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Ion acceleration and direct ion heating in three-component magnetic reconnection

Description: Ion acceleration and direct ion heating in magnetic reconnection are experimentally observed during counterhelicity merging of two plasma toroids. Plasma ions are accelerated up to order of the Alfen speed through contraction of the reconnected field-lines with three-components. The large increase in ion thermal energy (from 10 eV up to 200 eV) is attributed to the direct conversion of the magnetic energy into the unmagnetized ion population. This observation is consistent with the magnetohydrodynamic and macro-particle simulations.
Date: March 1, 1996
Creator: Ono, Y.; Yamada, M. & Akao, T.
Partner: UNT Libraries Government Documents Department

Ion-beam-driven resonant ion cyclotron instability

Description: The resonant ion-beam-driven electrostatic ion cyclotron instability is identified. Measured dispersion relation and onset vs. beam energy and density agree with numerical calculations based on a theory which includes beam acoustic terms. After amplitude saturation, velocity space diffusion of the beam ions is observed. (auth)
Date: November 1, 1975
Creator: Hendel, H.W.; Yamada, M.; Seiler, S.W. & Ikezi, H.
Partner: UNT Libraries Government Documents Department

Characteristics of confinement and fusion reactivity in JT-60U high-{beta}{rho} and TFTR supershot regimes with deuterium neutral beam injection

Description: The high performance regimes achieved in JT-60U and TFTR have produced peak DD fusion neutron rates up to 5.6 {times} 10{sup 16}/s for similar heating beam powers, in spite of considerable differences in machine operation and plasma configuration. A common scaling for the DD fusion neutron rate (S{sub DD} {proportional_to} P{sub abs}{sup 2.0} H{sub ne} V{sub p}{sup {minus}0.9}) is obtained, where P{sub abs} and H{sub ne} are the absorbed beam power and beam fueling peaking factor, respectively, and V{sub p} is the plasma volume. The maximum stored energy obtained in each machine has been up to 5.4 MJ in TFTR and 8.7 MJ in JT-60U. Further improvements in the fusion neutron rate and the stored energy are limited by the {beta}-limit in Troyon range, {beta}{sub N} {approximately} 2.0--2.5. A common scaling for the stored energy (W{sub tot} {proportional_to} P{sub abs}V{sub p}H{sub ne}{sup 0.2}) is also proposed.
Date: March 1, 1995
Creator: Park, H.K.; Bell, M.G. & Yamada, M.
Partner: UNT Libraries Government Documents Department

Global stability study of the ultra low aspect ratio tokamak, ULART

Description: By introducing a slender current carrying conductor through the geometric center axis of the Tokyo University TS-3 device, the authors have generated ultra low aspect ratio tokamak (ULART) configurations with aspect ratio as low as 1.05. In this extreme limit they study the transition of the spheromak (q{sub edge} = 0, I{sub tf} = 0) to a ULART plasma (q{sub edge} = 2--20). The authors investigate the global MHD characteristics of ULART by comparing theoretical results with the experimental data obtained. A small current in the center conductor (compared with the plasma current) is found to significantly improve the overall MHD stability characteristics of the formed plasmas by effectively stabilizing the global tilt/shift mode. Theoretical calculations of the threshold toroidal field current required for stability and the growth rates of the tilt/shift modes agree well with the TS-3 data.
Date: February 1, 1996
Creator: Yamada, M.; Pomphrey, N.; Morita, A.; Ono, Y. & Katsurai, M.
Partner: UNT Libraries Government Documents Department

Nonlinear and Non-ideal Effects on FRC Stability

Description: New computational results are presented which advance the understanding of the stability properties of the Field-Reversed Configuration (FRC). We present results of hybrid and two-fluid (Hall-MHD) simulations of prolate FRCs in strongly kinetic and small-gyroradius, MHD-like regimes. The n = 1 tilt instability mechanism and stabilizing factors are investigated in detail including nonlinear and resonant particle effects, particle losses along the open field lines, and Hall stabilization. It is shown that the Hall effect determines the mode rotation and change in the linear mode structure in the kinetic regime; however, the reduction in the growth rate is mostly due to the finite Larmor radius effects. Resonant particle effects are important in the large gyroradius regime regardless of the separatrix shape, and even in cases when a large fraction of the particle orbits are stochastic. Particle loss along the open field lines has a destabilizing effect on the tilt mode and contributes to the ion spin up in toroidal direction. The nonlinear evolution of unstable modes in both kinetic and small-gyroradius FRCs is shown to be considerably slower than that in MHD simulations. Our simulation results demonstrate that a combination of kinetic and nonlinear effects is a key for understanding the experimentally observed FRC stability properties.
Date: October 21, 2002
Creator: Belova, E.V.; Davidson, R.C.; Ji, H. & Yamada, M.
Partner: UNT Libraries Government Documents Department

Experimental Investigation of the Neutral sheet Profile During Magnetic Reconnection

Description: During magnetic reconnection, a ''neutral sheet'' current is induced, heating the plasma. The resultant plasma thermal pressure forms a stationary equilibrium with the opposing magnetic fields. The reconnection layer profile holds significant clues about the physical mechanisms which control reconnection. On the Magnetic Reconnection Experiment [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)], a quasi steady-state and axisymmetric neutral sheet profile has been measured precisely using a magnetic probe array with spatial resolution equal to one quarter of the ion gyro-radius. It was found that the reconnecting field profile fits well with a Harris-type profile [E. G. Harris, Il Nuovo Cimento 23, 115 (1962)], B(x) approximately tanh(x/delta). This agreement is remarkable since the Harris theory does not take into account reconnection and associated electric fields and dissipation. An explanation for this agreement is presented. The sheet thickness delta is found to be approximately 0.4 times the ion skin depth, which agrees with a generalized Harris theory incorporating non-isothermal electron and ion temperatures and finite electric field. The detailed study of additional local features of the reconnection region is also presented.
Date: November 1, 1999
Creator: Trintchouk, F.; Ji, H.; Yamada, M.; Kulsrud, R.; Hsu, S. & Carter, T.
Partner: UNT Libraries Government Documents Department

Measurement of Lower-hybrid Drift Turbulence in a Reconnecting Current Sheet

Description: We present a detailed study of fluctuations in a laboratory current sheet undergoing magnetic reconnection. The measurements reveal the presence of lower-hybrid-frequency range fluctuations on the edge of current sheets produced in the Magnetic Reconnection Experiment (MRX). The measured fluctuation characteristics are consistent with theoretical predictions for the lower-hybrid drift instability (LHDI). Our observations suggest that the LHDI does not provide any significant turbulent resistivity in MRX current sheets.
Date: June 20, 2001
Creator: Carter, T.A.; Ji, H.; Trintchouk, F.; Yamada, M. & Kulsrud, R.M.
Partner: UNT Libraries Government Documents Department

Magnetic Reconnection with Sweet-Parker Characteristics in Two-dimensional Laboratory Plasmas

Description: Magnetic reconnection has been experimentally studied in a well-controlled, two-dimensional laboratory magnetohydrodynamic plasma. The observations are found to be both qualitatively and quantitatively consistent with a generalized Sweet-Parker model which incorporates compressibility, downstream pressure, and the effective resistivity. The latter is significantly enhanced over its classical values in the collisionless limit. This generalized Sweet-Parker model also applies to the case in which an unidirectional, sizable third magnetic component is present.
Date: January 1, 1999
Creator: Carter, T.; Hsu, S.; Ji, H.; Kulsrud, R.; Yamada, M. & al, et
Partner: UNT Libraries Government Documents Department

Measurement of the Transverse Spitzer Resistivity during Collisional Magnetic Reconnection

Description: Measurement of the transverse resistivity was carried out in a reconnecting current sheet where the mean free path for the Coulomb collision is smaller than the thickness of the sheet. In a collisional neutral sheet without a guide field, the transverse resistivity is directly related to the reconnection rate. A remarkable agreement is found between the measured resistivity and the classical value derived by L. Spitzer. In his calculation the transverse resistivity for the electrons is higher than the parallel resistivity by a factor of 1.96. The measured values have verified this theory to within 30% errors.
Date: September 18, 2000
Creator: Trintchouk, F.; Yamada, M.; Ji, H.; Kulsrud, R.M. & Carter, T.A.
Partner: UNT Libraries Government Documents Department

Numerical Study of Global Stability of Oblate Field-Reversed Configurations

Description: Global stability of the oblate (small elongation, E < 1) Field-Reversed Configuration (FRC) has been investigated numerically using both three-dimensional magnetohydrodynamic (MHD) and hybrid (fluid electrons and kinetic ions) simulations. For every non-zero value of the toroidal mode number n, there are three MHD modes that must be stabilized. For n = 1, these are the interchange, the tilt and the radial shift; while for n > 1 these are the interchange and two co-interchange modes with different polarization. It is shown that the n = 1 tilt mode becomes an external mode when E < 1, and it can be effectively stabilized by close-fitting conducting shells, even in the small Larmor radii (MHD) regime. The tilt mode stability improves with increasing oblateness, however at suffciently small elongations the radial shift mode becomes more unstable than the tilt mode. The interchange mode stability is strongly profile dependent, and all n * 1 interchange modes can be stabilized for a class of pressure profile with separatrix beta larger than 0.035. Our results show that all three n = 1 modes can be stabilized in the MHD regime, but the stabilization of the n > 1 co-interchange modes still remains an open question.
Date: October 27, 2000
Creator: Belova, E.V.; Jardin, S.C.; Ji, H.; Yamada, M. & Kulsrud, R.
Partner: UNT Libraries Government Documents Department

Experimental Verification of the Kruskal-Shafranov Stability Limit in Line-Tied Partial Toroidal Plasmas

Description: The stability properties of partial toroidal flux ropes are studied in detail in the laboratory, motivated by ubiquitous arched magnetic structures found on the solar surface. The flux ropes studied here are magnetized arc discharges formed between two electrodes in the Magnetic Reconnection Experiment (MRX) [Yamada et al., Phys. Plasmas, 4, 1936 (1997)]. The three dimensional evolution of these flux ropes is monitored by a fast visible light framing camera, while their magnetic structure is measured by a variety of internal magnetic probes. The flux ropes are consistently observed to undergo large-scale oscillations as a result of an external kink instability. Using detailed scans of the plasma current, the guide field strength, and the length of the flux rope, we show that the threshold for kink stability is governed by the Kruskal-Shafranov limit for a flux rope that is held fixed at both ends (i.e., qa = 1).
Date: July 19, 2011
Creator: Oz, E.; Myers, C. E.; Yamada, M.; Ji, H.; Kulsrud, R. M. & Xie, J.
Partner: UNT Libraries Government Documents Department

Experimental Study of Ion Heating and Acceleration During Magnetic Reconnection

Description: Ion heating and acceleration has been studied in the well-characterized reconnection layer of the Magnetic Reconnection Experiment [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)]. Ion temperature in the layer rises substantially during null-helicity reconnection in which reconnecting field lines are anti-parallel. The plasma out flow is sub-Alfvonic due to a downstream back pressure. An ion energy balance calculation based on the data and including classical viscous heating indicates that the ions are heated largely due to non-classical mechanisms. The Ti rise is much smaller during co-helicity reconnection in which field lines reconnect obliquely. This is consistent with a slower reconnection rate and a smaller resistivity enhancement over the Spitzer value. These observations indicate strongly that non-classical dissipation mechanisms can play an important role both in heating the ions and in facilitating the reconnection process.
Date: October 24, 2000
Creator: Hsu, S.C.; Carter, T.A.; Fiksel, G.; Ji, H.; Kulsrud, R.M. & Yamada, M.
Partner: UNT Libraries Government Documents Department

Global Stability of the Field Reversed Configuration

Description: New computational results are presented which provide a theoretical basis for the stability of the Field Reversed Configuration (FRC). The FRC is a compact toroid with negligible toroidal field in which the plasma is confined by a poloidal magnetic field associated with toroidal diamagnetic current. Although many MHD modes are predicted to be unstable, FRCs have been produced successfully by several formation techniques and show surprising macroscopic resilience. In order to understand this discrepancy, we have developed a new 3D nonlinear hybrid code (kinetic ions and fluid electrons), M3D-B, which is used to study the role of kinetic effects on the n = 1 tilt and higher n modes in the FRC. Our simulations show that there is a reduction in the tilt mode growth rate in the kinetic regime, but no absolute stabilization has been found for s bar less than or approximately equal to 1, where s bar is the approximate number of ion gyroradii between the field null and the separatrix. However, at low values of s bar, the instabilities saturate nonlinearly through a combination of a lengthening of the initial equilibrium and a modification of the ion distribution function. These saturated states persist for many Alfven times, maintaining field reversal.
Date: November 15, 2000
Creator: Belova, E.V.; Jardin, S.C.; Ji, H.; Kulsrud, R.M.; Park, W. & Yamada, M.
Partner: UNT Libraries Government Documents Department

Local Measurement of Non-Classical Ion Heating During Magnetic Reconnection

Description: Local ion temperature is measured directly in the well-characterized reconnection layer of a laboratory plasma. These measurements demonstrate definitively that ions are heated due to reconnection and that more than half of the reconnected field energy is converted to ion kinetic energy. Neither classical Ohmic dissipation nor thermalization of energetic flows is sufficient to account for the energy converted, suggesting the importance of non-classical dissipation mechanisms such as wave-particle interactions.
Date: November 1, 1999
Creator: Fiskel, G.; Ji, H.; Yamada, M.; Kulsrud, R.M.; Hsu, S.C. & Carter, T.A.
Partner: UNT Libraries Government Documents Department

Numerical Study of the Formation, Ion Spin-up and Nonlinear Stability Properties of Field-reversed Configurations

Description: Results of three-dimensional numerical simulations of field-reversed configurations (FRCs) are presented. Emphasis of this work is on the nonlinear evolution of magnetohydrodynamic (MHD) instabilities in kinetic FRCs and the new FRC formation method by the counter-helicity spheromak merging. Kinetic simulations show nonlinear saturation of the n = 1 tilt mode, where n is the toroidal mode number. The n = 2 and n = 3 rotational modes are observed to grow during the nonlinear phase of the tilt instability due to the ion spin-up in the toroidal direction. The ion toroidal spin-up is shown to be related to the resistive decay of the internal flux, and the resulting loss of particle confinement. Three-dimensional MHD simulations of counter-helicity spheromak merging and FRC formation show good agreement with results from the SSX-FRC experiment. Simulations show formation of an FRC in about 30 Alfven times for typical experimental parameters. The growth rate of the n = 1 tilt mode is shown to be significantly reduced compared to the MHD growth rate due to the large plasma viscosity and field-line-tying effects.
Date: November 12, 2004
Creator: Belova, E.V.; Davidson, R.C.; Ji, H.; Yamada, M.; Cothran, C.D.; Brown, M.R. et al.
Partner: UNT Libraries Government Documents Department

Quantitative Study Of Guide Field Effects on Hall Reconnection In A Laboratory Plasma

Description: The effect of guide field on magnetic reconnection is quantitatively studied by systematically varying an applied guide field in the Magnetic Reconnection Experiment (MRX). The quadrupole field, a signature of two-fluid reconnection at zero guide field, is significantly altered by a finite guide field. It is shown that the reconnection rate is significantly reduced with increasing guide field, and this dependence is explained by a combination of local and global physics: locally, the in-plane Hall currents are reduced, while globally guide field compression produces an increased pressure both within and downstream of the reconnection region. __________________________________________________
Date: April 17, 2012
Creator: Tharp, T. D.; Yamada, M.; Ji, H.; Lawrence, E.; Dorfman, S. & Myers, C.
Partner: UNT Libraries Government Documents Department

Numerical Study of Field-reversed Configurations: The Formation and Ion Spin-up

Description: Results of three-dimensional numerical simulations of field-reversed configurations (FRCs) are presented. Emphasis of this work is on the nonlinear evolution of magnetohydrodynamic (MHD) instabilities in kinetic FRCs, and the new FRC formation method by counter-helicity spheromak merging. Kinetic simulations show nonlinear saturation of the n = 1 tilt mode, where n is the toroidal mode number. The n = 2 and n = 3 rotational modes are observed to grow during the nonlinear phase of the tilt instability due to the ion spin-up in the toroidal direction. The ion toroidal spin-up is shown to be related to the resistive decay of the internal flux, and the resulting loss of particle confinement. Three-dimensional MHD simulations of counter-helicity spheromak merging and FRC formation show good qualitative agreement with results from the SSX-FRC experiment. The simulations show formation of an FRC in about 20-30 Alfven times for typical experimental parameters. The growth rate of the n = 1 tilt mode is shown to be significantly reduced compared to the MHD growth rate due to the large plasma viscosity and field-line-tying effects.
Date: June 6, 2005
Creator: Belova, E. V.; Davidson, R. C.; Ji, H.; Yamada, M.; Cothran, C. D.; Brown, M. R. et al.
Partner: UNT Libraries Government Documents Department