Search NASA⌕ Search

SEARCH · Search NASA

Results for “CLOSED PLASMA DEVICES”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 records
All sourcesNASA NTRSDOE OSTI
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

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↗
DOE OSTI2011

Experimental Investigation of RF Sheath Rectification in ICRF and LH Heated Plasmas on Alcator C-Mod

Radio frequency (RF) rectification of the plasma sheath is being actively studied on C-Mod as a likely mechanism that leads to prohibitively high molybdenum levels in the plasma core of ion cyclotron RF (ICRF) heated discharges. We installed emissive, ion sensitive, Langmuir, and 3-D B-dot probes to quantify the plasma potentials ({Phi}{sub P}) in ICRF and lower hybrid (LH) heated discharges. Two probe sets were mounted on fixed limiter surfaces and one set of probes was mounted on a reciprocating (along the major radius) probe. Initial results showed that RF rectification is strongly dependent on the local plasma density and not on the local RF fields. The RF sheaths had a threshold-like appearance at the local density of {approx}10{sup 16} m-{sup 3}. Radial probe scans revealed that the RF sheaths peaked in the vicinity of the ICRF limiter surface, agreeing with a recent theory. The highest {Phi}{sub P}'s were observed on magnetic field lines directly mapped to the active ICRF antenna. Measurements in LH heated plasmas showed a strong {Phi}{sub P} dependence on the parallel index of refraction n{sub ||} of the launched LH waves: {Phi}{sub P} is greater at lower n{sub ||}. Little dependence was observed on the local plasma density.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Lower Hybrid Wave Induced SOL Emissivity Variation at High Density on the Alcator C-Mod Tokamak

Lower Hybrid Current Drive (LHCD) in the Alcator C-Mod tokamak provides current profile control for the generation of Advanced Tokamak (AT) plasmas. Non-thermal electron bremsstrahlung emission decreases dramatically at n-bar{sub e}>1{center_dot}10{sup 20}[m{sup -3}] for diverted discharges, indicating low current drive efficiency. It is suggested that Scrape-Off-Layer (SOL) collisional absorption of LH waves is the cause for the absence of non-thermal electrons at high density. VUV and visible spectroscopy in the SOL provide direct information on collision excitation processes. Deuterium Balmer-, Lyman- and He-I transition emission measurements were used for initial characterization of SOL electron-neutral collisional absorption. Data from Helium and Deuterium LHCD discharges were characterized by an overall increase in the emissivity as well as an outward radial shift in the emissivity profile with increasing plasma density and applied LHCD power. High-temperature, high-field (T{sub e} = 5keV,B{sub t} = 8T) helium discharges at high density display increased non-thermal signatures as well as reduced SOL emissivity. Variations in emissivity due to LHCD were seen in SOL regions not magnetically connected to the LH Launcher, indicating global SOL effects due to LHCD.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Lower hybrid current drive in a high density diverted tokamak

Experimental observations of LHCD at high density (n-bar{sub e}>10{sup 20}m{approx}{sup 3}) on the Alcator C-Mod tokamak are presented in this paper. Bremsstrahlung emission from relativistic fast electrons in the core plasma drops sharply in single null discharges well below the density limit previously observed on limited tokamaks ({omega}/{omega}{sub LH}{approx}2). Modeling and experimental evidence suggest that the absence of LH driven fast electrons at high density may be due to collisional absorption in the scrape off layer. Experiments show that the expected current drive density dependence is recovered for inner wall limited discharges across the range of densities scanned (0.5x10{sup 20} m{sup -3}<n-bar{sub e}<1.5x10{sup 20} m{sup -3}). Increasing T{sub e} in the periphery of the plasma (0.8<r/a<1.0) also results in a modest increase in non-thermal electron emission at high n-bar{sub e}. Ray tracing/Fokker-Planck simulations of these discharges predict the observed sensitivity to plasma position when the effects of collisional absorption in the SOL are included in the model.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Observation of Co and Counter Rotation Produced by Lower Hybrid Waves in Alcator C-Mod

Lower hybrid waves launched uni-directionally into tokamak plasmas impart momentum to the electrons. This momentum can be transferred to the ions, leading to substantial counter current rotation. Observations of LH-induced counter rotation have been previously reported [1], and the initial rate of increase has been found to be consistent with the calculated rate of wave momentum injection [2]. However, in recent experiments in Alcator C-Mod it has been found that application of LH waves to relatively low current (I{sub p}{approx}0.4-0.6 MA) plasmas can result in a co-current change of rotation, which implies a different mechanism than that described above. This appears to be linked to the so-called intrinsic rotation commonly observed in Alcator C-Mod and other tokamaks [3]. In addition to the change in direction at low current, some dependence on the magnetic configuration (USL vs. LSN) has been observed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Estimation of the ion toroidal rotation source due to momentum transfer from Lower Hybrid waves in Alcator C-Mod

Significant ion toroidal rotation (50km/s) has been measured by X-Ray spectroscopy for impurities in Alcator C-Mod during lower hybrid (LH) RF power injection. We investigate the relation between the computed toroidal momentum input from LH waves and the measured INITIAL change of ion toroidal rotation when the LH power is turned on. The relation may depend on the plasma current and magnetic configuration. Because of the fast build up time of the electron quasilinear plateau (<1 millisecond), the electron distribution function rapidly reaches steady state in which the electrons transfer momentum to the ions. The LH wave momentum input is computed from the self consistent steady state electron distribution function and a bounce-averaged quasilinear diffusion coefficient that are obtained by iterating a full wave code (TORLH) with a Fokker Plank code (CQL3D)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Investigating ICRF Core Physics with a Hybrid Method using {delta}f Particles

Particle-in-cell simulations of ICRF core physics yield a significant amount of noise that can overwhelm the low amplitude signals associated with ICRH. To overcome this noise, we employ a hybrid approach that uses a fluid model for electrons and the delta-f particle-in-cell (DFPIC) method for ions. Noise is reduced to a level well below the amplitude of the signal and time steps are permitted to go beyond the electron time scales. With this formulation we have explored minority heating scenarios in Alcator C-Mod and high harmonic heating scenarios in NSTX. We can also employ the fluid model for both electrons and ions to investigate cold plasma scenarios. Here we examine ion test particles with particular attention paid to particle paths near resonance layers. We present our technique for investigating full particle orbits from a single toroidal mode expansion.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Progress in LHCD modeling and experiments towards the AT regime on Alcator C-Mod

Electron internal transport barrier (ITB) formation is observed during fully non-inductive LHCD discharges on Alcator C-Mod. The formation is characterized by an abrupt increase of the central electron temperature after a few current penetration time has passed following LHCD turn-on. The ITB phase is often terminated by m = 2 mode activity. Kinetic equilibrium reconstruction and TRANSP/LSC analysis show shear reversal in the q profile during the ITB phase. TRANSP/LSC predictive analysis suggests that f{sub Bs}{approx}60% is possible if a similar temperature gradient is maintained at r/a{approx}0.5 with H-factor of 1.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
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

Fast-ions on Alcator C-Mod: Comparisons between Simulation and Experiment for Equilibrium and Evolving Distributions

Analysis of fast-ion distributions >100 T{sub e} is important for both ITER and a fusion reactor, as both will have a significant population of fusion-born alpha particles. Furthermore, energetic ions may display transport properties that differ from the bulk plasma. It is imperative to benchmark current simulation codes with measurements of highly energetic fast-ions on current devices. Experimental measurements of ICRF heated H-minority plasmas on Alcator C-Mod are performed with an upgraded Compact Neutral Particle Analyzer (CNPA). The CNPA consists of vertically-viewing photodiode detectors with viewing chords from r/a = {approx}0.18 to 0.6 and energies from 150 keV to 1.5 MeV. The CNPA measurements are compared to the output of a synthetic diagnostic that has been added to the Fokker-Planck solver CQL3D. CQL3D is coupled with the full-wave solver AORSA to evolve a self-consistent fast-ion distribution. Good agreement is obtained between simulation and experiment for fast ion distributions that are in steady-state (df/dt = 0). However, there are discrepancies between the experimental and simulated results for the time-dependent evolution of the fast-ion distribution. A possible explanation of the discrepancy is due to violation of certain assumptions of quasi-linear theory.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Minority Ion Measurements During ICRF Experiments in Alcator C-Mod

ICRF is the primary auxiliary heating in C-Mod where both H or {sup 3}He minority and mode conversion regimes are utilized. For transport analysis, the power deposition profile is critical and measuring the resulting fast ion distribution provides a direct means to constrain and validate ICRF simulations used to calculate power deposition. In mode conversion, measurement of the minority ion density, temperature, and velocity profiles is critical for the wave physics and may provide some insight into the fundamental physics of flow drive. Using active charge exchange, the He{sup +1} 4686A or H 6563A line is observed to find whether fast ion and relevant thermal ion measurements are practical. Results of these experiments yield fast ion and thermal ion measurements in D(He). A new analysis technique to extract information from high noise fast ion spectra is developed. A development path for improved D({sup 3}He) and D(H) is indicated.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

RF Plasma Heating in the PFRC-2 Device: Motivation, Goals and Methods

The motivation for using radio frequency, odd-parity rotating magnetic fields for heating field-reversed-configuration (FRC) plasmas is explained. Calculations are presented of the expected electron and ion temperatures in the PFRC-2 device, currently under construction.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Production of High Energy Tail Electrons by Electron Bernstein Waves during the Current Start-up Discharges in the LATE Device

Toroidal plasma current is started and ramped up by injecting microwave power in the electron cyclotron range of frequency without induction in the LATE device. Radial scanning with hard X-ray pulse height analysis reveals the production of high energy electrons with average energy {approx}100 keV in the radial region from R = 28 cm to 40.5 cm, which are heated by electron Bernstein wave. The radial profile of photon counts in the energy range from 25 to 200 keV is very similar to that of perpendicular pressure obtained by magnetic measurement and equilibrium analysis, suggesting that a significant portion of trapped electrons exists outside the last closed flux surface. The plasma current inside the LCFS is carried mainly by passing electrons, while some portion of the outside current may be generated as a result of the toroidal precession of trapped electrons.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Parasitic signals in the receiving band of the Sub-Harmonic Arc Detection system on JET ICRF Antennas

When testing the SHAD system on JET ICRF antennas, parasitic signals in the detection band (5-20MHz) were detected. We have identified emission from grid breakdown events in the Neutral Beam injectors, and Ion Cyclotron Emission from the plasma. Spurious signals in the band 4-10 MHz are also often observed at the onset of ELM events. Such parasitic signals could complicate the design and operation of SHAD in ICRF systems for fusion devices.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Slow Wave Excitation in the ICRF and HHFW Regimes

Theoretical considerations and high spatial resolution numerical simulations of radio frequency (rf) wave heating in tokamaks and in spherical toruses (ST) indicate that fast waves launched into tokamaks in the ion cyclotron range of frequencies (ICRF) or into spherical toruses in the high harmonic fast wave (HHFW) regime may excite a short wavelength slow mode inside of the plasma discharge due to the presence of hot electrons that satisfy the condition {omega}<k{sub ||}v{sub te}, where {omega} is the launched wave frequency, k{sub ||} is the local parallel component of the wave vector, and v{sub te} is the local electron thermal speed. This excited slow wave may be related to the electrostatic ion cyclotron wave that propagates for frequencies above the fundamental ion cyclotron frequency in warm plasmas or to a high frequency version of a kinetic Alfven wave. This slow wave, if physically real, would provide another path for rf power absorption in tokamaks and ST devices.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Scattering of Radio Frequency Waves by Edge Density Blobs in Tokamak Plasmas

The density blobs and fluctuations present in the edge region of magnetic fusion devices can scatter radio frequency (RF) waves through refraction and diffraction. The scattering can diffuse the rays in space and in wave-vector space. The diffusion in space can make the rays miss their intended target region, while the diffusion in wave-vector space can broaden the wave spectrum and modify the wave damping and current profile.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗