnitude of NTV torque is reduced by about 75% near the edge. We present comparisons of several models of tangential magnetic drifts, finding appreciable differences only for superbanana-plateau transport at small $E_r$. We find the scaling of calculated NTV torque with ripple magnitude to indicate that ripple-trapping may be a significant mechanism for NTV in ITER. The computed NTV torque without ferritic components is comparable in magnitude to the NBI and intrinsic turbulent torques and will likely damp rotation, but the NTV torque is significantly reduced by the planned ferritic inserts.
- Oct 10 2014 physics.plasm-ph arXiv:1410.2309v1We present the field-line modeling, design and construction of a prototype circular-coil tokamak-torsatron hybrid called Proto-CIRCUS. The device has a major radius R = 16 cm and minor radius a < 5 cm. The six "toroidal field" coils are planar as in a tokamak, but they are tilted. This, combined with induced or driven plasma current, is expected to generate rotational transform, as seen in field-line tracing and equilibrium calculations. The device is expected to operate at lower plasma current than a tokamak of comparable size and magnetic field, which might have interesting implications for disruptions and steady-state operation. Additionally, the toroidal magnetic ripple is less pronounced than in an equivalent tokamak in which the coils are not tilted. The tilted coils are interlocked, resulting in a relatively low aspect ratio, and can be moved, both radially and in tilt angle, between discharges. This capability will be exploited for detailed comparisons between calculations and field-line mapping measurements. Such comparisons will reveal whether this relatively simple concept can generate the expected rotational transform.