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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.

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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
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

Characterisation of SOL density fluctuations in front of the LHCD PAM launcher in Tore

The density fluctuations, modified by Lower Hybrid Wave (LHW), is analyzed in Tore Supra with reference to the injected LHW power, density and the gap between LCFS (Last Closed Flux Surface) and the PAM (passive-active-multijunction) launcher. The density fluctuations are measured with RF probes installed at the PAM launcher front. A density scan at nominal toroidal field (3.8 T) shows that the fluctuations rate stays nearly constant ({approx}50%) for <n{sub e}><3.5x10{sup 19}m{sup -3} and with LHW power up to 2MW. However, when increasing the density above <n{sub e}><{approx}4.2x10{sup 19}m{sup -3}, using strong gas puffing, the fluctuation rate increases to >70% and is characterized by strong negative spikes, with typical frequency >100kHz. These are most likely originating from acceleration of electrons in the LHW near field due to parasitic absorption, as evidenced on the IR images, showing hot spots on the side limiters.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Role of Combined NNBI and ICRH Heating in FAST H-mode plasmas

The combination of ICRH+NNBI in FAST allows the generation of fast ion populations with different velocity space anisotropy and radial profiles. These energetic ion populations can excite meso-scale fluctuations with the same characteristics of those expected in reactor conditions and, for this reason, FAST can address a number of important burning plasma physics issues. Numerical simulation and modeling of energetic particle physics are based on the use of transport codes that are iteratively coupled with a bi-dimensional full wave-quasi-linear solver for ICRH, in order to determinate the normalized supra-thermal population pressure ss{sub hot}. The value of ss{sub hot}. as well the energetic particle distribution functions can be used as initial condition for numerical simulation studies, investigating the destabilization and saturation of fast ion driven Alfvenic modes.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Library for RF Interactions in Orbit Following Codes

A new code-library has been developed to handle quasi-linear wave particle interactions in orbit following Monte Carlo codes, RFOF (RF interactions in Orbit Following codes). This library will enable a large number of orbit following codes to model fast ion acceleration during ICRF and Lower Hybrid heating. The RFOF consists of two main modules: one evaluates the resonance condition, the other the resulting RF acceleration. The resonance condition is tested at each step along the orbit and the location of the next upcoming resonance is predicted. When a particle reaches the resonance, a quasi-linear acceleration is calculated with a novel Monte Carlo technique that avoids the time-consuming evaluation of phase-space derivatives of the interaction strength. In RFOF the wave-particles interactions are assumed to be localized to a single point on the orbit. This is often valid for the ion cyclotron and lower hybrid frequency ranges, but prevents the treatment of bounce and precessional resonances. The RFOF has been developed within the European Task Force for Integrated Tokamak Modelling, enabling interaction between experts in different fields. As a result the code is designed with a simple and generic interface, with a minimum of assumptions on e.g. the geometry. Successful integration with the two orbit following codes, ASCOT and SPOT, has already been demonstrated.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Simulation Study of Current Drive Efficiency for KSTAR 5 GHz LHCD

Theoretical 5 GHz lower hybrid current drive (LHCD) efficiency using power spectrum given by 0-D Brambilla code and Lower Hybrid Simulation Code (LSC) have been studied for KSTAR. In LSC simulation, RF-driven current and current drive efficiency has been found to be deeply dependent on the profiles of the plasma density and temperature as well as on current profile in order to obtain hollow current profile favorable for advance tokamak operation mode and steady state operation. The peaked density and broad temperature profile control has been found to be efficient in current drive with maximum RF-driven current larger than 400 kA/MW with very high efficiency when the peak plasma density is ranged from 0.2 to 2.0x1020 m-3, and the peak electron temperature range of 2-20 keV together with toroidal field 2-3.5 T and Ip = 0.5-2 MA. The on-/off-axis current profile controllability is also investigated through parametric scan, and small negative magnetic shear is seen at the narrow region of the off-axis for very high temperature regime and for high BT and I{sub p}. In order to achieve the same for lower temperature regime Ip has to be lower and also for higher LH-power compromising with CD efficiency in this case.

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

Direct Electron Heating Observed by Fast Waves in ICRF Range on a Low-Density Low Temperature Tokamak ADITYA

Fast wave electron heating experiments are carried out on Aditya tokamak [R = 0.75 m, a = 0.25m,Bt = 0.75T,ne{approx}1-3E13/cc,Te{approx}250eV] with the help of indigenously developed 200 kW, 20-40 MHz RF heating system. Significant direct electron heating is observed by fast waves in hydrogen plasma with prompt rise in electron temperature with application of RF power and it increases linearly with RF power. A corresponding increase in plasma beta and hence increase in stored diamagnetic energy is also observed in presence of RF. We observe an improvement of energy confinement time from 2-4msec during ohmic heating phase to 3-6msec in RF heating phase. This improvement is within the ohmic confinement regime for the present experiments. The impurity radiation and electron density do not escalate significantly with RF power. The direct electron heating by fast wave in Aditya is also predicted by ion cyclotron resonance heating code TORIC.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Enhancing The Mode Conversion Efficiency In JET Plasmas With Multiple Mode Conversion Layers

The constructive interference effect described by Fuchs et al. [1] shows that the mode conversion and thereby the overall heating efficiency can be enhanced significantly when an integer number of fast wave wavelengths can be folded in between the high field side fast wave cutoff and the ion-ion hybrid layer(s) at which the ion Bernstein or ion cyclotron waves are excited. This effect was already experimentally identified in ({sup 3}He)-D plasmas [2] and was recently tested in ({sup 3}He)-H JET plasmas. The latter is an 'inverted' scenario, which differs significantly from the ({sup 3}He)-D scenarios since the mode-conversion layer is positioned between the low field side edge of the plasma and the ion-cyclotron layer of the minority {sup 3}He ions (whereas the order in which a wave entering the plasma from the low field side encounters these layers is inverted in a 'regular' scenario), and because much lower {sup 3}He concentrations are needed to achieve the mode-conversion heating regime. The presence of small amounts of {sup 4}He and D in the discharges gave rise to an additional mode conversion layer on top of the expected one associated with {sup 3}He-H, which made the interpretation of the results more complex but also more interesting: Three different regimes could be distinguished as a function of X[{sup 3}He], and the differing dynamics at the various concentrations could be traced back to the presence of these two mode conversion layers and their associated fast wave cutoffs. Whereas (1-D and 2-D) numerical modeling yields quantitative information on the RF absorptivity, recent analytical work by Kazakov [3] permits to grasp the dominant underlying wave interaction physics.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Plasma Current Start-up Experiment using Waves in the Lower Hybrid Frequency Range in TST-2

Noninductive plasma current (I{sub p}) start-up experiments using RF power in the lower hybrid frequency range are being conducted on the TST-2 spherical tokamak. The lower hybrid wave (LHW) has demonstrated efficient current drive in conventional tokamaks. However, in spherical tokamak (ST) plasmas with very high dielectric constants (;{epsilon}{approx}{omega}{sub pe}{sup 2}/{Omega}{sub e}{sup 2}>>1), accessibility of the LHW to the plasma core is severely limited. Our approach is to keep the plasma density low (such that {epsilon}{approx}1) during I{sub p} ramp-up. Once I{sub p} reaches a level sufficiently high for neutral beam current drive, plasma can be densified and transformed into an advanced tokamak plasma dominated by the self-driven bootstrap current. Initial plasma start-up experiments were performed on TST-2 using a combline antenna which excites a traveling fast wave. After formation of toroidal flux surfaces, RF power and vertical field were ramped up to increase I{sub p}. Up to 12 kA of Ip has been obtained by this method. Soft X-ray measurements indicate that the electron temperature increases as I{sub p} increases, and hard X-ray spectroscopy indicates that energetic electrons build up as I{sub p} is ramped up.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Neutron emission spectroscopy of DT plasmas at enhanced energy resolution with diamond detectors

This work presents measurements done at the Peking University Van de Graaff neutron source of the response of single crystal synthetic diamond (SD) detectors to quasi-monoenergetic neutrons of 14-20 MeV. The results show an energy resolution of 1% for incoming 20 MeV neutrons, which, together with 1% detection efficiency, opens up to new prospects for fast ion physics studies in high performance nuclear fusion devices such as SD neutron spectrometry of deuterium-tritium plasmas heated by neutral beam injection.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

Studying fast wave propagation and absorption at any cyclotron harmonic using a 2D finite element area coordinates wave equation solver

Fourier analysis in the poloidal direction is a standard ingredient in present-day 2D wave equation solvers describing radio frequency waves in hot tokamak plasmas. Although a powerful and elegant technique, Fourier analysis has the disadvantage that a large number of modes is needed to describe the field pattern on a magnetic surface if a short wavelength mode exists on any - even very small - subpart of the particle trajectory. The present paper examines the potential of a method that does not suffer from this drawback: a finite element technique relying on simple linear or cubic area base functions that are defined on irregular elementary surfaces of triangular shape. The wave equation is solved in its weak Galerkin variational form and for realistic 2D tokamak geometry, accounting for the toroidal curvature but assuming the toroidal angle is ignorable, allowing to study the wave pattern for each of the independent toroidal modes excited by the antenna individually.The locally uniform full hot plasma dielectric tensor to all orders in finite Larmor radius was adopted. As the main intended application is the study of fast wave behavior (heating and current drive) at arbitrary harmonics, the wave vector complex amplitude appearing in the dielectric tensor is determined through a local dispersion root evaluation. High frequency fast wave propagation and damping is provided as an illustration in view of possible application of this type of current drive in future high density reactor-like tokamaks.

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

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

High frequency fast wave current drive for DEMO

A steady-state tokamak reactor (SSTR) requires a high efficiency current drive system, from plug to driven mega-amps. RF systems working in the ion-cyclotron range of frequencies (ICRF) have high efficiency from plug to antenna but a limited current drive (CD) efficiency and centrally peaked CD profiles. The latter feature is not adequate for a SSTR where the current should be sufficiently broad to keep the central safety factor (possibly significantly) above 1. In addition, the fact that the fast wave (FW) is evanescent at the edge limits coupling, requiring high voltage operation, which makes the system dependent on plasma edge properties and prone to arcing, reducing its reliability. A possible way to overcome these weaknesses is to operate at higher frequency (10 times or more the cyclotron frequency). The advantages are: (1) The coupling can be much better (waves propagate in vacuum) if the parallel refractive index n{sub ||} is kept below one, (2) The FW group velocity tends to align to the magnetic field, so the power circumnavigates the magnetic axis and can drive off-axis current, (3) Due to the latter property, n{sub ||} can be upshifted along the wave propagation path, allowing low n{sub ||} launch (hence good coupling, large CD efficiency) with ultimately good electron absorption (which requires higher n{sub ||}. Note however that the n{sub ||} upshift is a self-organized feature, that electron absorption is in competition with {alpha}-particle absorption and that uncoupling of the FW from the lower hybrid resonance at the edge requires n{sub ||} slightly above one. The latter possibly counterproductive features might complicate the picture. The different aspects of this potentially attractive off-axis FWCD scheme are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Lower hybrid wave propagation in tokamaks in weak and strong absorption regimes

Lower hybrid (LH) waves have the attractive property of damping strongly via electron Landau resonance on relatively fast tail electrons at (2.5-3)xv{sub te}, where v{sub te} = (2T{sub e}/m{sub e}){sup 1/2}. The velocity at which damping occurs depends on the non-linear balance between quasilinear diffusion and collisions. For high efficiency current drive, a low parallel index of refraction, n{sub ||}, corresponding to a high phase velocity, is chosen. Depending on the plasma electron temperature this may put the wave propagation in a multi-pass regime. In cases of low parallel refractive index, ray tracing with no SOL has been shown to have differences with experiment and collision effects in the scrape off layer may be important. Using a coupled model of the full wave code, TORLH, and the Fokker-Planck code, CQL3D, the importance of full wave effects in weak and strong absorption regimes are studied.

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

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

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↗