Arash Ashourvan

Rectification of the lateral Casimir force in a vibrating noncontact rack and pinion

The nonlinear dynamics of a cylindrical pinion that is kept at a distance from a vibrating rack is studied, and it is shown that the lateral Casimir force between the two corrugated surfaces can be rectified. The effects of friction and external load are taken into account, and it is shown that the pinion can do work against loads of up to a critical value, which is set by the amplitude of the lateral Casimir force. We present a phase diagram for the rectified motion that could help its experimental investigations, as the system exhibits a chaotic behavior in a large part of the parameter space.

Spontaneous profile self-organization in a simple realization of drift-wave turbulence

We report the observation of a transport bifurcation that occurs by spontaneous self-organization of a drift-wave and shear flow system in a linear plasma device. As we increase the magnetic field above a threshold ( BCr = 1200 G), a global transition occurs, with steepening of mean density and ion pressure profiles, onset of strong E×B shearing, a reduction of turbulence, and improved turbulent radial particle transport. An abrupt transition appears in the graph of turbulent particle flux versus density gradient. Hysteresis in the density gradient further confirms this transport bifurcation. The total Reynolds work on the flow sharply increases above threshold. This correlates with the increase of density steepness, which suggests the Reynolds stress-driven flow that plays an essential role in density steepening and transport bifurcation. A change in turbulence feature from drift waves (DWs) to a mix of DWs and ion temperature gradients also coincides with the transport bifurcation. Interesting phenomena...

Transport matrix for particles and momentum in collisional drift waves turbulence in linear plasma devices

The relationship between the physics of turbulent transport of particles and azimuthal momentum in a linear plasma device is investigated using a simple model with a background density gradient and zonal flows driven by turbulent stresses. Pure shear flow driven Kelvin-Helmholtz instabilities (k∥=0) relax the flow and drive an outward (down gradient) flux of particles. However, instabilities at finite k∥ with flow enhanced pumping can locally drive an inward particle pinch. The turbulent vorticity flux consists of a turbulent viscosity term, which acts to reduce the global vorticity gradient and the residual vorticity flux term, accelerating the zonal flows from rest. Moreover, we use the positivity of the production of fluctuation potential enstrophy to obtain a constraint relation, which tightly links the vorticity transport to the particle transport. This relation can be useful in explaining the experimentally observed correlation between the presence of E×B flow shear and the measured inward particle ...

On the emergence of macroscopic transport barriers from staircase structures

This paper presents a theory for the formation and evolution of coupled density staircases and zonal shear profiles in a simple model of drift-wave turbulence. Density, vorticity, and fluctuation potential enstrophy are the fields evolved in this system. Formation of staircase structures is due to inhomogeneous mixing of generalized potential vorticity (PV), resulting in the sharpening of density and vorticity gradients in some regions, and weakening them in others. When the PV gradients steepen, the density staircase structure develops into a lattice of mesoscale “jumps,” and “steps,” which are, respectively, the regions of local gradient steepening and flattening. The jumps merge and migrate in radius, leading to the development of macroscale profile structures from mesoscale elements. The positive feedback process, which drives the staircase formation occurs via a Rhines scale dependent mixing length. We present extensive studies of bifurcation physics of the global state, including results on the glob...

Nonlinear Trivelpiece-Gould waves: Frequency, functional form, and stability

This paper considers the frequency, spatial form, and stability of nonlinear Trivelpiece-Gould (TG) waves on a cylindrical plasma column of length L and radius rp, treating both traveling waves and standing waves, and focussing on the regime of experimental interest in which L/rp≫1. In this regime, TG waves are weakly dispersive, allowing strong mode-coupling between Fourier harmonics. The mode coupling implies that linear theory for such waves is a poor approximation even at fairly small amplitude, and nonlinear theories that include a small number of harmonics, such as three-wave parametric resonance theory, also fail to fully capture the stability properties of the system. It is found that nonlinear standing waves suffer jumps in their functional form as their amplitude is varied continuously. The jumps are caused by nonlinear resonances between the standing wave and nearly linear waves whose frequencies and wave numbers are harmonics of the standing wave. Also, the standing waves are found to be unsta...

Effect of squeeze on electrostatic Trivelpiece-Gould wave damping

We present a theory for increased damping of Trivelpiece-Gouid plasma modes on a nonneutral plasma column, due to application of a Debye shielded cylindrically symmetric squeeze potential φ1. We present two models of the effect this has on the plasma modes: a 1D model with only axial dependence, and a 2D model that also keeps radial dependence in the squeezed equilibrium and the mode. We study the models using both analytical and numerical methods. For our analytical studies, we assume that φ1/T≪1, and we treat the Debye shielded squeeze potential as a perturbation in the equilibrium Hamiltonian. Our numerical simulations solve the 1D Vlasov-Poisson system and obtain the frequency and damping rate for a self-consistent plasma mode, making no assumptions as to the size of the squeeze. In both the 1D and 2D models, damping of the mode is caused by Landau resonances at energies En for which the particle bounce frequency ωb(En) and the wave frequency ω satisfy ω=nωb(En). Particles experience a non-sinusoidal ...

Active spectroscopy measurements of the deuterium temperature, rotation, and density from the core to scrape off layer on the DIII-D tokamak (invited)

Main-ion charge exchange recombination spectroscopy (MICER) uses the neutral beam induced D α spectrum to measure the local deuterium ion (D+) temperature, rotation, and density, as well as parameters related to the neutral beams, fast ions, and magnetic field. An edge MICER system consisting of 16 densely packed chords was recently installed on DIII-D, extending the MICER technique from the core to the pedestal and steep gradient region of H-mode plasmas where the D+ and commonly measured impurity ion properties can differ significantly. A combination of iterative collisional radiative modeling techniques and greatly accelerated spectral fitting allowed the extension of this diagnostic technique to the plasma edge where the steep gradients introduce significant diagnostic challenges. The importance of including the fast ion D α emission in the fit to the spectrum for the edge system is investigated showing that it is typically not important except for cases which can have significant fast ion fractions near the plasma edge such as QH-mode. Example profiles from an Ohmic L-mode and a high power ITER baseline case show large differences in the toroidal rotation of the two species near the separatrix including a strong co-current D+ edge rotation. The measurements and analysis demonstrate the state of the art in active spectroscopy and integrated modeling for diagnosing fusion plasmas and the importance of direct main ion measurements.

Bounce harmonic Landau damping of plasma waves

We present measurements of bounce harmonic Landau damping due to z-variations in the plasma potential, created by an azimuthally symmetric “squeeze” voltage Vs applied to the cylindrical wall. Traditional Landau damping on spatially uniform plasma is weak in regimes where the wave phase velocity vph≡ω/k is large compared to the thermal velocity. However, z-variations in plasma density and potential create higher spatial harmonics, which enable resonant wave damping by particles with bounce-averaged velocities vph/n, where n is an integer. In our geometry, the applied squeeze predominantly generates a resonance at vph/3. Wave-coherent laser induced fluorescence measurements of particle velocities show a distinctive Landau damping signature at vph/3, with amplitude proportional to the applied Vs. The measured (small amplitude) wave damping is then proportional to Vs2, in quantitative agreement with theory over a range of 20 in temperature. Significant questions remain regarding “background” bounce harmonic ...

Collisionless and collisional effects on plasma waves from a partition squeeze

A simple 1D model is presented for the heating caused by cylindrically-symmetric plasma waves in a non-neutral plasma column due to the addition of a symmetric squeeze potential applied to the center of the column. We study this model by using analytical techniques and by using a numerical grids method solution, and we compare the results of this model to previous work (Ashourvan and Dubin (2014)). squeeze divides the plasma into passing and trapped particles; the latter cannot pass over the squeeze potential. In collisionless theory, enhanced heating is caused by additional bounce harmonics induced by the squeeze in the particle distribution, leading to Landau resonances at energies En for which the bounce frequency ωb(E) and wave frequency ωm satisfy ωm = nωb(En). As a result, heating is substantially higher than the case with no squeeze, even when ωm is much greater than the thermal bounce frequency ωb(T ). Adding collisions to the theory creates a boundary layer at the separatrix between trapped and passing particles that further enhances the heating at small ωm/kmvs , where km is the axial wavenumber and vs is the velocity at the separatrix. However, at large ωm/vs , the heating from the separatrix boundary layer is only a small correction to the heating from collisionless resonances in the trapped particle distribution function.

Validation of the kinetic-turbulent-neoclassical theory for edge intrinsic rotation in DIII-D

In a recent kinetic model of edge main-ion (deuterium) toroidal velocity, intrinsic rotation results from neoclassical orbits in an inhomogeneous turbulent field [T. Stoltzfus-Dueck, Phys. Rev. Lett. 108, 065002 (2012)]. This model predicts a value for the toroidal velocity that is co-current for a typical inboard X-point plasma at the core-edge boundary (ρ ∼ 0.9). Using this model, the velocity prediction is tested on the DIII-D tokamak for a database of L-mode and H-mode plasmas with nominally low neutral beam torque, including both signs of plasma current. Values for the flux-surface-averaged main-ion rotation velocity in the database are obtained from the impurity carbon rotation by analytically calculating the main-ion—impurity neoclassical offset. The deuterium rotation obtained in this manner has been validated by direct main-ion measurements for a limited number of cases. Key theoretical parameters of ion temperature and turbulent scale length are varied across a wide range in an experimental database of discharges. Using a characteristic electron temperature scale length as a proxy for a turbulent scale length, the predicted main-ion rotation velocity has a general agreement with the experimental measurements for neutral beam injection (NBI) powers in the range PNBI < 4 MW. At higher NBI power, the experimental rotation is observed to saturate and even degrade compared to theory. TRANSP-NUBEAM simulations performed for the database show that for discharges with nominally balanced—but high powered—NBI, the net injected torque through the edge can exceed 1 Nm in the counter-current direction. The theory model has been extended to compute the rotation degradation from this counter-current NBI torque by solving a reduced momentum evolution equation for the edge and found the revised velocity prediction to be in agreement with experiment. Using the theory modeled—and now tested—velocity to predict the bulk plasma rotation opens up a path to more confidently projecting the confinement and stability in ITER.In a recent kinetic model of edge main-ion (deuterium) toroidal velocity, intrinsic rotation results from neoclassical orbits in an inhomogeneous turbulent field [T. Stoltzfus-Dueck, Phys. Rev. Lett. 108, 065002 (2012)]. This model predicts a value for the toroidal velocity that is co-current for a typical inboard X-point plasma at the core-edge boundary (ρ ∼ 0.9). Using this model, the velocity prediction is tested on the DIII-D tokamak for a database of L-mode and H-mode plasmas with nominally low neutral beam torque, including both signs of plasma current. Values for the flux-surface-averaged main-ion rotation velocity in the database are obtained from the impurity carbon rotation by analytically calculating the main-ion—impurity neoclassical offset. The deuterium rotation obtained in this manner has been validated by direct main-ion measurements for a limited number of cases. Key theoretical parameters of ion temperature and turbulent scale length are varied across a wide range in an experimental data...