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  • 11
    Electronic Resource
    Electronic Resource
    Stability of negative central magnetic shear discharges in the DIII-D tokamak : The 38th annual meeting of the Division of Plasma Physics (DPP) of the American Physical Society (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 1783-1791 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Discharges with negative central magnetic shear (NCS) hold the promise of enhanced fusion performance in advanced tokamaks. However, stability to long wavelength magnetohydrodynamic modes is needed to take advantage of the improved confinement found in NCS discharges. The stability limits seen in DIII-D [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] experiments depend on the pressure and current density profiles and are in good agreement with stability calculations. Discharges with a strongly peaked pressure profile reach a disruptive limit at low beta, βN=β(I/aB)−1≤2.5 (% m T/MA), caused by an n=1 ideal internal kink mode or a global resistive instability close to the ideal stability limit. Discharges with a broad pressure profile reach a soft beta limit at significantly higher beta, βN=4 to 5, usually caused by instabilities with n〉1 and usually driven near the edge of the plasma. With broad pressure profiles, the experimental stability limit is independent of the magnitude of negative shear but improves with the internal inductance, corresponding to lower current density near the edge of the plasma. Understanding of the stability limits in NCS discharges has led to record DIII-D fusion performance in discharges with a broad pressure profile and low edge current density. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872366
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  • 12
    Electronic Resource
    Electronic Resource
    Nonstationary signal analysis of magnetic islands in plasmas (1999)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 4545-4551 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Rotating magnetic islands produce fluctuations on a variety of diagnostics in magnetic fusion energy plasmas. The analysis of these fluctuations requires the calculation of the amplitude, phase, and frequency of the oscillations. These three spectral quantities generally evolve in time, necessitating nonstationary signal analysis techniques. The Hilbert transform offers an efficient and accurate method of calculating these three quantities from one diagnostic signal. This feature allows the Hilbert transform to determine the success of the active rotation control of magnetic islands, and to calculate the profile of the diagnostic measurements in a frame of reference co-rotating with the magnetic island. Comparisons to quadrature and spectrogram techniques demonstrate the accuracy of the Hilbert transform method. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1150110
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  • 13
    Electronic Resource
    Electronic Resource
    Tomographic analysis of the evolution of plasma cross sections (1986)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 57 (1986), S. 1557-1566 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A diagnostic has been developed for measurements of the evolution of plasma density distributions in two dimensions using the principles of computed emisson tomography. The system measures the near-infrared emission along several chords through the plasma cross section and the emission profile is then reconstructed using a maximum entropy based algorithm. This paper will present details of the reconstruction algorithm, the data collection system, the verification of the diagnostic using both physical and numerical phantom sources, evidence for the correlation between n2 and the infrared emission intensity, and initial measurements on a tokamak plasma.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1138584
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  • 14
    Electronic Resource
    Electronic Resource
    High poloidal beta equilibria in the Tokamak Fusion Test Reactor limited by a natural inboard poloidal field null (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 2277-2284 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent operation of the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Nucl. Fusion Research 1, 51 (1986)] has produced plasma equilibria with values of Λ≡βp eq+li/2 as large as 7, εβp dia≡2μ0ε〈p⊥〉/〈〈Bp〉〉2 as large as 1.6, and Troyon normalized diamagnetic beta [Plasma Phys. Controlled Fusion 26, 209 (1984); Phys. Lett. 110A, 29 (1985)], βNdia≡108〈βt⊥〉aB0/Ip as large as 4.7. When εβp dia(approximately-greater-than)1.25, a separatrix entered the vacuum chamber, producing a naturally diverted discharge that was sustained for many energy confinement times, τE. The largest values of εβp and plasma stored energy were obtained when the plasma current was ramped down prior to neutral beam injection. The measured peak ion and electron temperatures were as large as 24 and 8.5 keV, respectively. Plasma stored energy in excess of 2.5 MJ and τE greater than 130 msec were obtained. Confinement times of greater than 3 times that expected from L-mode predictions have been achieved. The fusion power gain QDD reached a value of 1.3×10−3 in a discharge with Ip=1 MA and εβp dia=0.85. A large, sustained negative loop voltage during the steady-state portion of the discharge indicates that a substantial noninductive component of Ip exists in these plasmas. Transport code analysis indicates that the bootstrap current constitutes up to 65% of Ip. Magnetohydrodynamic (MHD) ballooning stability analysis shows that, while these plasmas are near, or at the βp limit, the pressure gradient in the plasma core is in the first region of stability to high-n modes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859647
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  • 15
    Electronic Resource
    Electronic Resource
    Growth rate, saturation, and radial transport measurements for the trapped ion instability (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 293-304 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The dissipative trapped ion instability (DTII) is observed as an m=1 mode localized to the mirror cell of the Columbia Linear Machine (CLM) [Plasma Phys. 24, 185 (1982)]. A gated feedback diagnostic allows the direct measurement of the DTII linear growth rate. The DTII growth rate is measured as a function of trapped fraction, keeping all other parameters fixed, and clearly displays the scaling with trapped fraction predicted by the radially local linear dispersion relation, including ion Landau damping. The saturated DTII mode amplitude is measured concurrently with the growth rate. The saturated amplitude displays a nearly linear dependence on the measured growth rate, normalized to the DTII real frequency, and is nearly proportional to the square root of the measured growth rate. A striped particle collector is used to measure the ambipolar radial plasma flux as the trapped fraction is varied. Radial diffusion coefficients are calculated from the measured radial particle fluxes and display a scaling intermediate between the Kadomtsev weak and strong turbulence predictions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859195
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  • 16
    Electronic Resource
    Electronic Resource
    Study of a collisionless, curvature and rotationally driven, trapped particle instability (1988)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 1773-1786 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The Columbia Linear Machine [Plasma Phys. 24, 185 (1982)] has been redesigned to enter the parameter regime of the collisionless, curvature driven, trapped particle instability. The changes that were made reduced the neutral collisionality by over a factor of 2 and the Coulomb collisionality by an order of magnitude, and increased the curvature drive by nearly an order of magnitude. With the new machine configuration, a strong (ñ/N(approximately-greater-than)60%), coherent, m=1 fluctuation is observed which is identified as a collisionless, curvature and rotationally driven, trapped particle instability. The mode is localized strongly to the region of trapping. The radial structure of the mode is broad and the mode exhibits magnetohydrodynamiclike body motion of the plasma column. Furthermore, the mode displays the expected parametric dependences of increasing mode amplitude with increasing mirror ratio and rf power and decreasing mirror cell length. A theory whose predictions are in fair agreement with the experiment has been developed for the mode in the presence of a dc radial electric field. Preliminary experiments designed to test the concept of charge overshoot stabilization of trapped particle modes show results consistent with that idea. A measurement of the growth rate of the instability has been made and is in fair agreement with the theoretical prediction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866666
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  • 17
    Electronic Resource
    Electronic Resource
    High poloidal beta long-pulse experiments in the Tokamak Fusion Test Reactorf| (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2525-2531 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experiments have been performed in the Tokamak Fusion Test Reactor [D. M. Meade et al. in Plasma Physics Controlled Nuclear Fusion Research, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 9] with neutral beam injection of up to 4 sec. duration, which is comparable to the time scale for resistive redistribution of the plasma current profile. These plasmas were created using a rapid decrease of the plasma current which initially created a plasma with enhanced stability and confinement. As the current profile evolved, a significantly reduced beta limit was observed. The high εβp plasmas had up to 90% of the current driven noninductively which significantly broadened the current profile during the long pulse lengths. These experiments demonstrated that high βN plasmas could not be sustained for times longer than the resistive relaxation of the outer current region which at early times after the current ramp-down carried negative current. At later times in lower βN discharges, beta collapses were sometimes observed as the current profile broadened at βN∼1.5. The appearance of disruptions was consistent with the predictions of ideal magnetohydrodynamics (MHD) stability analyses.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860739
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  • 18
    Electronic Resource
    Electronic Resource
    Investigation of ballooning modes in high poloidal beta plasmas in the Tokamak Fusion Test Reactorf| (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2571-2577 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Medium-n (toroidal mode number) instabilities with ballooning characteristics were investigated using x-ray and electron-cyclotron emission (ECE) imaging techniques in the Tokamak Fusion Test Reactor (TFTR) [R. J. Hawryluk et al., Plasma Phys. Controlled Fusion 33, 1509 (1991)]. The poloidal mode numbers were determined by fitting the data to soft x-ray signals simulated with a wave-packet model, and the toroidal mode numbers were determined from the phase difference between the soft x-ray and ECE signals. The modes are identified as ideal ballooning modes from the observation that the identified mode numbers are much higher than that of the usual kink or tearing modes, the mode has a strong ballooning characteristic, the growth rate is consistent with the theoretical prediction, and the equilibrium is theoretically predicted to be marginal to the infinite n ballooning mode. The ballooning mode accompanies a β collapse which happens near the Troyon limit in TFTR. The ballooning mode has also been observed just before a major disruption in TFTR.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860692
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  • 19
    Electronic Resource
    Electronic Resource
    Initial high beta operation of the HBT-EP Tokamak (1993)
    Springer
    Journal of fusion energy 12 (1993), S. 303-310 
    Details
    ISSN: 1572-9591
    Keywords: High beta operation ; HBT-EP tokamak
    Source: Springer Online Journal Archives 1860-2000
    Topics: Energy, Environment Protection, Nuclear Power Engineering
    Notes: Abstract HBT-EP is a new research tokamak designed and built to investigate passive and active feedback techniques to control MHD instabilities. In particular, HBT-EP will be able to test techniques to control fast MHD instabilities occurring at high Troyon-normalized beta, βN ≡ βBa/Ip [Tm/MA], since it is equipped with a thick, close-fitting, and adjustable conducting shell. The major goals of the initial operation of HBT-EP have been the achievement of high beta operation (βN ∼ 3) using only ohmic heating and the observation of MHD instabilities. By using a unique fast startup technique, we have successfully achieved these goals. A variety of MHD phenomena were observed during the high beta operation of HBT-EP. At modest beta (βN ≤ 2), discharges have been maintained for more than 10 msec, and these discharges exhibit saturated resistive instabilities. When βN approaches 3, major disruptions occur preceded by oscillating, growing precursors. During start-up, one or more minor disruptions are usually observed. A 1-D transport code has been used to simulate the evolution of the current profile, and these early minor instabilities are predicted to be double tearing modes. The simulation also reproduces the observed high beta operation when saturated neo-Alcator energy confinement scaling is assumed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01079674
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