Showing papers by "Princeton Plasma Physics Laboratory published in 2004"

Magnetic reconnection and mass acceleration in flare-coronal mass ejection events

Abstract: An observational relationship has been well established among magnetic reconnection, high-energy flare emissions and the rising motion of erupting flux ropes. In this paper, we verify that the rate of magnetic reconnection in the low corona is temporally correlated with the evolution of flare nonthermal emissions in hard X-rays and microwaves, all reaching their peak values during the rising phase of the soft X-ray emission. In addition, however, our new observations reveal a temporal correlation between the magnetic reconnection rate and the directly observed acceleration of the accompanying coronal mass ejection (CME) and filament in the low corona, thus establishing a correlation with the rising flux rope. These results are obtained by examining two well-observed two-ribbon flare events, for which we have good measurements of the rise motion of filament eruption and CMEs associated with the flares. By measuring the magnetic flux swept through by flare ribbons as they separate in the lower atmosphere, we infer the magnetic reconnection rate in terms of the reconnection electric field Erec inside the reconnecting current sheet (RCS) and the rate of magnetic flux convected into the diffusion region. For the X1.6 flare event, the inferred Erec is ~5.8 V cm-1 and the peak mass acceleration is ~3 km s-2, while for the M1.0 flare event Erec is ~0.5 V cm-1 and the peak mass acceleration is 0.2-0.4 km s-2.

Electromagnetic Fluctuations during Fast Reconnection in a Laboratory Plasma

Abstract: Experimental evidence for a positive correlation is established between the magnitude of electromagnetic fluctuations up to the lower-hybrid frequency range and enhancement of reconnection rates in a well-controlled laboratory plasma. The fluctuations belong to the right-hand polarized whistler wave branch, propagating obliquely to the reconnecting magnetic field, with a phase velocity comparable to the relative drift velocity between electrons and ions. The measured short coherence lengths indicate their strongly nonlinear nature.

High resolution Thomson scattering for Joint European Torus (JET)

Abstract: A Thomson scattering system is being developed for Joint European Torus with 15 mm spatial resolution and a foreseen accuracy for temperature better than 15% at a density of 1019 m−3. This resolution is required at the internal transport barrier and edge pedestal and it can not be fully achieved with the present light detection and ranging systems. The laser for this system is Nd:YAG, 5 Joule, 20 Hz. Scattering volumes from R=2.9 m to R=3.9 m are imaged onto 1 mm diameter fibers, with F/25 collection aperture. Two fibers are used per scattering volume. Using optical delay lines, three scattering volumes are combined in each of the 21 filter polychromators. The signals are recorded with transient digitizers, which allow the combined time delayed signals to be resolved. Knowledge of the time delay between signals allows the use of correlation techniques in determining signal levels. The ac output of the amplifier is used, which tolerates a higher level of background signal without affecting dynamic range. T...

Amplification of ultrashort laser pulses by a resonant Raman scheme in a gas-jet plasma

Abstract: Raman amplification of subpicosecond laser pulses up to 95 times is demonstrated at corresponding frequencies in a gas-jet plasma. The larger amplification is accompanied by a broader bandwidth and shorter pulse duration. Theoretical simulations show a qualitative agreement with the measurements, and the effects of the plasma conditions and laser intensities are discussed.

Measurement of the resistive wall mode stability in a rotating plasma using active mhd spectroscopy

Abstract: The stability of the resistive-wall mode (RWM) in DIII-D plasmas above the conventional pressure limit, where toroidal plasma rotation in the order of a few percent of the Alfve n velocity is sufficient to stabilize the n=1 RWM, has been probed using the technique of active MHD spectroscopy at frequencies of a few Hertz. The measured frequency spectrum of the plasma response to externally applied rotating resonant magnetic fields is well described by a single-mode approach and provides an absolute measurement of the damping rate and the natural mode rotation frequency of the stable RWM.

Trapped-particle instability leading to bursting in stimulated Raman scattering simulations

Abstract: Nonlinear, kinetic simulations of stimulated Raman scattering (SRS) under laser-fusion conditions present a bursting behavior. Different explanations for this regime have been given in previous studies: saturation of SRS by increased nonlinear Landau damping [K. Estabrook et al., Phys. Fluids B 1, 1282 (1989)], and detuning due to the nonlinear frequency shift of the plasma wave [H. X. Vu et al., Phys. Rev. Lett. 86, 4306 (2001)]. Another mechanism, also assigning a key role to the trapped electrons is proposed here: the breakup of the plasma wave through the trapped-particle instability.

Monitoring Alfven Cascades with Interferometry on the JET Tokamak

Abstract: A microwave interferometry technique is applied for the first time for detecting a discrete spectrum of Alfven cascade (AC) eigenmodes excited with fast ions in reversed magnetic shear plasmas of the Joint European Torus. The interferometry measurements of plasma density perturbations associated with ACs show an unprecedented frequency and time resolution superior to that obtained with external magnetic coils. The measurements of ACs are used for monitoring the evolution of the safety factor and density of rational magnetic surfaces in the region of maximum plasma current.

Equilibrium reconstruction in the Madison Symmetric Torus reversed field pinch

Abstract: A non-linear Grad–Shafranov toroidal equilibrium reconstruction code (MSTFit) has been developed for the Madison Symmetric Torus. This is the first such code applied to the unique magnetohydrodynamic (MHD) equilibrium of the reversed field pinch. A new set of toroidal Green's tables have been computed to impose the boundary condition of the close-fitting conducting shell. The non-linear fitting routine is sufficiently versatile for incorporating data from a variety of internal and external diagnostics, including a novel constraint based on orbits from a heavy ion beam probe diagnostic. Utilizing the full complement of internal and external magnetic and pressure diagnostics, MSTFit resolves accurately subtle changes in internal magnetic structure with implications on MHD stability. We show example equilibria that confirm conservation of magnetic helicity during relaxation and two-dimensional equilibrium effects.

Exploiting a transmission grating spectrometer

Abstract: The availability of compact transmission grating spectrometers now allows an attractive and economical alternative to the more familiar Czerny–Turner configuration for many high-temperature plasma applications. Higher throughput is obtained with short focal length refractive optics and stigmatic imaging. Many more spectra can be obtained with a single spectrometer since smaller, more densely packed optical input fibers can be used. Multiple input slits, along with a bandpass filter, can be used to maximize the number of spectra per detector, providing further economy. Curved slits can correct for the strong image curvature of the short focal length optics. Presented here are the governing grating equations for both standard and high-dispersion transmission gratings, defining dispersion, image curvature, and desired slit curvature, that can be used in the design of improved plasma diagnostics.

The resistive wall mode and feedback control physics design in NSTX

Abstract: One of the goals of the National Spherical Torus Experiment (NSTX) is to investigate the physics of global mode stabilization in a low aspect ratio device. NSTX has a major radius R0 = 0.86 m, a midplane half-width of 0.7 m, and an on-axis vacuum toroidal field B0 0.6 T and has reached a plasma current Ip = 1.5 MA. Experiments have established the wall-stabilized MHD operating space of the machine. The maximum βt and βN have reached 35% and 6.5%, respectively, with βN reaching 9.5li. Collapses in plasma toroidal rotation and βt have been correlated with violation of the n = 1 ideal MHD beta limit, β Nn o-wall, computed by the DCON stability code using timeevolving EFIT reconstructions of experimental discharges. The resistive wall mode (RWM) was observed over a wide range of βN when β Nn o-wall was exceeded. Plasma toroidal rotation damping during the RWM was rapid and global. Damping rates were more than five times larger than caused by low toroidal mode number rotating modes alone, which displayed a slower, diffusive rotation damping away from the rational surface. The rotation damping rate and dynamics depend on the applied toroidal field and the computed minimum value of the safety factor. The computed RWM perturbed field structure from experimental plasma reconstructions has been input to the VALEN feedback analysis code for quantitative comparison of experimental and theoretical RWM growth rates and to analyse the effectiveness of various active feedback stabilization designs. The computed RWM n = 1 mode growth rate, which depends on plasma equilibrium parameters such as βN and pressure profile peaking, agrees well with experimental growth rates in different operating regimes. Increasing βN in the ST initially improves mode coupling to the stabilizing wall; however, at the highest βN values reached, the ideal with-wall beta limit, βN wall, is approached, the effectiveness of the passive stabilizing plates is reduced, and the computed RWM growth rate approaches ideal MHD growth rates. Several active mode control designs were considered and evaluated. The most effective configuration is computed to provide stabilization at βN up to 94% of the ideal with-wall limit.

Scintillator probe for lost alpha measurements in JET

Abstract: Good confinement of alpha particles in a large magnetic fusion device is a precondition for building a magnetic fusion reactor. The direct measurement of alpha particle losses is of particular interest. Appropriate diagnostics are now being prepared for the Joint European Torus tokamak: a scintillator probe and a set of Faraday cups. Both systems are capable of measuring charged fusion products and ion cyclotron resonance heating tail ions. The design of the lost alpha particle scintillator probe is in the scope of this article. It will allow the detection of particles with a gyroradius between 20 and 140 mm (15% resolution) and a pitch angle between 30° and 86° (5% resolution). As scintillating material P56 will be used. The light emitted by the scintillator caused by charged particles that pass the collimator and hit the scintillator will be detected via a set of optical lenses and a coherent image fiber bundle with a charge coupled device camera and a photomultiplier array. In the following the present...

Aneutronic Fusion in a Degenerate Plasma

Abstract: In a Fermi-degenerate plasma, the electronic stopping of a slow ion is smaller than that given by the classical formula, because some transitions between the electron states are forbidden. The bremsstrahlung losses are then smaller, so that the nuclear burning of an aneutronic fuel is more efficient. Consequently, there occurs a parameter regime in which self-burning is possible. Practical obstacles in this regime that must be overcome before net energy can be realized include the compression of the fuel to an ultra dense state and the creation of a hot spot.

Numerical Transport Codes

Abstract: This paper gives a brief introduction on numerical transport codes. The relevant equations which are used in these codes are established, and on the basis of these equations, the necessary calculations needed to resolve them are pointed out. Finally, some examples are given, illustrating their application.

Zonal-flow dynamics and size scaling of anomalous transport.

Abstract: Nonlinear equations for the slow space-time evolution of the radial drift wave envelope and zonal flow amplitude have been self-consistently derived for a model nonuniform tokamak equilibrium within the coherent 4-wave drift wave-zonal flow modulation interaction model of Chen, Lin, and White [Phys. Plasmas 7 (2000) 3129]. Solutions clearly demonstrate turbulence spreading due to nonlinearly enhanced dispersiveness and, consequently, the device-size dependence of the saturated wave intensities and transport coefficients.

Scaling of the critical plasma rotation for stabilization of the n = 1 resistive wall mode (ideal kink) in the DIII-D tokamak

Abstract: Experiments in the DIII-D tokamak show that the n = 1 ideal kink can be stabilized by a resistive wall if the plasma is rotating fast enough. A database of the onset of the n = 1 resistive wall mode as a function of the equilibrium toroidal magnetic field, the plasma density and the toroidal rotation has been assembled for plasmas with beta between the theoretically predicted no wall and ideal wall stability limits. The critical rotation frequency is found to scale as the inverse of the Alfven time with ? ?A 0.02 (evaluated at the q = 2 surface at ? 0.6) or ? ?S 0.7, where ?S is the sound time. The dependence of ? ? A or ? ?S on ?N/?N,no wall from 1?2 is weak and suggests the plasmas are in the 'intermediate dissipation' regime.

LABORATORY MEASUREMENTS OF THE Fe XVII 2p-3s AND 2p-3d TRANSITIONS AND COMPARISON WITH SOLAR AND ASTROPHYSICAL OBSERVATIONS

Abstract: The L-shell emission spectrum of Fe XVII is measured in high-temperature laboratory plasmas, and the inferred intensities of the 3s → 2p transitions relative to those of the dominant 3d → 2p transition are compared to solar observations, as well as to observations of Capella, Procyon, Castor C, II Pegassi, and NGC 4636 with the Chandra and XMM-Newton X-ray observatories. The results from laboratory and astrophysical plasmas are in very good agreement, indicating that the collisional line formation processes found in low-density, high-temperature laboratory plasmas are a good description of those found in astrophysical plasmas. The laboratory observations disagree, however, to varying degrees with spectral modeling calculations. A review of existing laboratory measurements suggests that the intensity of the dominant 3d → 2p transition is overestimated by spectral modeling predications. By calibrating spectral models with laboratory data, especially by decreasing the strength of the dominant 3d → 2p transition, spectral models can be brought into agreement with the majority of solar and astrophysical observations. Without doing so, opacity effects may be grossly overestimated.

Towards the realization on JET of an integrated H-mode scenario for ITER

Abstract: ELMy H-mode experiments at JET in 2000/mid-2002 have focused on discharges with normalized parameters for plasma density, energy confinement and beta similar to those of the ITER Q(DT) = 10 referen ...

MHD ballooning instability in the plasma sheet

Abstract: [1] Based on the ideal MHD model the stability of ballooning modes is investigated by employing realistic 3D magnetospheric equilibria for the substorm growth phase. Our results show that without making compressibility approximations the ballooning modes are unstable for the entire plasma sheet where the equatorial βeq ≥ 1, and the most unstable modes are located in the strong cross-tail current sheet region in the near-Earth plasma sheet, which maps to the initial brightening location of the breakup arc in the ionosphere. However, the MHD βeq threshold is too low in comparison with observations by AMPTE/CCE, which show that prior to substorm onset a low frequency instability is excited only when βeq > 50. The difficulty can be mitigated by including kinetic effects, which greatly increase the stabilizing effects of field line tension and can enhance the βeq threshold to limit the unstable region to the cross-tail current sheet region.

Collective instabilities and beam-plasma interactions in intense heavy ion beams

Abstract: This paper presents a survey of the present theoretical understanding of collective processes and beam-plasma interactions affecting intense heavy ion beam propagation in heavy ion fusion systems. In the acceleration and beam transport regions, the topics covered include discussion of the conditions for quiescent beam propagation over long distances; the electrostatic Harris-type instability and the transverse electromagnetic Weibel-type instability in strongly anisotropic, one-component non-neutral ion beams; and the dipole-mode, electron-ion two-stream instability driven by an (unwanted) component of background electrons. In the plasma plug and target chamber regions, collective processes associated with the interaction of the intense ion beam with a charge-neutralizing background plasma are described, including the electrostatic electron-ion two-stream instability, the electromagnetic Weibel instability, and the resistive hose instability. Operating regimes are identified where the possible deleterious effects of collective processes on beam quality are minimized.

Energetic particle physics in JT-60U and JFT-2M

Abstract: Recent energetic particle physics research in JT-60U and JFT-2M is reported. Alfven eigenmodes (AEs) are investigated in reversed-shear (RS) plasmas in JT-60U where frequency sweeping (FS) modes are observed to follow the q-profile evolution. The RS-induced AE model can explain the FS of the modes within the context of an evolving q-profile. Enhanced energetic ion transport is also investigated with the appearance of modes in the toroidicity-induced AE range of frequency in JT-60U using a multi-channel neutron profile monitor and in JFT-2M using a lost ion probe. Additionally, the ripple loss in the complex toroidal field ripple due to ferritic steel inserts in JFT-2M is shown and compared with model analysis. The simulation code developed to predict ripple loss in JFT-2M will be of use in estimating the heat flux in the complex ripple field of a future device such as ITER.

Electrostatic probe apparatus for measurements in the near-anode region of Hall thrusters

Abstract: Near-anode processes in Hall-current plasma thrusters are largely uncharacterized in the experimental literature. In order to perform measurements in the near-anode region, the high potential of the anode relative to ground, small spatial variations of plasma properties, and the complicated thruster geometry are just some of the features that must be taken into consideration. A diagnostic apparatus for measurements in the near-anode region of Hall thrusters, comprising biased and emissive electrostatic probes, a high-precision positioning system, and low-noise electronic circuitry, was developed and tested. Test data for this apparatus indicate that radially inserted probes negligibly perturb the discharge. Accurate near-anode measurements of the plasma density, electron temperature, and plasma potential performed with this diagnostic have allowed the first experimental identification of the electron-repelling anode sheath predicted theoretically in Hall thrusters.

Confinement studies of auxiliary heated NSTX plasmas

Abstract: The confinement of auxiliary heated national spherical torus experiment discharges is discussed. From a database analysis, it is found that the energy confinement time in NBI heated plasmas with either L- or H-modes edge is up to 2.5 times the values predicted by the ITER97L scaling. A high power NBI heated H-mode discharge is discussed in detail. TRANSP calculations based on the kinetic profile measurements reproduce well the magnetically determined stored energy, but overestimate the measured neutron rate by 30%. Power balance calculations reveal that the ion thermal transport is above or near neoclassical levels, and significantly below the electron thermal transport, which constitutes the main power loss channel. Perturbative impurity injection techniques indicate the particle diffusivity is slightly above the neoclassical level in discharges with L-mode edge. High-harmonic fast-wave (HHFW) bulk electron heating is described and thermal transport is discussed. Thermal ion transport is found to be above the neoclassical level, and thermal electron transport remains the main loss mechanism. Evidence of an electron thermal internal transport barrier obtained with HHFW heating is presented. A description of H-mode discharges obtained during HHFW heating is made.

Investigation of performance limiting phenomena in a variable phase ICRF antenna in Alcator C-Mod

Abstract: High power density, phased antenna operation can often be limited by antenna voltage handling and/or impurity and density production. Using a pair of two-strap antennas for comparison, the performance of a four-strap, fast wave antenna is assessed for a variety of configurations and antenna phases in Alcator C-Mod. To obtain robust voltage handling, the antenna was reconfigured to eliminate regions where the RF E-field is parallel to B or to reduce the RF E-field to <1.0 MV m−1. To limit impurity generation, BN tiles were used to replace the original Mo tiles, a BN clad septum was inserted to limit field line connection length, and BN–metal interfaces were shielded from the plasma. With these modifications, the antenna heating efficiency and impurity generation are nearly identical to those of the two-strap antennas and independent of antenna phase in L-mode discharges. This antenna has achieved 11 MW m−2 in both heating and current drive phases in both L-mode and H-mode discharges.

Design and characterization of a neutralized-transport experiment for heavy-ion fusion

Abstract: In heavy-ion inertial-confinement fusion systems, intense beams of ions must be transported from the exit of the final-focus magnet system through the fusion chamber to hit spots on the target with radii of about 2 mm For the heavy-ion-fusion power-plant scenarios presently favored in the US, a substantial fraction of the ion-beam space charge must be neutralized during this final transport The most effective neutralization technique found in numerical simulations is to pass each beam through a low-density plasma after the final focusing To provide quantitative comparisons of these theoretical predictions with experiment, the Virtual National Laboratory for Heavy Ion Fusion has completed the construction and has begun experimentation with the neutralized-transport experiment The experiment consists of three main sections, each with its own physics issues The injector is designed to generate a very high-brightness, space-charge-dominated potassium beam, while still allowing variable perveance by a beam aperturing technique The magnetic-focusing section, consisting of four pulsed quadrupoles, permits the study of magnet tuning, as well as the effects of phase-space dilution due to higher-order nonlinear fields In the final section, the converging ion beam exiting the magnetic section is transported through a drift region with plasma sources for beam neutralization, and the final spot size is measured under various conditions of neutralization In this paper, we discuss the design and characterization of the three sections in detail and present initial results from the experiment

Effect of gas fuelling location on H-mode access in NSTX

Abstract: The dependence of H-mode access on the poloidal location of the gas injection source has been investigated in NSTX. We find that gas fuelling from the centre stack midplane area produces the most reproducible H-mode access, with generally the lowest L–H threshold power in the lower single-null configuration. The edge toroidal rotation velocity of C2+ is largest (in the direction of the plasma current) just before the L–H transition with centre stack midplane fuelling and then reverses direction after the L–H transition. Simulation of these results with a guiding centre Monte Carlo neoclassical transport code (XGC) is qualitatively consistent with the trends in the measured velocities. Double-null discharges exhibit H-mode access with gas fuelling from either the centre stack midplane or centre stack top locations, indicating a reduced sensitivity of H-mode access to fuelling location in that shape.

Tritium Removal from Carbon Plasma Facing Components

Abstract: Tritium removal is a major unsolved development task for next-step devices with carbon plasma facing components. The 2–3 order of magnitude increase in duty cycle and associated tritium accumulation rate in a next-step tokamak will place unprecedented demands on tritium removal technology. The associated technical risk can be mitigated only if suitable removal techniques are demonstrated on tokamaks before the construction of a next-step device. This article reviews the history of codeposition, the tritium experience of TFTR and JET and the tritium removal rate required to support ITER's planned operational schedule. The merits and shortcomings of various tritium removal techniques are discussed with particular emphasis on oxidation and laser surface heating.

Saturated state of the nonlinear small-scale dynamo.

Abstract: We consider the problem of incompressible, forced, nonhelical, homogeneous, and isotropic MHD turbulence with no mean magnetic field and large magnetic Prandtl number. This type of MHD turbulence is the end state of the turbulent dynamo, which generates folded fields with small-scale direction reversals. We propose a model in which saturation is achieved as a result of the velocity statistics becoming anisotropic with respect to the local direction of the magnetic folds. The model combines the effects of weakened stretching and quasi-two-dimensional mixing and produces magnetic-energy spectra in remarkable agreement with numerical results at least in the case of a one-scale flow. We conjecture that the statistics seen in numerical simulations could be explained as a superposition of these folded fields and Alfven-like waves that propagate along the folds.