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  • 1
    Electronic Resource
    Electronic Resource
    Thresholds and the role of the radial electric field in confinement bifurcations in the H-1 heliac (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 2390-2398 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The threshold conditions for the confinement bifurcations in the H-1 heliac [S. M. Hamberger et al., Fusion Technol. 17, 123 (1990)] are studied experimentally. The thresholds include the magnetic field, the rf power, and the neutral gas filling pressure. It is shown that in any combination of these parameters it is the radial electric field Er that is driven to a critical value before the bifurcation. A mechanism of the electric field formation is suggested, which is based on the balance of the electron and ion losses. The ion loss, at low magnetic field and high ion temperature observed in H-1, is dominated by the direct orbit loss mechanism. This is shown by modeling of the exact ion orbits and by comparing a qualitative picture of the Er formation with the experimental data. Relative efficiency of the electron and ion heating in the inner regions of plasma define the conditions for either low-to-high (L–H) or high-to low (H–L) confinement bifurcations in H-1. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872914
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  • 2
    Electronic Resource
    Electronic Resource
    Ion temperature and plasma flows in improved confinement mode in the H-1 heliac (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 3629-3634 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Low and improved confinement modes in the H-1 heliac [M. G. Shats et al., Phys. Rev. Lett. 77, 4190 (1996)] are studied experimentally in rf-sustained (〈100 kW, 7 MHz) argon discharges at low magnetic fields (〈0.15 T). Surprisingly high ion temperature, measured using a retarding field energy analyzer, is found which increases across the transition to improved confinement mode from 40 to 80 eV, while the electron density increases by about 50%. Both toroidal and poloidal plasma flow velocities do not change across the transition. The increase in a radial electric field in high mode is balanced on average by a corresponding increase in the ion pressure gradient. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872258
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  • 3
    Electronic Resource
    Electronic Resource
    Drift-wave-like density fluctuations in the Advanced Toroidal Facility (ATF) torsatron (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 398-413 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Density fluctuations in low-collisionality, low-beta (β∼0.1%), currentless plasmas produced with electron cyclotron heating (ECH) in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] have been studied using a 2 mm microwave scattering diagnostic. Pulsed gas puffing is used to produce transient steepening of the density profile from its typically flat shape; this leads to growth in the density fluctuations when the temperature and density gradients both point in the same direction in the confinement region. The wave number spectra of the fluctuations that appear during this perturbation have a maximum at higher k⊥ρs (∼1) than is typically seen in tokamaks. The in–out asymmetry of the fluctuations along the major radius correlates with the distribution of confined trapped particles expected for the ATF magnetic field geometry. During the perturbation, the relative level of the density fluctuations in the confinement region (integrated over normalized minor radii ρ from 0.5 to 0.85) increases from ñ/n∼1% when the density profile is flat to ñ/n∼3% when the density profile is steepened. These observations are in qualitative agreement with theoretical expectations for helical dissipative trapped-electron modes (DTEMs), which are drift-wave instabilities associated with particle trapping in the helical stellarator field; they suggest that trapped-electron instabilities may play a role in constraining the shape of the density profile in ATF, but have little effect on global energy confinement. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870966
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  • 4
    Electronic Resource
    Electronic Resource
    Overview of probe diagnostics on the H-1 heliac : Proceedings of the 12th topical conference on high temperature plasma diagnostics (1999)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 476-479 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A complex of probe diagnostics allows extensive characterization of the plasma parameters in the H-1 heliac. The probes used on H-1 include quadruple probes, poloidal and radial "fork" probes, a Mach probe, a 24-channel probe array, and a retarding field energy analyzer. This article provides details of the probe design and a brief description of the experimental techniques involved. Different techniques are compared. Sample results in two different modes of confinement in H-1 are presented. The capabilities of the probe diagnostics used in different combinations are discussed. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1149483
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  • 5
    Electronic Resource
    Electronic Resource
    Magnetic field mapping system on the H-1 heliac (abstract)a) : Proceedings of the tenth topical conference on high temperature plasma diagnostics (1995)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 66 (1995), S. 467-467 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The H-1 heliac recently brought into operation is a medium-sized 3-field-period heliac with major radius R0=1 m, plasma mean minor radius 〈a〉≤0.2 m and a wide range of rotational transforms 0.6≤(large-closed-square)(0)≤2.0. Electron beam mapping of the vacuum magnetic field was performed using new type of a fluorescent target (movable fluorescent rod array having a transparency about 98%). Up to 150–200 of toroidal transits were observed at each electron gun position. The spatial resolution of the system was about 3 mm. Electron collector probes were used for monitoring the positions of the magnetic surfaces in different toroidal field periods. Visible paths of the electron beam due to the excitation of the background gas (p×10−4 Torr) were used for identification of the toroidal transit numbers. This newly developed method gives an accuracy in measurement of the rotational transform of about 1.5%. Experimental surfaces and measured (large-closed-square) profiles show very good agreement with the computer model results. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146385
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  • 6
    Electronic Resource
    Electronic Resource
    Microwave scattering diagnostic on the Advanced Toroidal Facility torsatron (abstract)a) : Proceedings of the tenth topical conference on high temperature plasma diagnostics (1995)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 66 (1995), S. 841-841 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A 2 mm scattering diagnostic used to study density fluctuations of the electron cyclotron resonance heated plasma in the Advanced Toroidal Facility is described. A four-channel flexible antenna design was chosen to monitor fluctuations within the wave number range of 3〈k⊥〈20 cm−1 and to locate the scattering volume anywhere in the stellarator minor cross section. Special attention was paid to the system calibration and to the normalization of the k spectra. Analysis of the diagnostic noise showed that the main source of spurious signal was a scattering from high-level edge turbulence driven by the second-harmonic radiation from the gyrotron used for plasma heating. This spurious signal was suppressed using waveguide filters and rearrangement of the microwave circuit elements. Evidence in support of direct scattering from a well-defined plasma volume was found. Good correlation of the scattered waves' intensity with the density scale length was observed. Use of a dual-homodyne circuit configuration made is possible to determine that the plasma density fluctuations were propagating in the electron diamagnetic direction. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146242
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  • 7
    Electronic Resource
    Electronic Resource
    Magnetic field mapping system on the H-1 heliaca) (1995)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 66 (1995), S. 1163-1166 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The H-1 heliac recently brought into operation is a medium-sized 3-field-period heliac with major radius R0=1 m, plasma mean minor radius 〈a〉≤0.2 m and a wide range of rotational transforms 0.6≤ι–(0)≤2.0. Electron beam mapping of the vacuum magnetic field was performed using new type of a fluorescent target (movable fluorescent rod array having a transparency about 98%). Up to 150–200 toroidal transits were observed at each electron gun position. The spatial resolution of the system was about 3 mm. Electron collector probes were used for monitoring the positions of the magnetic surfaces in different toroidal field periods. Visible paths of the electron beam due to the excitation of the background gas (p∼10−4 Torr) were used for identification of the toroidal transit numbers. This newly developed method gives an accuracy in measurement of the rotational transform of about 1.5%. Experimental surfaces and measured ι(Slashthrough accent mark) profiles show very good agreement with the computer model results. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1145998
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  • 8
    Electronic Resource
    Electronic Resource
    Microwave scattering diagnostic on the Advanced Toroidal Facility torsatrona) (1995)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 66 (1995), S. 1221-1224 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A 2 mm scattering diagnostic used to study density fluctuations of the electron cyclotron resonance heated plasma in the Advanced Toroidal Facility is described. A four-channel flexible antenna design was chosen to monitor fluctuations within the wave number range of 3〈k⊥〈20 cm−1 and to locate the scattering volume anywhere in the stellarator minor cross section. Special attention was paid to the system calibration and to the normalization of the k spectra. Analysis of the diagnostic noise showed that the main source of spurious signal was a scattering from high-level edge turbulence driven by the second-harmonic radiation from the gyrotron used for plasma heating. This spurious signal was suppressed using waveguide filters and rearrangement of the microwave circuit elements. Evidence in support of direct scattering from a well-defined plasma volume was found. Good correlation of the scattered waves' intensity with the density scale length was observed. Use of a dual-homodyne circuit configuration made is possible to determine that the plasma density fluctuations were propagating in the electron diamagnetic direction. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146010
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  • 9
    Electronic Resource
    Electronic Resource
    Fluctuation studies using combined Mach/triple probe : Papers from the thirteenth topical conference on high temperature plasma diagnostics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 449-452 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A probe consisting of two poloidally separated triple probes and a Mach probe (TMT probe) has been designed and installed on the H-1 heliac to study fluctuations. Mach probes are shown to be sensitive to the fluctuations in the electron density, electron and ion temperatures, and ion drift velocity. If the ion Larmor radius is much larger than the characteristic probe dimension, then the Mach probe is insensitive to the magnetic field. When the Mach probe is oriented such that the two tips are separated radially, it becomes sensitive to the radial velocity of the ions. A model has been devised to allow the above mentioned time-resolved plasma parameters to be reconstructed from the data obtained using the TMT probe. One of the important results of these studies is that ion temperature fluctuations cannot be neglected. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1318244
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  • 10
    Electronic Resource
    Electronic Resource
    Collective microwave scattering diagnostic on the H-1 heliac : Papers from the thirteenth topical conference on high temperature plasma diagnostics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 352-354 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A multichannel microwave scattering diagnostic has been developed and installed on the H-1 heliac. The purpose of the new diagnostic is to study small-scale plasma fluctuations in H-1, which are believed to be responsible for the loss of particles and energy from the plasma. The diagnostic is a 132 GHz, four-channel superheterodyne system. The transmitter and receiver antennas (consisting of horns and focusing bispherical mirrors) are located inside the vacuum vessel of H-1. A radial resolution of Δr/a∼0.2 is achieved. The scattering volume is positioned in the density gradient region at r/a∼0.6. At present, the system is aligned to measure fluctuations in the poloidal wave number range from approximately 10 to 25 cm−1. The use of the heterodyne detection system allows the fluctuation propagation direction to be determined. The low frequency bandwidth of the system is 1 MHz. The instrument sensitivity is about Ps/Pi∼10−6. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1315635
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