Showing papers by "Princeton Plasma Physics Laboratory published in 1991"

Excitation of toroidal Alfvén eigenmodes in TFTR.

Abstract: Deuterium neutral beams with energies up to 110 keV were injected into TFTR (Tokamak Fusion Test Reactor) plasmas at low magnetic field such that the beam injection velocities were comparable to the Alfven velocity. Excitation of toroidal Alfven eigenmodes was observed by Mirnov coils and beam emission spectroscopy. 10 refs., 4 figs.

Considerations of ion‐temperature‐gradient‐driven turbulence

Abstract: The ion‐temperature‐gradient‐driven instability is considered in this paper. Physical pictures are presented to clarify the nature of the instability. The saturation of a single eddy is modeled by a simple nonlinear equation. It is shown that eddies that are elongated in the direction of the temperature gradient are the most unstable and have the highest saturation amplitudes. In a sheared magnetic field, such elongated eddies twist with the field lines. This structure is shown to be an alternative to the usual Fourier mode picture in which the mode is localized around the surface where k∥ =0. These elongated twisting eddies, which are an integral part of the ‘‘ballooning mode’’ structure, could survive in a torus. The elongated eddies are shown to be unstable to secondary instabilities that are driven by the large gradients in the long eddy. It is argued that the ‘‘mixing length’’ is affected by this nonlinear process, and is unlikely to be a linear eigenmode width.

Approximate formula for radiofrequency current drive efficiency with magnetic trapping

Abstract: A functional form is presented for the efficiency of current drive in toroidal geometry with waves at frequencies below the electron cyclotron frequency By fitting constants in order to duplicate numerical results for the efficiency, an accurate function is obtained which will be useful for computer calculations of current drive

H-mode confinement in JET with enhanced performance by pellet peaked density profiles

Abstract: The combination of two regimes of enhanced performance, the H-mode and the pellet enhanced performance (PEP) mode, has been achieved in JET. The strong enhancement of the central plasma parameters, obtained with pellet injection and subsequent auxiliary heating, is found to persist well into the H-mode phase. A characteristic of the PEP regime is that an improvement of the fusion reactivity over non-pellet discharges is obtained under the condition of nearly equal electron and ion temperatures. A maximum neutron production rate of 0.95 ? 10l6 s?1 was obtained in a double-null X-point discharge with 2.5 MW of neutral beam heating and 9 MW of ion cyclotron resonance heating, with central ion and electron temperatures of about 10 keV and a central deuterium density of 8.0 ? 1019 m?3. The corresponding fusion product nD(0)?ETi(0) is between 7.0 and 8.6 ? 1020 m?3?s?keV. The enhanced neutron production is predominantly of thermonuclear (Maxwellian) origin. The compatibility of these regimes is an important issue in the context of tokamak ignition strategies. Several technical developments on JET have played a role in the achievement of this result: (1) the use of low voltage plasma breakdown (0.15 V/m) to permit pellet injection in an X-point configuration before the formation of a q = 1 surface; (2) the elimination of ICRH specific impurities with antenna Faraday screens made of solid beryllium; (3) the use of a novel system of plasma radial position control that stabilizes the coupling resistance of the ion cyclotron heating system.

The ARIES-I Tokamak Reactor Study†

Abstract: The ARIES research program is a multi-institutional effort to develop several visions of tokamak reactors with enhanced economic, safety, and environmental features. Three ARIES visions are currently planned for the ARIES program. The ARIES-I design is a DT-burning reactor based on modest extrapolation from the present tokamak physics data base; ARIES-II is a DT-burning reactor which will employ potential advances in physics; and ARIES-III is a conceptual D-3He reactor. The first design to be completed is ARIES-I, a 1000 MWe power reactor. The key features of ARIES-I are: (1) a passively safe and low environmental impact design because of choice of low activation material throughout the fusion power core, (2) an acceptable cost of electricity, (3) a plasma with performance as close as possible to present-day experimental achievements, (4) a high performance, low activation, SiC composite blanket cooled by He, and (5) an advanced Rankine power cycle as planned for near term coal-fired plants. The ARIES-I research has also identified key physics and technology areas with the highest leverage for achieving attractive fusion power system.

Overview of TFTR transport studies

Abstract: A review of TFTR plasma transport studies is presented. Parallel transport and the confinement of suprathermal ions are found to be relatively well described by theory. Cross-field transport of the thermal plasma, however, is anomalous with the momentum diffusivity being comparable to the ion thermal diffusivity and larger than the electron thermal diffusivity in neutral beam heated discharges. Perturbative experiments have studied nonlinear dependencies in the transport coefficients and examined the role of possible nonlocal phenomena. The underlying turbulence has been studied using microwave scattering, beam emission spectroscopy and microwave reflectometry over a much broader range in k perpendicular to than previously possible. Results indicate the existence of large-wavelength fluctuations correlated with enhanced transport.

Observation of temperature-dependent transport in the TFTR tokamak.

Abstract: Local particle and heat transport coefficients have been measured in a temperature scan of neutral-beam--heated plasmas with {ital n}, {ital I}{sub {ital p}}, and {ital B}{sub {ital c}{ital p}{ital h}{ital i}} held constant. The electron transport is ascertained from a flux analysis of a small density perturbation, and the heat transport is obtained from the equilibrium power balance. The transport coefficients vary as {ital T}{sub {ital e}}{sup {alpha}}, where {alpha}=1.5--2.5. The observed temperature dependence is predicted by numerical calculations of anomalous transport due to trapped-particle drift-type microinstabilities.

Analysis of sawtooth oscillations using simultaneous measurement of electron cyclotron emission imaging and x-ray tomography on TFTR.

Abstract: High-resolution electron cyclotron emission and x-ray image reconstructions have been made simultaneously during the sawtooth crash in the fast rotating plasma with neutral beam injection heating on TFTR. The measured x-ray emission is identical as metal impurity radiation. The results suggest that the sawtooth crash is a full reconnection process of the TFTR sawteeth. The crescent shaped ``hot spot'' in the x-ray emissivity is found not to present flux surfaces. New features are observed during the sawtooth crash, such as an oval-shaped hot spot and a cooler region between the island and the hot spot.

An analytic solution of high‐beta equilibrium in a large aspect ratio tokamak

Abstract: An analytic solution of the high‐beta (eβp∼βq2/e≫1) equilibrium of a large aspect ratio tokamak is presented. Two arbitrary flux functions, the pressure profile p(ψ) and the safety factor profile q(ψ), specify the equilibrium. The solution splits into two asymptotic regions: the core region where ψ is a function of the major radius alone and a narrow boundary layer region adjoining the conducting wall. The solutions in the two regions are asymptotically matched to each other. For monotonic pressure profiles, the Shafranov shift is equal to the minor radius. For beta much bigger than 1, the solution contains a region (in place of the magnetic axis) of zero magnetic field and constant pressure. At high beta the quantity βI, which is essentially proportional to the pressure over the total current squared, is largely independent of pressure. The important ramifications of limited βI for high‐beta reactors are discussed. Generalizations to shaped cross sections and hollow pressure profiles are outlined. The p...

Efficiency of Lower Hybrid Current Drive at 2.45 GHz in ASDEX

Abstract: The experimental lower hybrid current drive efficiency at zero electric field is found to agree with theory if the spectrum of the lower hybrid waves is fully accessible to the central region of the plasma. For broad or very low parallel wave number spectra, N∥, for which the plane wave accessibility condition is not fully satisfied, the observed efficiency is higher than predicted. At non-zero electric field, the measurements agree with theories for an electric field dependent efficiency. The model can also be applied to experiments in which a symmetric spectrum of waves is launched, or to cases where the current is driven opposite to the DC electric field if runaway electrons can be neglected. A shift of the deposition zone by superposition of a low N∥ current drive spectrum and a high N∥ symmetric spectrum does not change the current drive efficiency.

Impurity transport during the H-mode in DIII-D

Abstract: In H-mode plasmas in DIII-D, large modulations in spectroscopically measured impurity densities have been observed during shots with giant edge localized modes (ELMs). These spectral modulations have been analysed with the MIST impurity transport code. This analysis indicates that impurities are alternately flowing towards the plasma centre and then away from it. This alternating flow is correlated with ELM produced changes in the electron density. The electron density oscillations are extreme, causing the density profile to switch from hollow (just before an ELM) to centrally peaked (just after an ELM). Neoclassical convection, dependent on ion density gradients, causes impurities to concentrate most heavily where the electron density is largest and can explain the modulating impurity behaviour. Anomalous diffusion, D 1.0 × 104 cm2/s, reduces the degree of impurity peaking. As the plasma current increases, the increase in hollowness of electron density profiles can account for the observed decrease in central impurity accumulation. Transport of cobalt, injected by laser ablation, has also been studied; cobalt transport variations are consistent with the ELM induced changes seen in intrinsic impurity transport. The transport results may be consistent with neoclassical impurity convective fluxes and suggest that impurity accumulation in tokamaks will occur unless the electron density profile is flat or particle confinement is low.

Enhanced carbon influx into TFTR supershots

Abstract: Under some conditions, a very large influx of carbon into TFTR occurs during neutral beam injection into low recycling plasmas (the supershot regime). These carbon 'blooms' result in serious degradation of plasma parameters. The sources of this carbon have been identified as hot spots on the TFTR bumper limiter at or near the last closed flux surface. Two separate temperature thresholds have been identified. One threshold, at about 1650°C, is consistent with radiation enhanced sublimation (RES). The other, at about 2300°C, appears to be thermal sublimation of carbon from the limiter. The carbon influx can be quantitatively accounted for by taking laboratory values for RES rates, making reasonable assumptions about the extent of the blooming area and assuming unity carbon recycling at the limiter. Such high carbon recycling is expected, and it is shown that, in target plasmas at least, it is observed on TFTR. The sources of the carbon blooms are sites which have either loosely attached fragments of limiter material (caused by damage) or surfaces that are nearly perpendicular to the magnetic field lines. Such surfaces may have local power depositions two orders of magnitude higher than usual. The TFTR team modified the limiter during the opening of winter 1989–1990. The modifications greatly reduced the number and magnitude of the blooms, so that they are no longer a problem.

Plasma kinetic control in a tokamak

Abstract: Plasma kinetic (temperature and density) control is developed in terms of optimal control theory. This involved solving a distributed parameter control problem for which no previous general theory or techniques could be practically implemented. The methodology used involves reducing the one-dimensional particle and energy transport partial differential equations into a system of ordinary differential equations. The latter are linearized and put into standard control theory format. This technique can be generalized to any distributed parameter system whose variables can be modeled as simple analytic functions of the spatial coordinates. This distributed parameter control technique shows excellent control characteristics when applied to realistic plasma temperature and density profiles. Tests were made on temperatures and densities which had been perturbed about 10% below their desired value and profiles that were significantly more peaked than the required shape. Results were obtained for a simplified model problem with specific empirical transport coefficients and a one species plasma. >

Stability of internal kink modes with energetic trapped particles

Abstract: An analysis of the stability of ideal internal kink modes in high temperature tokamaks is presented. The theory includes kinetic effects associated with energetic trapped particles, bulk ion diamagnetic drifts as well as transit resonances. A corresponding dispersion relation is derived and analysed both analytically and numerically. The roles and implications of various kinetic effects are delineated and comparisons with previously obtained results made.

Particle transport in pellet fuelled JET plasmas

Abstract: Multipellet fuelling experiments have been performed on the Joint European Torus (JET) and the resulting density profile evolution has been analysed. A local particle transport analysis is carried out using a 1½–D radial particle transport code to model the experimentally determined evolution of the electron density profile. Plasmas in the limiter configuration, with Ohmic heating, ion cyclotron resonance frequency (ICRF) heating and neutral beam injection are analysed. Ohmic and ICRF heated discharges with peaked density profiles generated by deep pellet penetration are usually characterized by a low core particle diffusivity and can be satisfactorily modelled without any anomalous convective (pinch) term. The evolution of the density profile in auxiliary heated discharges with enhanced levels of magnetohydrodynamic fluctuations shows a rapid decay of the peaked central density that can be modelled with a temperature dependent increase in the diffusivity. Particle balance calculations of effective particle diffusivity in the core region yield values as low as 0.04 m2/s in discharges with highly peaked density profiles. The determined particle diffusivity in all cases is 10–100 times larger than the neoclassical diffusivity. The dependence of the particle diffusivity on density profile peakedness correlates well with the effective thermal diffusivity from power balance calculations.

Fusion policy advisory committee: final report

Abstract: Presentation des grandes orientations de la politique americaine de recherche et developpement en matiere de fusion nucleaire controlee

Monte Carlo Calculation of Delayed Gamma Dose Rate in Complex Geometry Using the Concept of Effective Delayed Gamma Production Cross Section

Abstract: With appropriate approximations, the delayed gamma dose rate problem can be formulated in terms of the effective delayed gamma production cross section. The coupled neutron-delayed gamma transport equations take the same form as the coupled neutron-prompt gamma transport equations and they can, therefore, be solved directly in the same manner. This eliminates the flux coupling step required in conventional calculations and makes it easier to handle complex, multidimensional problems, especially those that call for Monte Carlo calculations. Mathematical formulation and solution algorithms are derived. The advantages of this method in complex geometry are illustrated by its application in the Monte Carlo solution of a practical design problem.

Image reconstruction methods for the PBX-M pinhole camera.

Abstract: We describe two methods that have been used to reconstruct the soft x-ray emission profile of the PBX-M tokamak from the projected images recorded by the PBX-M pinhole camera [ Proc. Soc. Photo-Opt. Instrum. Eng.691, 111 ( 1986)]. Both methods must accurately represent the shape of the reconstructed profile while also providing a degree of immunity to noise in the data. The first method is a simple least-squares fit to the data. This has the advantage of being fast and small and thus easily implemented on the PDP-11 computer used to control the video digitizer for the pinhole camera. The second method involves the application of a maximum entropy algorithm to an overdetermined system. This has the advantage of allowing the use of a default profile. This profile contains additional knowledge about the plasma shape that can be obtained from equilibrium fits to the external magnetic measurements. Additionally the reconstruction is guaranteed positive, and the fit to the data can be relaxed by specifying both the amount and distribution of noise in the image. The algorithm described has the advantage of being considerably faster for an overdetermined system than the usual Lagrange multiplier approach to finding the maximum entropy solution [ J. Opt. Soc. Am.62, 511 ( 1972);Rev. Sci. Instrum.57, 1557 ( 1986)].

Three-Dimensional Monte Carlo Calculations of Nuclear Heating in CIT and Comparisons with Results from Simpler Models

Abstract: In the Compact Ignition Tokamak (CIT), significant nuclear heating results from the neutron and γ interactions in structural components near the plasma and it affects their thermo-mechanical behavi...

Charge states of many-electron atoms in a strong laser field

Abstract: The nonresonant, multiphoton ionization of many-electron atoms is explained theoretically within the Thomas-Fermi model with the inclusion of an electromagnetic field. Numerical results are presented for the charge state and induced dipole moment versus the strength of the applied electric field, which, because of the scaling properties of the model, can be expressed as universal functions of the field. Our results compare favorably with those from recent ionization experiments performed on noble gases using laser fields.

Sawtooth stabilization by electron cyclotron heating at the q=1 surface in the WT-3 tokamak

Abstract: Sawtooth oscillations (STO) in the Ohmically heated (OH) WT‐3 tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1988 (IAEA, Vienna, 1989), Vol. 1, p. 563] are strongly modified or suppressed by localized electron cyclotron resonance heating (ECH) near the q=1 surface, where q refers to the safety factor. The effect of ECH is much stronger when it is applied on the high‐field side, as compared to the low‐field side. Complete suppression of the STO is achieved for the duration of the ECH when it is applied on the high‐field side, in a low‐density plasma, provided the ECH power exceeds a threshold value. This threshold decreases with the increasing safety factor at the limiter qL and the complete stabilization of STO can be obtained even when ECH is applied on the low‐field side in the high‐qL region. It is noticeable that these complete stabilizations can be obtained only by the localized ECH with the elliptic antenna. The STO stabilization is attributed to a modification of the current density...

Four-wave mixing and phase conjugation in plasmas using ionization nonlinearities.

Abstract: Four-wave mixing in plasmas using ponderomotive and thermal nonlinearities has been extensively studied. Plasmas have received considerable attention since they become more efficient four-wave-mixing and phase-conjugation media at longer wavelengths (far infrared, 10 \ensuremath{\mu}m, to radio wave, 10 m). In this paper, the prospect of using an ionization nonlinearity in weakly ionized plasmas for degenerate four-wave mixing and phase conjugation is discussed. Like the thermal pressure nonlinearity, the ionization nonlinearity results from the heating of the plasma by the beat wave. However, as the local temperature increases, more neutral species are ionized by electron-impact ionization to form a beat-wave grating structure; instead of pushing the electrons into a grating structure (as with the thermal and ponderomotive nonlinearities), the electron density grating is created directly by ionization. Numerical estimates of the phase-conjugate reflectivity indicate reflectivities in the range of ${10}^{\mathrm{\ensuremath{-}}4--}$${10}^{\mathrm{\ensuremath{-}}3}$ are possible in a weakly ionized steady-state gas-discharge plasma. The reflectivity is limited to this range due to the onset of an ionization instability (striations). Larger reflectivities may be possible for pulsed plasma sources.

Magnetic fluctuations in currentless plasmas in the ATF torsatron

Abstract: Measurements of poloidal magnetic field fluctuations outside currentless, finite-beta (β ≤ 0.5%, β0 ≤ 3%) plasmas with peaked pressure profiles in the Advanced Toroidal Facility (ATF) torsatron reveal bands of small (θ/B ~ 10−5), coherent fluctuations in the frequency range 5-60 kHz. The geometrical structure of these fluctuations shows n = 1 toroidal mode symmetry, with inferred poloidal mode numbers of m = 2 and 3. The coherent Bθ amplitudes increase with B for B 0.3%, and then saturate and begin to decrease with B for B 0.3%.

Bootstrap current and Ware pinch in drift‐wave turbulent transport

Abstract: Quasilinear theory is used to calculate contributions to the bootstrap current, Ware pinch, and parallel resistivity due to drift‐wave fluctuations. It is shown that drift‐wave oscillations can produce large anomalous cross‐field fluxes, but have little effect on parallel transport. Specifically, the anomaly in the bootstrap current and Ware pinch is proportional to the spectral average of k⊥k∥, which vanishes because of the parity properties of the drift‐wave eigenfunction. Implications for recent perturbative experiments on tokamak fusion test reactor (TFTR) [Phys. Rev. Lett. 66, 421 (1990)] are discussed.

ITER helium ash accumulation

Abstract: Many studies have shown the importance of the ratio {upsilon}{sub He}/{upsilon}{sub E} in determining the level of He ash accumulation in future reactor systems. Results of the first tokamak He removal experiments have been analysed, and a first estimate of the ratio {upsilon}{sub He}/{upsilon}{sub E} to be expected for future reactor systems has been made. The experiments were carried out for neutral beam heated plasmas in the TEXTOR tokamak, at KFA/Julich. Helium was injected both as a short puff and continuously, and subsequently extracted with the Advanced Limiter Test-II pump limiter. The rate at which the He density decays has been determined with absolutely calibrated charge exchange spectroscopy, and compared with theoretical models, using the Multiple Impurity Species Transport (MIST) code. An analysis of energy confinement has been made with PPPL TRANSP code, to distinguish beam from thermal confinement, especially for low density cases. The ALT-II pump limiter system is found to exhaust the He with maximum exhaust efficiency (8 pumps) of {approximately}8%. We find 1<{upsilon}{sub He}/{upsilon}{sub E}<3.3 for the database of cases analysed to date. Analysis with the ITER TETRA systems code shows that these values would be adequate to achieve the required He concentration with the present ITERmore » divertor He extraction system.« less