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  • 1
    Electronic Resource
    Electronic Resource
    A basic experiment on isotope scaling of transport (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 4690-4693 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: There is considerable divergence between theoretical predictions and tokamak experimental results for the isotopic mass effects on transport. This motivated a basic physics experiment in the Columbia Linear Machine [G. A. Navratil, J. Slough, and A. K. Sen, Phys. Plasmas 24, 184 (1982)]. Experiments were done using two different gases: hydrogen and deuterium. Measurements of particle transport driven by an E×B rotationally driven mode were made via the cross correlation of potential and density fluctuations. Unlike in tokamaks, the plasma parameter profiles and fluctuation spectra were maintained nearly invariant for both gases, thus enabling a meaningful comparison and estimation of scaling. The first series of data reveal smaller transport values for the deuterium plasma and a scaling that ranges as D⊥∝Ai−0.6 to D⊥∝Ai−0.8. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1398571
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  • 2
    Electronic Resource
    Electronic Resource
    Studies of transport scaling and reduction under feedback (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 2933-2939 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A feedback control system using an ion beam as a remote suppressor has been previously shown to be very effective in suppressing plasma instabilities in the Columbia Linear Machine [G. A. Navratil et al., Plasma Phys. 24, 184 (1982)]. The first experimental measurements for the effect of this feedback system on anomalous particle transport, as determined from the cross-correlation of density and potential fluctuations is presented. It is shown that feedback reduces transport due to a rotational E×B mode by up to a factor of 3 in this experiment. Also, it was found that particle transport scales linearly with fluctuation amplitude and feedback control does not alter this scaling. Last, the experimentally observed scaling of particle transport does not agree with any theoretical predictions. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872425
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  • 3
    Electronic Resource
    Electronic Resource
    Feedback control of multimode magnetohydrodynamic instabilities via neutral beams (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 2956-2962 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In most scenarios of confinement degradation due to MHD (magnetohydrodynamic) fluctuations in both tokamaks and reversed field pinches several MHD modes are involved. This is the motivation for the development of a multimode feedback scheme in the present paper, in contrast to the past work. The scheme is based on modal (state) feedback, where each mode is unscrambled out of the sensor signal, which is a superposition of all mode information and then individually acted upon by a unique gain and phase. Finally, all these individually processed mode signals are electronically summed and impressed on the accelerator grid of a neutral beam as a single control signal. It is shown that this process can lead to the stabilization of all unstable modes without destabilization of any stable modes, in contrast to previous feedback experiments. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873020
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  • 4
    Electronic Resource
    Electronic Resource
    Modification of tokamak edge turbulence using feedbackf| (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1606-1611 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Using active feedback, the turbulent fluctuation levels have been reduced by as much as a factor of 2 in the edge of the Texas Experimental Tokamak (TEXT) [K. W. Gentle, Nucl. Fusion Technol. 1, 479 (1981)]. A probe system was used to drive a suppressor wave in the TEXT limiter shadow. A decrease in the local turbulence-induced particle flux has been seen, but a global change in the particle transport at the present time has not been observed. By changing the phase shift and gain of the feedback network, the amplitude of the turbulence was increased by a factor of 10.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870661
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  • 5
    Electronic Resource
    Electronic Resource
    Remote feedback stabilization of tokamak instabilitiesf| (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1479-1488 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A novel remote suppressor consisting of an injected ion beam has been used for the stabilization of plasma instabilities. A collisionless curvature-driven trapped-particle instability, an E×B flute mode and an ion temperature gradient (ITG) instability have been successfully suppressed down to noise levels using this scheme. Furthermore, the first experimental demonstration of a multimode feedback stabilization with a single sensor–suppressor pair has been achieved. Two modes (an E×B flute and an ITG mode) were simultaneously stabilized with a simple state-feedback-type method where more "state'' information was generated from a single-sensor Langmuir probe by appropriate signal processing. The above experiments may be considered as paradigms for controlling several important tokamak instabilities. First, feedback suppression of edge fluctuations in a tokamak with a suitable form of insulated segmented poloidal limiter sections used as Langmuir-probe-like suppressors is proposed. Other feedback control schemes are proposed for the suppression of electrostatic core fluctuations via appropriately phased ion density input from a modulated neutral beam. Most importantly, a scheme to control major disruptions in tokamaks via feedback suppression of kink (and possibly) tearing modes is discussed. This may be accomplished by using a modulated neutral beam suppressor in a feedback loop, which will supply a momentum input of appropriate phase and amplitude. Simple theoretical models predict modest levels of beam energy, current, and power.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870698
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  • 6
    Electronic Resource
    Electronic Resource
    Experimental study of growth rate scaling and quasicoherent states in plasma turbulence (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 3577-3586 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experimental results of the scaling of linear growth rate with the electrostatic fluctuation levels are presented. A quadratic scaling was obtained for the ion temperature gradient-driven instability (ITG), characteristic of weak turbulence. For the rotationally driven E×B mode, a linear scaling was obtained, characteristic of strong turbulence. Moreover, the linear growth rate scaling for the E×B mode exhibited two distinct behaviors, corresponding to a broadening and a narrowing of the spectral width as a function of increasing fluctuation level. The spectral broadening is associated with the usual turbulent broadening, while the spectral narrowing is attributed to the presence of coherent structures via self-organization. Radial harmonics were detected and shown to contribute to the spread in the spectral width. These structures were also studied via a bispectral analysis. The evolution to the quasicoherent state was also found to result in the presence of a fewer number of radial harmonics in the saturated state. It revealed the presence of three-wave coupling involving the radial and azimuthal harmonics, suggesting a possible mechanism for the nonlinear saturation of the mode and anomalous transport.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870893
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  • 7
    Electronic Resource
    Electronic Resource
    Compact ion beam source for feedback control of plasma instabilities (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 4427-4431 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A compact ion beam source has been constructed for use in the Columbia Linear Machine for feedback control of plasma instabilities. This source has been used in a feedback configuration to stabilize the collisionless trapped particle instability [Phys. Rev. Lett. 67, 204 (1991)]. The source was based on an E×B hot cathode, magnetron-type design. It utilized the background magnetic field and a radial discharge voltage to obtain the azimuthal E×B drifts. The pressure inside the discharge chamber was maintained in the mTorr range via differential pumping. The source had a stable operating window in gas pressure of over 50% about the chosen operating parameters. Typical source plasma parameters were a plasma potential of 100 V, an electron temperature of 10 eV, and a plasma density of 109–1010 cm−3. The power dissipated was around 30 W. The ion beam energy was typically 100 eV with an energy spread of 20 eV and it could be modulated 100%. In addition, external control of the ion beam energy was provided via the anode bias.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143691
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  • 8
    Electronic Resource
    Electronic Resource
    A hybrid ion temperature gradient and Kelvin–Helmholtz instability (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 4772-4779 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is shown that Kelvin–Helmholtz drive due to shear in parallel flow can modestly enhance the ion temperature gradient (ITG) driven modes in the Columbia Linear Machine [R. Scarmozzino et al., Phys. Fluids 31, 1773 (1988)]. However, this modest enhancement does not substantially alter the large discrepancy between our experimental results and the theoretical estimates of stabilizing levels of transverse flow shear, necessary for stabilization of ITG modes. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1409345
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  • 9
    Electronic Resource
    Electronic Resource
    Control and diagnostic uses of feedback : The 41st annual meeting of the division of plasma physics of the american physical society (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 1759-1766 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent results on multimode feedback control of magnetohydrodynamic (MHD) modes and a variety of diagnostic uses of feedback are summarized. First, is the report on reduction and scaling of transport under feedback. By controlling the fluctuation amplitudes and consequently the transport via feedback, it is found that the scaling of the diffusion coefficient is linear with root-mean-square rms fluctuation level. The scaling appears not to agree with any generic theory. A variety of other diagnostic uses of feedback have been developed. The primary goal is an experimental methodology for the determination of dynamic models of plasma turbulence, both for better transport understanding and more credible feedback controller designs. A specific motivation is to search for a low-order dynamic model, suitable for the convenient study of both transport and feedback. First, the time series analysis method is used for the determination of chaotic attractor dimension of plasma fluctuations. For E×B rotational flute modes it is found to be close to three, indicating that a low-order dynamic model may be adequate for transport prediction and feedback controller design. Second, a new method for direct experimental determination of nonlinear dynamical models of plasma turbulence using feedback has been developed. Specifically, the process begins with a standard three-wave coupling model and introduces a variable feedback gain. The power spectrum, delayed power spectrum, and bispectrum of fluctuations are then experimentally obtained. By varying the feedback gain continuously, an arbitrary number of numerical equations for a fixed number of unknowns can be generated. Their numerical solution yields the linear dispersion, as well as nonlinear coupling coefficients. This method has been successfully applied for E×B rotationally driven flute modes. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873996
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  • 10
    Electronic Resource
    Electronic Resource
    Experimental determination of attractor dimension of E×B turbulence (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 4492-4498 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The time series analysis method for chaotic attractor dimension by Grassberger and Procaccia [P. Grassberger and I. Procaccia, Physica D 9, 189 (1983)] was applied to fluctuations due to an E×B rotational flute mode in the Columbia Linear Machine (CLM) [G. A. Navratil, J. Slough, and A. K. Sen, Plasma Phys. 24, 184 (1982)]. After frequency domain filtering, the results show that the fluctuations carry a dimension close to three. A dimension of three supports a possible three-wave mode coupling process revealed in bi-coherence studies. The Grassberger Procaccia method was also performed on cases where partial feedback suppression was applied to the instability. It is shown that feedback does not alter the attractor dimension of the instability, supporting previous findings that the linear feedback only affects the linear drive of the instability, and not its non-linear saturation mechanism. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1315306
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