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  1. A non-modal analytical method to predict turbulent properties applied to the Hasegawa-Wakatani model

    Friedman, B. ; Carter, T. A. - Physics of Plasmas
    Linear eigenmode analysis often fails to describe turbulence in model systems that have non-normal linear operators and thus nonorthogonal eigenmodes, which can cause fluctuations to transiently grow faster than expected from eigenmode analysis. When combined with energetically conservative nonlinear mode mixing, transient growth can lead to sustained turbulence even in the absence of eigenmode instability. Since linear operators ultimately provide the turbulent fluctuations with energy, it is useful to define a growth rate that takes into account non-modal effects, allowing for prediction of energy injection, transport levels, and possibly even turbulent onset in the subcritical regime. Here, we define suchmore » a non-modal growth rate using a relatively simple model of the statistical effect that the nonlinearities have on cross-phases and amplitude ratios of the system state variables. In particular, we model the nonlinearities as delta-function-like, periodic forces that randomize the state variables once every eddy turnover time. Furthermore, we estimate the eddy turnover time to be the inverse of the least stable eigenmode frequency or growth rate, which allows for prediction without nonlinear numerical simulation. Also, we test this procedure on the 2D and 3D Hasegawa-Wakatani model [A. Hasegawa and M. Wakatani, Phys. Rev. Lett. 50, 682 (1983)] and find that the non-modal growth rate is a good predictor of energy injection rates, especially in the strongly non-normal, fully developed turbulence regime.« less

  2. Progress in the Long $${\rm Nb}_{3}{\rm Sn}$$ Quadrupole R&D by LARP

    Ambrosio, G. ; Andreev, N. ; Anerella, M. ; ... - IEEE Transactions on Applied Superconductivity
    After the successful test of the first long Nb3Sn quadrupole (LQS01) the US LHC Accelerator Research Program (LARP, a collaboration of BNL, FNAL, LBNL and SLAC) is assessing training memory, reproducibility, and other accelerator quality features of long Nb3Sn quadrupole magnets. LQS01b (a reassembly of LQS01 with more uniform and higher pre-stress) was subjected to a full thermal cycle and reached the previous plateau of 222 T/m at 4.5 K in two quenches. A new set of four coils, made of the same type of conductor used in LQS01 (RRP 54/61 by Oxford Superconducting Technology), was assembled in the LQS01more » structure and tested at 4.5 K and lower temperatures. The new magnet (LQS02) reached the target gradient (200 T/m) only at 2.6 K and lower temperatures, at intermediate ramp rates. The preliminary test analysis, here reported, showed a higher instability in the limiting coil than in the other coils of LQS01 and LQS02.« less

  3. Transverse instability of a bunched beam with space charge and wakefield

    Balbekov, V. - Physical Review Special Topics - Accelerators and Beams

  4. On cosmic acceleration without dark energy

    Kolb, Edward W. ; Matarrese, Sabino ; Riotto, Antonio - New Journal of Physics
    We elaborate on the proposal that the observed acceleration of the Universe is the result of the backreaction of cosmological perturbations, rather than the effect of a negative-pressure dark-energy fluid or a modification of general relativity. Through the effective Friedmann equations describing an inhomogeneous Universe after smoothing, we demonstrate that acceleration in our local Hubble patch is possible even if fluid elements do not individually undergo accelerated expansion. This invalidates the no-go theorem that there can be no acceleration in our local Hubble patch if the Universe only contains irrotational dust. We then study perturbatively the time behavior of general-relativisticmore » cosmological perturbations, applying, where possible, the renormalization group to regularize the dynamics. We show that an instability occurs in the perturbative expansion involving sub-Hubble modes. Whether this is an indication that acceleration in our Hubble patch originates from the backreaction of cosmological perturbations on observable scales requires a fully non-perturbative approach.« less

  5. Finite orbit energetic particle linear response to toroidal Alfven eigenmodes

    Berk, H. L. ; Breizman, B. N. ; Ye, Huanchun - Physics Letters. A
    The linear response of energetic particles to the TAE modes is calculated taking into account their finite orbit excursion from the flux surfaces. The general expression reproduces the previously derived theory for small banana width: when the banana width $$\Delta_b$$ is much larger than the mode thickness $$\Delta_m$$, we obtain a new compact expression for the linear power transfer. When $$\Delta_m/\Delta_b$$ $$\gg 1$$, the banana orbit effect reduces the power transfer by a factor of $$\Delta_m/\Delta_b$$ from that predicted by the narrow orbit theory. A comparison is made of the contribution to the TAE growth rate of energetic particles withmore » a slowing-down distribution arising from an isotropic source, and a balance-injected beam source when the source speed is close to the Alfven speed. For the same stored energy density, the contribution from the principal resonances $$(|v_{||}| = v_A)$$ is substantially enhanced in the beam case compared to the isotropic case, while the contribution at the higher sidebands $$(|v_1|-v_A/(2\ell-1)$$ with $$\ell≥2)$$ is substantially reduced. 10 refs.« less

  6. Global confinement and discrete dynamo activity in the MST reversed-field pinch

    Hokin, S. ; Almagri, A. ; Assadi, S. ; ... - Physics of Fluids. B, Plasma Physics
    Results obtained on the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 131 (1991)] after installation of the design poloidal field winding are presented. Values of βθe0≡2μ0ne0Te0/B2θ(a)~12% are achieved in low-current (I=220 kA) operation; here, ne0 and Te0 are central electron density and temperature, and Bθ(a) is the poloidal magnetic field at the plasma edge. An observed decrease in βθe0 with increasing plasma current may be due to inadequate fueling, enhanced wall interaction, and the growth of a radial field error at the vertical cut in the shell at high current. Energy confinement time varies little with plasma current,more » lying in the range of 0.5–1.0 msec. Strong discrete dynamo activity is present, characterized by the coupling of m=1, n=5–7 modes leading to an m=0, n=0 crash (m and n are poloidal and toroidal mode numbers). The m=0 crash generates toroidal flux and produces a small (2.5%) increase in plasma current.« less

  7. Steady-State Magnetic Diffusion from Resistive Interchange Modes in a Plasma

    Manheimer, Wallace M. - Physical Review Letters
    Here, it is shown that a likely nonlinear state for resistive interchange modes is one in which velocity vortex structure is balanced in Ohmic dissipation. Macroscopically this is manifested as magnetic diffusion and anomalous energy transport. The results are discussed and compared with recent experiments.
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