Francois Anderegg - Academia.edu (original) (raw)

Papers by Francois Anderegg

APS Division of Plasma Physics Meeting Abstracts, 2020

Bulletin of the American Physical Society, 2015

Bulletin of the American Physical Society, 2016

(Italy)-We present quantitative measurements of parametric wave-wave coupling rates and decay ins... more (Italy)-We present quantitative measurements of parametric wave-wave coupling rates and decay instabilities in the range 10 meV <T <1 eV. These experiments are conducted on a laser cooled pure ion plasmas confined in a cylindrical Penning-Malmberg trap. The axisymmetric, standing plasma waves have near-acoustic dispersion ω(k z) ∝ k z − αk 3 z , discretized by the axial wave number k z = m z (π/L p). The parametric coupling rates are measured between m z = 2 waves with large amplitude δn 2 /n 0 , and small amplitude m z = 1 waves, which have a small frequency detuning ∆ω = 2ω 1 − ω 2. On cold plasmas, the parametric coupling rates Γ ∝ (δn 2 /n 0) are consistent with cold fluid, 3-wave instability theory, and the decay instability occurs when Γ >∆ω/2. In contrast, at higher temperatures, the m z = 2 wave is more unstable. The instability threshold is reduced from the cold fluid prediction as the plasma temperature is increased, which is in qualitative agreement with Vlasov simulations, but is not yet understood theoretically.

Physical Review Letters, 2019

We observe plasma heating due to collisional diffusion across a separatrix when a magnesium ion c... more We observe plasma heating due to collisional diffusion across a separatrix when a magnesium ion column in a Penning-Malmberg trap is cyclically pushed back and forth across a partial trapping barrier. The barrier is an externally applied axisymmetric "squeeze" potential, which creates a velocity separatrix between trapped and passing particles. Weak ion-ion collisions then cause separatrix crossings, leading to irreversible heating. The heating rate scales as the square root of the oscillation rate times the collision frequency, and thus can be dominant for low-collisionality plasmas. The particle velocity distribution function is measured with coherent Laser Induced Fluorescence, and shows passing and trapped particles having an out-of-phase response to the forced plasma oscillations.

We observe damping of Langmuir modes in Mg^+ ion plasmas with different-mass ion impurities, and ... more We observe damping of Langmuir modes in Mg^+ ion plasmas with different-mass ion impurities, and compare to nascent theory treating inter-species drag and bulk viscosity. The cylindrical ion plasmas have density n ˜10^7cm-3, length Lp˜10cm, and radius Rp˜0.5cm in a field of B = 3Tesla, with plasma temperatures controlled over the range 10-5 < T < 1eV. For T >=0.1eV, damping rates agree closely with Landau theory for the standing mθ= 0, kz= 1 Langmuir mode at frequency f ˜20kHz. The damping from 10-2eV < T < 0.1eV is not yet understood. For T <=10-2eV, damping rates 10 < γ< 10^3 increase with (controlled) impurity fraction, and increase with decreasing temperature as expected for collisional drag, as γT-3/2. For T < 10-3eV, a decrease in γ is observed; and theory must include effects of strong magnetization, ion-ion correlations, spatial isotope separation, and bulk viscosity. Additionally, the wave damping is generally dependent on initial amplitude at the...

For azimuthally symmetric plasma modes in a magnesium ion plasma, confined in a 3 Tesla Penning-M... more For azimuthally symmetric plasma modes in a magnesium ion plasma, confined in a 3 Tesla Penning-Malmberg trap with a density of n ˜10^7cm-3, we measure a damping rate of 2s-1< gamma< 10^4s-1 over a wide range in temperature (5 x10-6 eV< T < 5eV) and aspect ratio (0.25 < alpha< 25), with a wave amplitude of deltan / n ˜5%. Changing the aspect ratio, alpha= Lp/ 2rp, of the plasma column, alters the frequency of the mode from 16 KHz to 192 KHz. The oscillatory fluid displacement is small compared to the wavelength of the mode; in contrast, the fluid velocity, deltavf, can be large compared to v. The real part of the frequency satisfies a linear dispersion relation. In long thin plasmas (alpha> 10) these modes are Trivelpiece-Gould (TG) modes, and for smaller values of alpha they are Dubin spheroidal modes. However the damping appears to be non-linear; initially large waves have weaker exponential damping, which is not yet understood. Recent theory ootnotetextM.W. A...

Archimedes Technology Group has recently begun operating a full scale demonstration unit of its P... more Archimedes Technology Group has recently begun operating a full scale demonstration unit of its Plasma Mass Filter^1 technology. The Filter is a cylindrical device that employs an axial magnetic and a radial electric field to produce a low-pass mass filter for ions. Ions with mass above a tunable cutoff mass are expelled from the plasma. This technology could significantly reduce the volume of high level radioactive waste at the Hanford Site in Richland, Washington. This complex waste will be converted into a vapor utilizing an ICP torch and subsequently ionized by RF power. Initial separation experiments have utilized an argon/xenon plasma with densities of about 10^18 m-3 and magnetic fields up to 0.1T. Ring electrodes create the required electric field to expel the heavy xenon ions from the plasma, which is observed using optical emission spectroscopy. The scaling of the separation with the cut-off mass will be presented and compared to theory. ^1T. Ohkawa, Plasma Mass Filter, U....

Physical Review Letters, 2016

This paper presents the first experimental confirmation of a new theory predicting enhanced drag ... more This paper presents the first experimental confirmation of a new theory predicting enhanced drag due to long-range collisions in a magnetized plasma. The experiments measure damping of Langmuir waves in a multi-species pure ion plasma, which is dominated by interspecies collisional drag in certain regimes. The measured damping rates in these regimes exceed classical predictions of collisional drag damping by as much as an order of magnitude, but agree with the new theory.

Note: Proc. 17th EPS Conference on Controlled Fusion and Plasma Heating, Amsterdam, The Netherlan... more Note: Proc. 17th EPS Conference on Controlled Fusion and Plasma Heating, Amsterdam, The Netherlands, June 1990, 14B, Part IV, 1811 - 1814 (1990) Reference CRPP-CONF-1990-011 Record created on 2008-05-13, modified on 2017-05-12

Bulletin of the American Physical Society, 2013

Submitted for the DPP13 Meeting of The American Physical Society Measurements of Plasma Wave Deca... more Submitted for the DPP13 Meeting of The American Physical Society Measurements of Plasma Wave Decay to Longer Wavelengths 1 F. ANDEREGG, M. AFFOLTER, C.F. DRISCOLL, UCSD-We measure the decay of plasma waves to longer wavelengths, for both "standard" Langmuir waves with v phase ≫ v, and for the lower phase velocity "EAW" modes with v phase ∼ v. These are θ-symmetric standing modes on pure ion or pure electron plasma columns with discrete wavenumbers k z = m z (π/L p). A large amplitude m z = 2 Langmuir wave causes phase-locked exponential growth of the m z = 1 wave when they are near resonant, at growth rates Γ e ∝ δn 2 /n 0 consistent with cold fluid theory. For larger detuning ∆ω ≡ 2ω 1 − ω 2 , mode amplitude A 1 is observed to "bounce" at rate ∆ω, with amplitude excursions ∆A 1 ∝ δn 2 /n 0 also consistent with cold fluid theory; but A 1 often exhibits a slower overall growth, as yet unexplained by theory. In contrast, a large amplitude m z = 2 EAW mode generally causes either strong phaselocked m z = 1 growth or no growth at all, apparently because the EAW "frequency fungibility" enables ∆ω = 0, and EAW mode damping is strong until the velocity distribution F (v phase) is "flattened."

Bulletin of the American Physical Society, 2014

Submitted for the DPP14 Meeting of The American Physical Society Enhanced Collision Rates in Corr... more Submitted for the DPP14 Meeting of The American Physical Society Enhanced Collision Rates in Correlated Plasmas 1 C.F. DRISCOLL, F. ANDEREGG, D.H.E. DUBIN, T.M. O'NEIL, UCSD-Experiments on cryogenic pure ion plasmas corroborate the Salpeter collisional enhancement 2 factor g(Γ) when the correlation parameter Γ ≡ e 2 /aT is large. This factor enhances the perp-to-parallel collision rate in the magnetized plasmas described here, 3 and also enhances the nuclear reaction rates in dense stellar interiors. The enhancement is caused by plasma screening of the repulsive Coulomb potential, enabling closer collisions for a given particle energy. The Salpeter theory assumes thermal equilibrium screening, whereas various dynamical theories suggest other factors. Prior experiments 4 corroborate the predicted g ∼ exp(Γ) enhancement with enhancements as large as g ∼ 10 11. Current theory is considering the effects of parallel collisions in multi-species ion plasmas, and cyclotron modes and energy exchange. Current experiments with improved laser cooling and plasma stability will provide more accurate tests of equilibrium theory in the sensitive regime of Γ ≤ 1, and may also approach the (classical) pycnonuclear regime.

Bulletin of the American Physical Society, 2017

Diego-We present quantitative measurements of a decay instability to lower frequencies of near-ac... more Diego-We present quantitative measurements of a decay instability to lower frequencies of near-acoustic waves. These experiments are conducted on pure ion plasmas confined in a cylindrical Penning-Malmberg trap. The axisymmetric, standing plasma waves have near-acoustic dispersion, discretized by the axial wave number k z = m z (π/L p). The nonlinear coupling rates are measured between large amplitude m z = 2 (pump) waves and small amplitude m z = 1 (daughter) waves, which have a small frequency detuning ∆ω = 2ω 1 − ω 2. Classical 3-wave parametric coupling rates are proportional to pump wave amplitude as Γ ∝ (δn 2 /n 0), with oscillatory energy exchange for Γ < ∆ω/2 and decay instability for Γ > ∆ω/2. Experiments on cold plasmas agree quantitatively for oscillatory energy exchange, and agree within a factor-oftwo for decay instability rates. However, nascent theory suggest that this latter agreement is merely fortuitous, and that the instability mechanism is trapped particles. Experiments at higher temperatures show that trapped particles reduce the instability threshold below classical 3-wave theory predictions.

Advanced Textbooks in Physics, 2016

AIP Conference Proceedings, 1995

Cyclotron modes varying as exp(ilθ), with l=1, 2 and 3, have been observed in an unneutralized Mg... more Cyclotron modes varying as exp(ilθ), with l=1, 2 and 3, have been observed in an unneutralized Mg ion plasma. The l=1 mode is observed to be down‐shifted from the corresponding cyclotron frequency, while the l≥2 modes are found to be up‐shifted. Good agreement is found between the observed down‐shifts of the l=1 modes of Mg+ and Mg++ and the predictions of a multi‐species cold plasma theory. The down‐shifts depend on the composition and size of the plasma, and the relative abundance of each ion can thus be determined.

Note: Proc. of the Invited papers, 1987 Int. Conference on Plasma Physics, Kiev, USSR, April 1987... more Note: Proc. of the Invited papers, 1987 Int. Conference on Plasma Physics, Kiev, USSR, April 1987, Vol. 1, 441 - 461 (1987) Reference CRPP-CONF-1988-022 Record created on 2008-05-13, modified on 2017-05-12

Note: 31st Annual Meeting, APS, Div. of Plasma Physics, Reference CRPP-CONF-1989-020 Record creat... more Note: 31st Annual Meeting, APS, Div. of Plasma Physics, Reference CRPP-CONF-1989-020 Record created on 2008-05-13, modified on 2017-05-12