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Integrated shell approach to vertical position control on PBX-M

Description: The PBX-M device produces highly shaped discharges that, because of the negative external magnetic field decay index required, are vertically unstable. Vertical positional stability in PBX-M has been achieved by directly controlling the n = 0 component of the eddy current in the passive shell instead of the commonly used function of magnetic flux signals. Because the active coil is controlled via currents in the passive shell we call this an ``integrated shell`` approach to vertical position control. We present results of these experiments and make comparisons between the two methods of control.
Date: March 1, 1995
Creator: Hatcher, R.E. & Okabayashi, M.
Partner: UNT Libraries Government Documents Department

Influence of radial electric field on Alfven-type instabilities

Description: The influence of the large scale radial electric field, E{sub r}{sup (0)} on the frequency of shear-Alfven-type instability is analyzed. A frozen-in-flux constraint and the moderate-{beta} ion gyrokinetic equation are used in the derivation. The analysis indicates that the frequency predicted by a theory with E{sub r}{sup (0)} effect should be Doppler-shifted by k {center_dot} V{sub E} for comparison to the experimentally observed frequency. A specific example of the practical relevance of the result is given regarding possible identification of the edge-localized-mode-associated magnetic activity recently observed in PBX-M tokamak experiment.
Date: March 1, 1994
Creator: Hahm, T.S. & Tang, W.M.
Partner: UNT Libraries Government Documents Department

Spreading of wave-driven currents in a tokamak

Description: Lower hybrid current drive (LHCD) in the tokamak Princeton Beta Experiment-Modification (PBX-M) is computed with a dynamic model in order to understand an actual discharge aimed at raising the central q above unity. Such configurations offer advantages for steady-state operation and plasma stability. For the particular parameters of this PBX-M experiment, the calculation found singular profiles of plasma current density J and safety factor q developing soon after LHCD begins. Smoothing the lower hybrid-driven current and power using a diffusion-Eke equation and a velocity-independent diffusivity for fast-electron current brought the model into reasonable agreement with the measurements if D{sub fast} {approx} 1.0 m{sup 2}/s. Such a value for D{sub fast} is in the range suggested by other work.
Date: January 1996
Creator: Ignat, D. W.; Kaita, R.; Jardin, S. C. & Okabayashi, M.
Partner: UNT Libraries Government Documents Department

Measurement of the hot electrical conductivity in the PBX-M tokamak

Description: A new method for the analysis of tokamak discharges in which the plasma current is driven by the combination of high-power rf waves and a dc electric field is presented. In such regimes, which are the most usual in rf current drive experiments, it is generally difficult to separate the different components of the plasma current, i.e., purely Ohmic, purely noninductive and cross terms. If the bilinear (in wave power and electric field) cross term is the dominant one, an explicit relation between the loop voltage drop and the injected power can be found. This relation involves two parameters, the purely rf current drive efficiency and the hot (power dependent) electrical conductivity. These can be simultaneously determined from a simple two-parameter fit, if the loop voltage drop is measured at several rf power levels. An application to lower hybrid current drive experiments in the PBX-M tokamak is presented. It is shown that the method also allows the independent evaluation of the average power absorption fraction and n{sup {parallel}} upshift.
Date: January 1, 1997
Creator: Giruzzi, G.; Barbato, E.; Cardinali, A. & Bernabei, S.
Partner: UNT Libraries Government Documents Department

Experimental and Theoretical Investigation of Synergy between Ion Bernstein and Lower Hybrid Waves in PBX-M

Description: The synergistic behavior of lower hybrid and ion Bernstein waves on the Princeton Beta Experiment-Modified tokamak [Phys. Fluids B 2, 1271 (1990)] is experimentally studied using a 2-D hard X-ray camera. The hard X-ray bremsstrahlung emission from suprathermal electrons, generated with lower hybrid current drive, is enhanced during ion Bernstein wave power injection. This enhancement is observed in limited regions of space suggesting the formation of localized current channels. The effects on plasma electrons during combined application of these two types of waves are theoretically investigated using a quasilinear model. The numerical code simultaneously solves the 3-D (R, Z, {Phi}) toroidal wave equation for the electric field (in the WKBJ approximation) and the Fokker-Planck equation for the distribution function in two dimensions (v{sub parallel}, v{sub perpendicular}) with an added quasilinear diffusion coefficient. The radial profile of the non-inductively generated current density, the transmitted power traces and the total power damping curve are calculated. The beneficial effects of a combined utilization of ion Bernstein and lower hybrid waves on the current drive are emphasized. The numerical results are compared with the experimental observations.
Date: February 1, 1998
Creator: Cardinali, A.; Post-Zwicker, A.; Paoletti, F.; Bernabei, S.; Goeler, S. Von & Tighe, W.
Partner: UNT Libraries Government Documents Department

Radially localized measurements of superthermal electrons using oblique electron cyclotron emission

Description: It is shown that radial localization of optically tin Electron Cyclotron Emission from superthermal electrons can be imposed by observation of emission upshifted from the thermal cyclotron resonance in the horizontal midplane of a tokamak. A new and unique diagnostic has been proposed and operated to make radially localized measurements of superthermal electrons during Lower Hybrid Current Drive on the PBX-M tokamak. The superthermal electron density profile as well as moments of the electron energy distribution as a function of radius are measured during Lower Hybrid Current Drive. The time evolution of these measurements after the Lower Hybrid power is turned off are given and the observed behavior reflects the collisional isotropization of the energy distribution and radial diffusion of the spatial profile.
Date: May 1, 1996
Creator: Preische, S.; Efthimion, P.C. & Kaye, S.M.
Partner: UNT Libraries Government Documents Department

Measurement of electron energy distribution from X-rays diagnostics - foil techniques used with the hard X-ray camera on PBX-M

Description: A half-screen foil technique is used with the Hard X-ray Camera on the PBX-M tokamak to determine the energy distribution of the suprathermal electrons generated during lower hybrid current drive. The ratio of perpendicular to parallel temperature of the suprathermal electrons is deduced from the anisotropy of the bremsstrahlung emission utilizing Abel inversion techniques. Results from lower hybrid current drive discharges are discussed.
Date: December 31, 1995
Creator: Goeler, S. von; Bell, R.; Bernabei, S.; Davis, W. & Ignat, D.
Partner: UNT Libraries Government Documents Department

A folded waveguide ICRF antenna for PBX-M and TFTR

Description: The folded waveguide (FWG) antenna is an advanced ICRF launcher under development at ORNL that offers many significant advantages over current-strap type antennas. These features are particularly beneficial for reactor-relevant applications such as ITER and TPX. Previous tests of a development folded waveguide with a low density plasma load have shown a factor of 5 increase in power capability over loop antennas into similar plasma conditions. The performance and reliability of a FWG with an actual tokamak plasma load must now be verified for further acceptance of this concept. A 58 MHz, 4 MW folded waveguide is being designed and built for the PBX-M and TFTR tokamaks at Princeton Plasma Physics Laboratory. This design has a square cross-section that can be installed as either a fast wave (FW) or ion-Bernstein wave (IBW) launcher by 90{degree} rotation. Two new features of the design are: a shorter quarter-wavelength resonator configuration and a rear-feed input power coupling loop. Loading calculations with a standard shorting plate indicate that a launched power level of 4 MW is possible on either machine. Mechanical and disruption force analysis indicates that bolted construction will withstand the disruption loads. An experimental program is planned to characterize the plasma loading, heating effectiveness, power capability, impurity generation and other factors for both FW and IBW cases. High power tests of the new configuration are being performed with a development FWG unit on RFTF at ORNL.
Date: September 1, 1995
Creator: Bigelow, T.S.; Carter, M.D. & Fogelman, C.H.
Partner: UNT Libraries Government Documents Department

Stabilization of the external kink and control of the resistive wall mode in tokamaks

Description: One promising approach to maintaining stability of high beta tokamak plasmas is the use of a conducting wall near the plasma to stabilize low-n ideal MHD instabilities. However, with a resistive wall, either plasma rotation or active feedback control is required to stabilize the more slowly growing resistive wall modes (RWMs). Experiments in the DIII-D, PBHX-M, and HBT-EP tokamaks have demonstrated that plasmas with a nearby conducting wall can remain stable to the n = 1 ideal external kink above the beta limit predicted with the wall at infinity, with durations in DIII-D up to 30 times {tau}{sub w}, the resistive wall time constant. More recently, detailed, reproducible observation of the n = 1 RWM has been possible in DIII-D plasmas above the no-wall beta limit. The DIII-D measurements confirm characteristics common to several RWM theories. The mode is destabilized as the plasma rotation at the q = 3 surface decreases below a critical frequency of 1 to 7 kHz. The measured mode growth times of 2 to 8 ms agree with measurements and numerical calculations of the dominant DIII-D vessel eigenmode time constants, {tau}{sub w}. From its onset, the RWM has little or no toroidal rotation and rapidly reduces the plasma rotation to zero. Both DIII-D and HBT-EP have adopted the smart shell concept as an initial approach to control of these slowly growing RWMs; external coils are controlled by a feedback loop designed to make the resistive wall appear perfectly conducting by maintaining a net zero radial field at the wall. Initial experiment results from DIII-D have yielded encouraging results.
Date: January 1999
Creator: Garofalo, A. M.; Turnbull, A. D. & Strait, E. J.
Partner: UNT Libraries Government Documents Department

Princeton Plasma Physics Laboratory annual report, October 1, 1993-- September 30, 1994

Description: The Tokamak Fusion Test Reactor (TFTR) project is well into the experimental phase of its deuterium-tritium (D-T) program, with the objective to derive the maximum amount of experimental data on the behavior of tokamak plasmas containing a significant population of energetic alpha particles. Since the initial D-T experiments in December 1993, the operational performance of the TFTR, as well as the required tritium-handling and machine maintenance procedures in an activated environment, have improved markedly, so that D-T operation has now become essentially routine, while fully conforming with all of the safety and environmental requirements. During the D-T phase, the machine and auxiliary-systems parameters have also been increased, most notably the toroidal field (to 5.6 T) and the neutral-beam power (to 40 MW). The radio-frequency power in the ion-cyclotron-range of frequencies (ICRF) has been increased to 11 MW.
Date: December 31, 1994
Partner: UNT Libraries Government Documents Department

Soft x-ray imaging system for measurement of noncircular tokamak plasmas

Description: A soft x-ray camera and image processing system has been constructed to provide measurements of the internal shape of high temperature tokamak plasmas. The camera consists of a metallic-foil-filtered pinhole aperture and a microchannel plate image intensifier/convertor which produces a visible image for detection by a CCD TV camera. A wide-angle tangential view of the toroidal plasma allows a single compact camera to view the entire plasma cross section. With Be filters 12 to 50 ..mu..m thick, the signal from the microchannel plate is produced mostly by nickel L-line emissions which orignate in the hot plasma core. The measured toroidal image is numerically inverted to produce a cross-sectional soft x-ray image of the plasma. Since the internal magnetic flux surfaces are usually isothermal and the nickel emissivity depends strongly on the local electron temperature, the x-ray emission contours reflect the shape of the magnetic surfaces in the plasma interior. Initial results from the PBX tokamak experiment show clear differences in internal plasma shapes for circular and bean-shaped discharges.
Date: August 1, 1986
Creator: Fonck, R.J.; Reusch, M.; Jaehnig, K.P.; Hulse, R. & Roney, P.
Partner: UNT Libraries Government Documents Department

Measurement of emitted power in the divertor region in PBX

Description: In strongly indented PBX plasmas, radiated power profiles are calculated by combining data obtained from two bolometer arrays in order to study poloidal asymmetries arising from plasma indentation and characterize emission from the divertor region. A compact, 15-channel bolometer array that views the plasma tangentially along the midplane complements a 19-channel array that scans the plasma vertically in a poloidal plane. Assuming that radiated power density is constant along a magnetic flux surface, the contributions to the irradiance viewed by the poloidal array from the region inside the separatrix can be calculated from the midplane measurements. The difference between this contribution and the measured poloidal distribution is assumed to originate in the expanded boundary divertor. In general, the total radiated power loss constitutes 40% of the total input power, and is independent of beam geometry. However, the radiation profiles in the main plasma and divertor region depend on operating conditions such as beam geometry and gas puffing rates. Radiation from the main plasma accounts for 20% of the input power and radiation from the divertor region accounts for 20%. Accumulation of impurities during neutral-beam-heated discharges can cause peak radiation levels to exceed 1 W/cm/sup 3/, leading to a thermal collapse of the plasma.
Date: September 1, 1986
Creator: Paul, S.F.; Fonck, R.J. & Schmidt, G.L.
Partner: UNT Libraries Government Documents Department

Dynamic modeling of transport and positional control of tokamaks

Description: We describe here a numerical model of a free boundary axisymmetric tokamak plasma and its associated control systems. The plasma is modeled with a hybrid method using two-dimensional velocity and flux functions with surface-averaged MHD equations describing the evolution of the adiabatic invariants. Equations are solved for the external circuits and for the effects of eddy currents in nearby conductors. The method is verified by application to several test problems and used to simulate the formation of a bean-shaped plasma in the PBX experiment.
Date: October 1, 1985
Creator: Jardin, S.C.; Pomphrey, N. & DeLucia, J.
Partner: UNT Libraries Government Documents Department

Operation of a tangential bolometer on the PBX tokamak

Description: A compact 15-channel bolometer array that views plasma emission tangentially across the midplane has been installed on the PBX tokamak to supplement a 19-channel poloidal array which views the plasma perpendicular to the toroidal direction. By comparing measurements from these arrays, poloidal asymmetries in the emission profile can be assessed. The detector array consists of 15 discrete 2-mm x 2-mm Thinistors, a mixed semiconductor material whose temperature coefficient of resistance is relatively high. The accumulated heat incident on a detector gives rise to a change in the resistance in each active element. Operated in tandem with an identical blind detector, the resistance in each pair is compared in a Wheatstone bridge circuit. The variation in voltage resulting from the change in resistance is amplified, stored on a CAMAC transient recorder during the plasma discharge, and transferred to a VAX data acquisition computer. The instantaneous power is obtained by digitally smoothing and differentiating the signals in time, with suitable compensation for the cooling of the detector over the course of a plasma discharge. The detectors are ''free standing,'' i.e., they are supported only by their electrical leads. Having no substrate in contact with the detector reduces the response time and increases the time it takes for the detector to dissipate its accumulated heat, reducing the compensation for cooling required in the data analysis. The detectors were absolutely calibrated with a tungsten-halogen filament lamp and were found to vary by +-3%. The irradiance profiles are inverted to reveal the radially resolved emitted power density from the plasma, which is typically in the 0.1 to 0.5 W/cm/sup 3/ range.
Date: April 1, 1987
Creator: Paul, S.F.; Fonck, R.J. & Schmidt, G.L.
Partner: UNT Libraries Government Documents Department

Simplified eight-shot pneumatic pellet injector for plasma fueling applications on the Princeton Beta Experiment (PBX) and on the Advanced Toroidal Facility (ATF)

Description: Plasma fueling via injection of solid hydrogenic pellets has expanded the operating range for tokamaks and stellarators to higher densities than attainable with gas puffing. Pellet injection has also resulted in improved plasma energy confinement in tokamak discharges for which the pellet or pellets penetrate deep into the plasma core. The eight-shot pneumatic pellet injector described herein has been developed for use on the Princeton Beta Experiment (PBX) and on the Advanced Toroidal Facility (ATF) for routine plasma fueling and for confinement optimization studies. The injector is based upon the so-called ''pipe-gun'' concept, which generates deuterium and hydrogen pellets by direct condensation in the gun barrel tubes, segments of which are cooled below the hydrogen triple point temperature by contact with a liquid helium cooled block. Control of the pellet length is achieved both by regulating the deuterium fill pressure and by establishing temperature gradients along the barrel tubes. This injector features eight independent gun barrel assemblies mounted around the perimeter of a single cold block, each coupled to an ORNL-designed fast propellant valve. Thus, the injector is capable of injecting arbitrarily programmable sequences of up to eight pellets of sizes ranging from 1 mm to 3 mm at speeds up to 1500 m/s. 10 refs., 8 figs.
Date: January 1, 1988
Creator: Schuresko, D.D.; Cole, M.J.; Fisher, P.W.; Qualls, A.L.; Bauer, M.L.; Wysor, R.B. et al.
Partner: UNT Libraries Government Documents Department

Some effects of MHD activity on impurity transport in the PBX tokamak

Description: The effects of MHD activity on intrinsic impurity transport are studied in ohmic discharges of the Princeton Beta Experiment (PBX) by measuring of the Z/sub eff/ profile from visible bremsstrahlung radiation and the spectral line intensities from ultraviolet spectroscopy. A diffusive/convective transport model, including an internal disruption model, is used to simulate the data. The Z/sub eff/ profile with no MHD activity is fitted with a strong inward convection, characterized by a peaking parameter c/sub v/ (= -a/sup 2/v/2rD) = 11 (3.5, +4.5). At the onset of MHD activity (a large m = 1 n = 1 oscillation followed by sawteeth), this strongly peaked profile is flattened and subsequently reaches a new quasi-equilibrium shape. This profile is characterized by reduced convection (c/sub v/ = 3.6 (-1.1, +1.6), D = 1.4 (-0.7, +5.6) x 10/sup 4/ cm/sup 2//s), in addition to the particle redistribution which accompanies the sawtooth internal disruptions. 10 figs.
Date: October 1, 1985
Creator: Ida, K.; Fonck, R.J.; Hulse, R.A. & LeBlanc, B.
Partner: UNT Libraries Government Documents Department

Neoclassical transport coefficients for tokamaks with bean-shaped flux surfaces

Description: Simple analytic representations of the neoclassical transport coefficients for indented flux surfaces are presented. It is shown that a transport coefficient for an indented flux surface can be expressed in terms of a linear combination of the previously known transport coefficients for two nonindented flux surfaces. Numerical calculations based on actual equilibria from the PBX-M tokamak indicate that, even for modestly indented flux surfaces, the ion neoclassical thermal transport can be over a factor of two smaller than in a circular plasma with the same midplane radius or with the equivalent areas. 6 refs., 5 figs., 1 tab.
Date: November 1, 1990
Creator: Chang, C.S. (New York Univ., NY (USA). Courant Inst. of Mathematical Sciences Korea Advanced Inst. of Science and Technology, Seoul (Republic of Korea)) & Kaye, S.M. (Princeton Univ., NJ (USA). Plasma Physics Lab.)
Partner: UNT Libraries Government Documents Department

PBX experimental results in 1984

Description: Experimental activities of the PBX project during its first seven months of operation are summarized.
Date: July 1, 1985
Creator: Takahashi, H.; Bol, K.; Buchenauer, D.; Couture, P.; Fishman, H.; Fonck, R. et al.
Partner: UNT Libraries Government Documents Department

Post-disruptive plasma loss in the Princeton Beta Experiment (PBX)

Description: The free-boundary, axisymmetric tokamak simulation code TSC is used to model the transport time scale evolution and positional stability of PBX. A disruptive thermal quench will cause the plasma column to move inward in major radius. It is shown that the plasma can then lose axisymmetric stability, causing it to displace exponentially off the midplane, terminating the discharge. We verify the accuracy of the code by modeling several controlled experiments shots in PBX.
Date: July 1, 1986
Creator: Jardin, S.C.; DeLucia, J.; Okabayashi, M.; Pomphrey, N.; Reusch, M.; Kaye, S. et al.
Partner: UNT Libraries Government Documents Department

PBX: the Princeton beta experiment

Description: A rearrangement of the divertor coils in PDX will enable a test in 1984 of the MHD stability properties of deeply indented bean-shaped plasmas. The goal is a beta of 10%. Indentation is expected to counter the deterioration of MHD stability against pressure driven modes that is occasioned by the larger aspect ratios typical of anticipated reactor oriented devices. Indeed, as shown by M. Chance et al., indentation may offer direct access to the second region of stability for ballooning modes, and numerical analyses with PEST show the internal kink to be stabilized completely with even relatively modest indentation. The internal kink is implicated in the loss of beam ions in PDX. In this report the theoretical basis for the forthcoming experiment, and the design considerations underlying the modification from PDX to PBX, are described in detail. Additional theoretical material, including an analysis of particle orbits in an indented tokamak plasma, is appended.
Date: September 1, 1983
Creator: Bol, K.; Chance, M. & Dewar, R.
Partner: UNT Libraries Government Documents Department

Observation of neoclassical-like impurity transport in the q less than or equal to 1 region of the PBX tokamak

Description: Charge-dependent convective impurity transport is observed in the central core of PBX H-mode discharges by measuring radial profiles for both low- and high-Z intrinsic impurities in the presence of strong sawtooth activity. Transport coefficients are derived by simulating the data with a diffusive/convective transport model which includes an internal disruption model. The time evolving Z/sub eff/ profile and central metallic densities are fitted during the quiescent phase between internal disruptions to yield a diffusion coefficient of D = 1 x 10/sup 3/ cm/sup 2//s for all species and an inward convective speed of v = 2.2 x 10/sup 2/ (r/a) cm/s for oxygen and carbon and v = 1.1 x 10/sup 3/ (r/a) cm/s for metals. These transport coefficients are in quantitative agreement with simple cylindrical neoclassical estimates for the region within q less than or equal to 1. 17 refs., 4 figs.
Date: February 1, 1986
Creator: Ida, K.; Fonck, R.J.; Sesnic, S.; Hulse, R.A. & LeBlanc, B.
Partner: UNT Libraries Government Documents Department

High beta plasmas in the PBX tokamak

Description: Bean-shaped configurations favorable for high ..beta.. discharges have been investigated in the Princeton Beta Experiment (PBX) tokamak. Strongly indented bean-shaped plasmas have been successfully formed, and beta values of over 5% have been obtained with 5 MW of injected neutral beam power. These high beta discharges still lie in the first stability regime for ballooning modes, and MHD stability analysis implicates the external kink as responsible for the present ..beta.. limit.
Date: April 1, 1986
Creator: Bol, K.; Buchenauer, D.; Chance, M.; Couture, P.; Fishman, H.; Fonck, R. et al.
Partner: UNT Libraries Government Documents Department

High time resolution ion temperature profile measurements on PBX

Description: Ion temperature profiles with a time resolution of 2 to 5 ms have been measured on PBX by charge-exchange-recombination spectroscopy (CXRS) and a neutral-particle charge-exchange analyzer (NPA). The sightlines of both diagnostics crossed the trajectory of a near-perpendicular heating beam, which enhanced the local neutral density (proportional to signal strength) and provided spatial resolution. The time resolution of these two independent techniques is sufficient to see sawtooth oscillations and other MHD activity. Effects of these phenomena on the toroidal rotation velocity profile, v/sub phi/(r), are clearly observed by CXRS. For example, a sharp drop in the central v/sub phi/ occurs at the sawtooth crash, followed by a linear rise during the quiescent phase. The NPA results are compared with those from CXRS.
Date: May 1, 1986
Creator: Gammel, G.; Kaita, R.; Fonck, R.; Jaehnig, K. & Powell, E.
Partner: UNT Libraries Government Documents Department