Engineering topics

Porkolab, M

Publications and source records attributed to Porkolab, M.

6 records

DOE OSTI2012

Measurements of ion cyclotron range of frequencies mode converted wave intensity with phase contrast imaging in Alcator C-Mod and comparison with full-wave simulations

Radio frequency waves in the ion cyclotron range of frequencies (ICRF) are widely used to heat tokamak plasmas. In ICRF heating schemes involving multiple ion species, the launched fast waves convert to ion cyclotron waves or ion Bernstein waves at the two-ion hybrid resonances. Mode converted waves are of interest as actuators to optimise plasma performance through current drive and flow drive. In order to describe these processes accurately in a realistic tokamak geometry, numerical simulations are essential, and it is important that these codes be validated against experiment. In this study, the mode converted waves were measured using a phase contrast imaging technique in D-H and D-{sup 3}He plasmas. The measured mode converted wave intensity in the D-{sup 3}He mode conversion regime was found to be a factor of {approx}50 weaker than the full-wave predictions. The discrepancy was reduced in the hydrogen minority heating regime, where mode conversion is weaker.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2012

Ohmic energy confinement saturation and core toroidal rotation reversal in Alcator C-Mod plasmas

Ohmic energy confinement saturation is found to be closely related to core toroidal rotation reversals in Alcator C-Mod tokamak plasmas. Rotation reversals occur at a critical density, depending on the plasma current and toroidal magnetic field, which coincides with the density separating the linear Ohmic confinement regime from the saturated Ohmic confinement regime. The rotation is directed co-current at low density and abruptly changes direction to counter-current when the energy confinement saturates as the density is increased. Since there is a bifurcation in the direction of the rotation at this critical density, toroidal rotation reversal is a very sensitive indicator in the determination of the regime change. The reversal and confinement saturation results can be unified, since these processes occur in a particular range of the collisionality.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2011

ICRF Mode Conversion Flow Drive Experiments on Alcator C-Mod

We have carried out a detailed study of the dependence of ICRF mode conversion flow drive (MCFD) on plasma and RF parameters. The flow drive efficiency is found to depend strongly on the {sup 3}He concentration in D({sup 3}He) plasmas, a key parameter separating the ICRF minority heating regime and mode conversion regime. At +90 deg. antenna phasing (waves in the co-I{sub p} direction) and dipole phasing, the driven flow is in the co-I{sub p} direction, and the change of the rotation velocity increases with both P{sub RF} and I{sub p}, and scales unfavorably vs. plasma density and antenna frequency. When MCFD is applied to I-mode plasmas, the plasma rotation increases until the onset of MHD modes triggered by large sawtooth crashes. Very high performance I-mode plasmas with H{sub ITER98,y2}{approx}1.4 and T{sub e0}{approx}8 keV have been obtained in these experiments.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2011

ICRF Mode Conversion Studies with Phase Contrast Imaging and Comparisons with Full-Wave Simulations

Waves in the ion cyclotron range of frequencies (ICRF) are widely used to heat toka-mak plasmas. In a multi-ion-species plasma, the FW converts to ion cyclotron waves (ICW) and ion Bernstein waves (IBW) around the ion-ion hybrid resonance (mode conversion). The mode converted wave is of interest as an actuator to optimise plasma performance through flow drive and current drive. Numerical simulations are essential to describe these processes accurately, and it is important that these simulation codes be validated. On Alcator C-Mod, direct measurements of the mode converted waves have been performed using Phase Contrast Imaging (PCI), which measures the line-integrated electron density fluctuations. The results were compared to full-wave simulations AORSA and TORIC. AORSA is coupled to a Fokker-Planck code CQL3D for self-consistent simulation of the wave electric field and the minority distribution function. The simulation results are compared to PCI measurements using synthetic diagnostic. The experiments were performed in D-H and D-{sup 3}He plasmas over a wide range of ion species concentrations. The simulations agreed well with the measurements in the strong absorption regime. However, the measured fluctuation intensity was smaller by 1-2 orders of magnitudes in the weakly abosorbing regime, and a realistic description of the plasma edge including dissipation and antenna geometry may be required in these cases.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2011

Recent Results on Coupling Fast Waves to High Performance Plasmas on DIII-D

Fast Waves (FWs) at 60 MHz and 90 MHz are used in DIII-D for central electron heating and current drive. Coupling of FWs to high-performance discharges is limited by low antenna loading in these regimes. To extend the application of high-power FWs to such regimes, methods of increasing the antenna loading in these regimes are needed. A systematic study of loading enhancement techniques has been carried out in DIII-D, including reduction of the antenna/plasma distance, gas puffing into the far scrape-off layer (SOL), and control of other parameters that affect the particle balance in the far SOL. Quantitative understanding of the physics of the loading resistance and its dependence on edge density profiles is demonstrated. The core FW heating efficiency appeared to be {approx}100% in the Advanced Inductive regime, consistent with the high first-pass direct electron absorption of {approx}75% that is predicted by the ray-tracing code GENRAY in this high electron beta regime.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2011

Rotation Reversal Bifurcation and Energy Confinement Saturation in Tokamak Ohmic L-Mode Plasmas

Direction reversals of intrinsic toroidal rotation have been observed in diverted Alcator C-Mod Ohmic L-mode plasmas following electron density ramps. For low density discharges, the core rotation is directed cocurrent, and reverses to countercurrent following an increase in the density above a certain threshold. Such reversals occur together with a decrease in density fluctuations with 2 cm{sup -1}{<=}k{sub {theta}}{<=}11 cm{sup -1} and frequencies above 70 kHz. There is a strong correlation between the reversal density and the density at which the Ohmic L-mode energy confinement changes from the linear to the saturated regime.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗