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Showing papers by "Princeton Plasma Physics Laboratory published in 1990"


Journal ArticleDOI
TL;DR: In this paper, two new scaling expressions for tokamak L-mode energy confinement are proposed, namely a power law scaling and an offset-linear scaling, based on an analysis of the ITER Lmode ECC database.
Abstract: On the basis of an analysis of the ITER L-mode energy confinement database, two new scaling expressions for tokamak L-mode energy confinement are proposed, namely a power law scaling and an offset-linear scaling. The analysis indicates that the present multiplicity of scaling expressions for the energy confinement time τE in tokamaks (Goldston, Kaye, Odajima-Shimomura, Rebut-Lallia, etc.) is due both to the lack of variation of a key parameter combination in the database, fs = 0.32 R a−0.75 k0.5 ~ A a0.25k0.5, and to variations in the dependence of τE on the physical parameters among the different tokamaks in the database. By combining multiples of fs and another factor, fq = 1.56 a2 kB/RIp = qeng/3.2, which partially reflects the tokamak to tokamak variation of the dependence of τE on q and therefore implicitly the dependence of τE on Ip and ne, the two proposed confinement scaling expressions can be transformed to forms very close to most of the common scaling expressions. To reduce the multiplicity of the scalings for energy confinement, the database must be improved by adding new data with significant variations in fs, and the physical reasons for the tokamak to tokamak variation of some of the dependences of the energy confinement time on tokamak parameters must be clarified.

504 citations


Journal ArticleDOI
TL;DR: It is found that plasmas of antiparallel helicity merge much faster than those of parallel helicity, and the reconnection rate is proportional to the initial relative velocity of the two plasma tori, suggesting that magnetic reconnection is a forced phenomenon.
Abstract: Magnetic reconnection phenomena are investigated taking into account all three vector components of the magnetic field in a laboratory experiment. Two toroidal magnetized plasmas carrying identical toroidal currents and poloidal field configurations are made to collide, thereby inducing magnetic reconnection. The direction of the toroidal field plays an important role in the merging process. It is found that plasmas of antiparallel helicity merge much faster than those of parallel helicity. It is also found that the reconnection rate is proportional to the initial relative velocity of the two plasma tori, suggesting that magnetic reconnection, in the present experiment, is a forced phenomenon.

146 citations


Journal ArticleDOI
TL;DR: In this article, it was shown theoretically that flux surfaces near the separatrix of properly elongated and triangulated plasmas may be moved into the connection region between the first and second stability regions for ideal MHD ballooning modes, when q is high and the shear is low near the plasma surface.
Abstract: The occurrence of giant edge localized modes (ELMs) in DIII-D has previously been correlated with the violation of the ballooning stability criterion at the plasma edge. These results are extended in the paper. It is demonstrated theoretically that flux surfaces near the separatrix of properly elongated and triangulated plasmas may be moved into the connection region between the first and second stability regions for ideal MHD ballooning modes, when q is high and the shear is low near the plasma surface. The edge flux surfaces are then predicted to have no limit to the sustainable pressure gradient. Experimentally, giant ELMs disappear in these highly shaped plasmas. However, the edge pressure gradient does not increase and 'grassy' ELM behaviour appears instead. These results lend further support to the hypothesis that giant ELMs in DIII-D are triggered by ideal ballooning mode instabilities, but they indicate that giant ELM and grassy ELM behaviour may arise from somewhat different mechanisms.

127 citations


Journal ArticleDOI
TL;DR: In this paper, an innovative fusion reactor suitable for D-3He fuel is proposed; it is based on a dipole magnetic field produced by a levitated superconducting coil, whose phase space density satisfies ∂f0(μ,J,ψ)/∂ψ = 0, where ψ is the flux function, has a steep enough pressure profile for an efficient fusion reaction yet is stable to low frequency instabilities for local beta exceeding unity.
Abstract: An innovative fusion reactor suitable for D-3He fuel is proposed; it is based on a dipole magnetic field produced by a levitated superconducting coil. The equilibrium plasma, whose phase space density satisfies ∂f0(μ,J,ψ)/∂ψ = 0, where ψ is the flux function, has a steep enough pressure profile for an efficient fusion reaction yet is stable to low frequency instabilities for local beta exceeding unity. At the outer wall, the plasma is stabilized by line tying or localized magnetic cusps which can be used for direct conversion. The fusion product confinement time can be controlled for ash removal by breaking the axisymmetry of the dipole magnetic field. A conceptual 70 MW reactor design is presented.

92 citations


Journal ArticleDOI
TL;DR: It is shown that the relaxation of this {ital extended} perturbation is consistent with the power-balance estimates of the local thermal diffusivity.
Abstract: Measurements on the TFTR tokamak of the electron-temperature-profile evolution and soft-x-ray emissivity on a fast (10-\ensuremath{\mu}sec) time scale during a sawtooth crash show that significant heat is deposited beyond the mixing (or reconnection) radius within 200 \ensuremath{\mu}sec following a sawtooth crash. This extended region in which electron heat is redistributed during the sawtooth crash substantially complicates the determination of heat transport properties from the subsequent heat pulse propagation. It is shown that the relaxation of this extended perturbation is consistent with the power-balance estimates of the local thermal diffusivity.

73 citations


Journal ArticleDOI
TL;DR: In this article, the shape evolution of a discharge in an elongated tokamak was shown to be entirely preprogrammed, using magnetic measurements made close to the vessel wall.
Abstract: Plasma shape and position control in elongated tokamaks is analysed, using the TSC code. The paper presents a new algorithm which allows the shape evolution of a discharge to be entirely preprogrammed. The algorithm computes poloidal field coil voltages as functions of time, using magnetic measurements made close to the vessel wall. No preprogrammed coil current wave forms are required. By simulating the start-up phase of typical TCV tokamak discharges, it is shown that the actual shape evolution follows closely the preprogrammed one. X-points can be specified at arbitrary positions. Active stabilization of the dominant vertical mode is achieved by a conventional PD feedback loop. The optimization of feedback coefficients, the trade-off between shape accuracy and power dissipation in the poloidal field coils, as well as the computing power requirements for implementing the algorithm in a real experiment are discussed.

66 citations


Journal ArticleDOI
TL;DR: In this paper, a three-dimensional non-linear toroidal MHD code, MH3D, has been used to study sawtooth oscillations in tokamaks.
Abstract: A three-dimensional non-linear toroidal MHD code, MH3D, has been used to study sawtooth oscillations in tokamaks. The simulation results at high temperature agree with the Kadomtsev model, but good agreement with experiment is obtained only if neoclassical resistivity is used. The result Δq0 0.2 is much larger than the value Δq0 0.01 obtained with classical resistivity. To understand this, a formula for q0(t) is derived. A functional dependence of the sawtooth period and a semi-empirical scaling law are presented. There is some evidence that the fast crash time is due to enhanced resistivity at the singular current sheet.

53 citations


Journal ArticleDOI
TL;DR: In this article, a plasma-enhanced CVD process was used to deposit thin amorphous carbon/boron films onto the first wall of TFTR and a series of plasma experiments was performed to test the effect of this first-wall modification on low-Z impurity (carbon, oxygen) behavior, suppression of metallic impurities, post-disruption recovery, hydrogenic recycling, and plasma density limits.

49 citations


Journal ArticleDOI
28 Sep 1990-Science
TL;DR: The measured rate of heat leakage from these plasmas is sufficiently small to be compatible with the requirements of a full-sized fusion power reactor, and improvements in other aspects of reactor performance are still needed.
Abstract: Fusion plasmas with reactorlike temperatures have been confined in magnetic-field configurations of the tokamak type. The measured rate of heat leakage from these plasmas is sufficiently small to be compatible with the requirements of a full-sized fusion power reactor. Improvements in other aspects of reactor performance are still needed, however, and the high cost of reactor- development steps has become an obstacle on the path to practical fusion power.

48 citations


Journal ArticleDOI
TL;DR: In this paper, the authors studied the magnetic reconnection in TFTR tokamak plasmas and found that the shape of the q profile is a good indicator of the presence of sawteeth.
Abstract: Magnetohydrodynamic (MHD) activity within three zones (core, half‐radius, and edge) of TFTR [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. 1, p. 51] tokamak plasmas are discussed. Near the core of the plasma column, sawteeth are often observed. Two types of sawteeth are studied in detail; one with complete, and the other with incomplete, magnetic reconnection. Their characteristics are determined by the shape of the q profile. Near the half‐radius the m/n=3/2 and 2/1 resistive ballooning modes are found to correlate with a beta collapse. The pressure and the pressure gradient at the mode rational surface are found to play an important role in stability. MHD activity is also studied at the plasma edge during limiter H modes. The edge localized modes (ELM’s) are found to have a precursor mode with a frequency between 50–200 kHz and a mode number m/n=1/0. The mode does not show a ballooning structure. While these instabilities have been studied on many other machines, ...

34 citations


Journal ArticleDOI
TL;DR: In this paper, the edge localized mode (ELM) activity is observed at all poloidal locations, with the largest amplitudes at coils on the inner major radius side, indicating that the mode is not outward ballooning in character.
Abstract: H-mode plasmas with high poloidal beta values in the PBX-M tokamak often exhibit periods of edge localized mode (ELM) activity, with each ELM preceded by a short duration (≤ 350 μs) burst of high frequency (200-250 kHz) magnetic activity. The burst grows on a time-scale of 10 μs and disappears just before the rapid increase in the Dα emission that is characteristic of the ELM. The burst of activity is observed at all poloidal locations, with the largest amplitudes at coils on the inner major radius side, indicating that the mode is not outward ballooning in character. Stability calculations indicate that this high frequency ELM precursor is probably the pressure driven ideal kink mode.

Journal ArticleDOI
TL;DR: In this article, the authors examined the control of plasma position, shape, and current in a tokamak fusion reactor using linear optimal control, which is a feasible technique for controlling advanced tokak fusion this article.
Abstract: This paper examines the control of plasma position, shape, and current in a tokamak fusion reactor using linear optimal control These advanced tokamaks are characterized by non-up-down symmetric coils and structure, thick structure surrounding the plasma, eddy currents, shaped plasmas, superconducting coils, vertically unstable plasmas, and hybrid function coils providing ohmic heating, vertical field, radial field, and shaping field Modes of the electromagnetic environment in a tokamak are derived and used to construct control gains that are tested in nonlinear simulations with initial perturbations The issues of applying linear optimal control to advanced tokamaks are addressed, including complex equilibrium control, choice of cost functional weights, the coil voltage limit, discrete control, and order reduction Results indicate that linear optimal control is a feasible technique for controlling advanced tokamaks where the more common classical control, relying on the scalar/orthogonalized description, will be severely strained

Journal ArticleDOI
TL;DR: In this paper, the authors show that the magnitude of density fluctuations approximately follow the mixing length prediction in both beam-heated and ohmic discharges and that the most important part of the {ital k} spectrum falls below the critical value of 0.5 at a scale unresolved in these measurements.
Abstract: Initial results from extraordinary mode microwave scattering on TFTR show that the magnitude of density fluctuations approximately follows the mixing length prediction in both beam-heated and ohmic discharges and that the most important part of the {ital k} spectrum falls below {ital k}{sub {perpendicular}}{rho}{sub {ital s}}=0.5 at a scale unresolved in these measurements. These results are consistent with most scattering measurements done in the ordinary mode on smaller devices. We observe only electron diamagnetic spectral shifts in ohmic discharges while beam-heated discharges display a shift which depends on the sign of the toroidal flow velocity. The spatial resolution of the system inferred from ray tracing is consistent with the measured toroidal velocity profiles and measured spectral shapes. There are some limitations on our measurements caused by a relatively large ghost'' component in the low-frequency spectrum which cannot be identified with simple scattering from the crossing region of the incident and scattering antenna patterns. This component has some of the average features of scattering in the central region but does not have the frequency offset expected for drift waves and does not exhibit a Doppler shift when the plasma is rotating. We show that this ghost feature can be explained bymore » non-Gaussian wings on the transmitting and receiving antennas which have a response to the high fluctuation levels at low values of {ital k}.« less

Journal ArticleDOI
TL;DR: A perturbation theory in the relative field strength to calculate the ionization rate has the advantage over the conventional Rayleigh-Schroedinger theory in that it effectively takes into account the continuum without having to deal with it directly.
Abstract: We introduce an alternative approach to the study of multiphoton ionization. The approach is based on consideration of the relative frequency instead of the relative field strength of the applied laser field as a small parameter (or, equivalently, a large number of photons). The choice is based on the characteristics of present-day lasers. We apply our approach specifically to an electron bound by a {delta}-function potential. In order to show a simpler situation first, we start by applying our analysis to the time-independent tunneling problem from such a potential due to an external electrostatic field. For this problem, we derive from first principles the well-known ionization rate for tunneling. Next, we derive with the same general formalism an analytical formula, asymptotic in the number of photons, for the multiphoton ionization rate, under strong field conditions, of an electron confined by a {delta}-function potential. We also obtain the ionization rate from a numerical solution of the corresponding equations. The comparison with the analytical expression is very good, even for a small number of photons. Our approach puts the results of Perelomov, Popov, and Terentev (Zh. Eksp. Teor. Fiz. {bold 50}, 1393 (1966) (Sov. Phys. JETP 23, 924 (1966))) on a rigorousmore » basis as well as extending them to a wider range of parameters. To cover analytically the few-photon case, we also develop a perturbation theory in the relative field strength to calculate the ionization rate. This perturbative approach has the advantage over the conventional Rayleigh-Schroedinger theory in that it effectively takes into account the continuum without having to deal with it directly. The comparison with the numerical results is also very good.« less

Journal ArticleDOI
TL;DR: Theoretical models for radial transport of energy and particles in tokamaks due to drift waves, rippling modes and resistive ballooning modes have been combined in a predictive transport code.
Abstract: Theoretical models for radial transport of energy and particles in tokamaks due to drift waves, rippling modes and resistive ballooning modes have been combined in a predictive transport code. The resulting unified model has been used to simulate low confinement mode (L-mode) energy confinement scalings. The dependence of global energy confinement on electron density for the resulting model is also described.

Journal ArticleDOI
TL;DR: In this article, the authors reviewed the carbon bloom phenomenon as it occurs on both JET and TFTR and observed that the source of the carbon is at localized (both toroidally and poloidally) hot spots on either the divertor plates or limiters.

Journal ArticleDOI
TL;DR: In this article, the control and evolution of highly elongated tokamak plasmas with large growth rates are simulated with the axisymmetric, resistive MHD code TSC in the geometry of the TCV tokak.
Abstract: The control and evolution of highly elongated tokamak plasmas with large growth rates are simulated with the axisymmetric, resistive MHD code TSC in the geometry of the TCV tokamak. Simulations show that pressure perturbations such as sawteeth and externally programmed displacements create initial velocity perturbations in addition to equilibrium changes. It is demonstrated that these perturbations may be stabilized by low power, rapid response coils inside the passively stabilizing vacuum vessel, together with slower shaping coils outside the vessel. This method works because coils inside the vessel do not have a flux penetration delay. Vertical disruption induced voltages and forces on the rapid coils and the vessel are investigated to establish coil and vessel design requirements. A model is proposed and used to estimate the additional vertical force due to poloidal currents, which is comparable to the force from toroidal currents.

Journal ArticleDOI
TL;DR: Ohmically and neutral beam injection (NBI) heated detached plasmas studies are being carried out on TFTR in order to help determine the practicality of this concept as mentioned in this paper.

Journal ArticleDOI
TL;DR: In this article, a series of simulations with the one-dimensional BALDUR transport code was performed for the ITER tokamak and it was shown that ignition can be sustained for 200 s with Rhelium = 095.

Journal ArticleDOI
TL;DR: In this article, a helical MHD perturbation has been found to accompany edge localized modes (ELMs) in the ASDEX tokamak, and it propagates in the direction of the electron diamagnetic drift.
Abstract: Magnetohydrodynamic (MHD) activity during edge localized modes (ELMs) has been monitored in the ASDEX tokamak. Besides a fast inward shift of the plasma column, a helical MHD perturbation has been found to accompany ELMs. For the discharges investigated, the helical instability has a poloidal mode number m of 3 or 4 and a toroidal mode number n of 1, and it propagates in the direction of the electron diamagnetic drift. It seems to be identical with the Toi mode, which has been observed to disappear during L-H transitions.

Journal ArticleDOI
TL;DR: The neutrals code DEGAS and the transport code SNAP were used to model recycling during steady state phases of Ohmic and neutral beam-heated discharges in TFTR as mentioned in this paper.

Journal ArticleDOI
TL;DR: In this article, the central impurity levels in DIII-D high current H-modes, as modelled using this technique, remain below those seen in L-mode (fractional nickel concentrations ≤ 0.02%) throughout the neutral beam heating pulse.
Abstract: Impurity concentration profiles have been determined for H-mode discharges in the DIII-D tokamak from measured ne, Te, Zeff and radiated emissivity profiles. The central impurity levels in DIII-D high current H-modes, as modelled using this technique, remain below those seen in L-modes (fractional nickel concentrations ≤0.02%) throughout the neutral beam heating pulse. In contrast to some other experiments (ASDEX [1], JET [2], JFT-2M [3]), the H-mode does not terminate because of excessive radiation in DIII-D discharges heated with co-injected neutral beams. For increasing plasma current, the global impurity concentrations decrease and the profiles become more dominated by edge radiation. H-modes as obtained with electron cyclotron heating and co-injected neutral beams at similar heating powers also have low impurity levels, but the impurity distribution is significantly more hollow in the case of neutral beam heating.

Journal ArticleDOI
TL;DR: In this paper, an assessment is made of the various linear and nonlinear mechanisms that are likely to play a role in the near field of Faraday shielded inductive antennas commonly used in ICRF heating experiments.

Journal ArticleDOI
TL;DR: In this article, a new detector has been designed and installed on TFTR for studying the confinement of MeV ions, especially for measuring the losses due to the toroidal field ripple, located just below the outer midplane where the peak in ripple induced losses is expected.
Abstract: A new detector has been designed and installed on TFTR for studying the confinement of MeV ions, especially for measuring the losses due to the toroidal field ripple. It is located just below the outer midplane where the peak in ripple‐induced losses is expected. The detector consists of a scintillator [ZnS(Ag)] and collimating apertures mounted on a radially movable probe. New design features of this detector are presented along with some of the first results.

Journal ArticleDOI
TL;DR: In this article, it is argued that fluid transport modeling by itself cannot lead to a complete understanding of transport, there must be a very strong collaboration among theory, experiment, and modeling on both the fluid and kinetic levels.
Abstract: The scope and detail of physics contained in computational models for fluid (density, momentum, energy) transport in toroidal plasmas have steadily increased during the past two decades. There has been considerable success in the development and verification of models for sources and sinks of particles, energy, momentum, and magnetic flux. Transport codes have collectively become very useful tools in interpreting experimental data and in providing guidance for new experiments. However, a more thorough understanding of the fundamental transport processes of magnetically confined plasmas and development of improved computational models are needed to enhance the predictive capabilities of transport codes. It is argued that fluid transport modeling by itself cannot lead to a complete understanding of transport—there must be a very strong collaboration among theory, experiment, and modeling on both the fluid and kinetic levels.

Journal ArticleDOI
TL;DR: In this paper, the carbon concentration in TFTR discharges with high power neutral beam heating has been measured by charge exchange recombination spectroscopy (CXRS) of the C5+ n = 3-4 transition in the extreme ultraviolet region.
Abstract: The carbon concentration in the central region of TFTR discharges with high power neutral beam heating has been measured by charge exchange recombination spectroscopy (CXRS) of the C5+ n = 3–4 transition in the extreme ultraviolet region. The carbon concentrations were deduced from absolute measurements of the line brightness using a calculation of the beam attenuation and the appropriate cascade corrected line excitation rates. As a result of the high ion temperatures (20–30 keV) in most of the discharges, the contribution of beam halo neutrals to the line brightness was significant and therefore had to be included in the modelling of the data. Carbon concentrations have been measured in discharges with plasma currents Ip in the range 1.0-1.6 MA and beam power in the range 2.6–30 MW, including a number of supershots. The results are in good agreement with carbon concentrations deduced from the visible bremsstrahlung Zeff and with metallic impurity concentrations measured by X-ray pulse height analysis, demonstrating the reliability of the atomic rates used in the beam attenuation and line excitation calculations. Carbon is the dominant impurity species in these discharges; the oxygen concentration measured via CXRS in a high beam power case was 0.0006 of ne, compared to 0.04 for carbon. Trends with plasma current and beam power in the carbon concentration and the inferred deuteron concentration are presented. The carbon concentration is independent of plasma current and decreases from 0.13 at 2.6 MW beam power to 0.04 at 30 MW, while the deuteron concentration increases from 0.25 to 0.75 over the same range of beam power. These changes are primarily the result of beam particle fuelling, as the carbon density did not vary significantly with beam power. The time evolutions of the carbon and deuteron concentrations during two high power beam pulses, one which exhibited a carbon bloom (a sudden influx of carbon due to local heating of the limiter) and one which did not, are compared. In both types of discharge, the carbon concentration decreases early in the beam pulse as a result of beam particle fuelling, and the carbon density rises slowly during the beam pulse until the start of the bloom. The electron density rise during the bloom is primarily due to the increase in the carbon density.

Journal ArticleDOI
TL;DR: In this article, the balance between the deuterium in the limiter surface with that in the plasma, and the balance of input power with the radiated power and power deposited on the Limiter was analyzed.

Journal ArticleDOI
TL;DR: In this paper, the authors describe a system for the production of high flux neutral beams of low energy (5-10 eV) atoms, for use in beam-surface interaction and surface modification studies.
Abstract: We describe a new system for the production of high flux neutral beams of low energy (5-10 eV) atoms, for use in beam-surface interaction and surface modification studies. This source can recreate the flux of superthermal atomic oxygen and other neutral species to which spacecraft surfaces are exposed in low Earth orbit. The system is based upon a magnetically confined (3-4 kG) co-axial plasma source driven by 1 kW RF at 2.45 GHz. The beam is. produced by the acceleration of plasma ions onto a negatively biased plate of high-Z metal (Mo or Pt); the ions are neutralized and reflected by the surface, retaining a large fraction of their incident kinetic energy, forming a beam of superthermal atoms. The source can provide neutral flux > 5 × 10 16 /cm2 s and fluence > 1020 cm2 in a five hour exposure. Such a high fluence source can provide accelerated simulation testing of materials and coatings for space applications. To date, samples of carbon film, carbon-based paint (Z-302), Kapton, mylar, teflon,...

Journal ArticleDOI
TL;DR: In this paper, boundary plasma measurements on TFTR have been made during a NBI power scan in the range Ptot = 1-20 MW in the L-mode regime, and the results obtained during NBI heating in TFTR are very similar to recent JET observations during ICRF heating and suggest that the form of heating plays little role in the behaviour of discharge parameters.

Journal ArticleDOI
TL;DR: In this paper, a 2D Monte-Carlo simulation of the neutral atom densities in the divertor, divertor throat and pump duct of ITER was made using the DEGAS code.