M. Affolter

Cyclotron mode frequencies and resonant absorption in multi-species ion plasmasa)

Cyclotron mode frequencies are studied on trapped rigid-rotor multi-species ion plasmas. Collective effects and radial electric fields shift the mode frequencies away from the “bare” cyclotron frequencies 2πFc(s)≡(qsB/Msc) for each species s. These frequency shifts are measured on the distinct cyclotron modes ( m=0,1, and 2) with cos(mθ) azimuthal dependence. We find that for radially uniform plasmas the frequency shifts corroborate a simple theory expression, in which collective effects enter only through the E × B rotation frequency fE and the species fraction δs. The m = 1 center-of-mass mode is in agreement with a simple “clump” model. Additionally, ultra-cold ion plasmas exhibit centrifugal separation by mass, and additional frequency shifts are observed, in agreement with a more general theory.

Measurements of correlations enhanced collision rates in the mildly correlated regime (Γ ∼ 1)

We measure the perpendicular-to-parallel collision rate ν⊥|| in laser cooled, magnetized ion plasmas in the mildly correlated regime of Γ∼ 1 and find collision rates enhanced by exp(Γ). This ν⊥|| enhancement due to correlations is directly analogous to the enhancement of fusion collisions in hot dense stellar plasmas, as first analyzed by Salpeter [Aust. J. Phys. 7, 373 (1954)]. The enhancement is caused by screening of the repulsive Coulomb potential between charges, allowing closer collisions for a given relative energy. The measurements indicate that the screening is done by thermal particles and allows us to rule out dynamical screening theories, which predict no enhancement to the collisions rate for Γ≲ 1.

Bounce harmonic Landau damping of plasma waves

We present measurements of bounce harmonic Landau damping due to z-variations in the plasma potential, created by an azimuthally symmetric “squeeze” voltage Vs applied to the cylindrical wall. Traditional Landau damping on spatially uniform plasma is weak in regimes where the wave phase velocity vph≡ω/k is large compared to the thermal velocity. However, z-variations in plasma density and potential create higher spatial harmonics, which enable resonant wave damping by particles with bounce-averaged velocities vph/n, where n is an integer. In our geometry, the applied squeeze predominantly generates a resonance at vph/3. Wave-coherent laser induced fluorescence measurements of particle velocities show a distinctive Landau damping signature at vph/3, with amplitude proportional to the applied Vs. The measured (small amplitude) wave damping is then proportional to Vs2, in quantitative agreement with theory over a range of 20 in temperature. Significant questions remain regarding “background” bounce harmonic ...

Space Charge Frequency Shifts of the Cyclotron Modes in Multi-Species Ion Plasmas

AbstractShifts of the cyclotron frequency away from the “bare” cyclotron frequency are observed to be proportional to the total ion density through the E × B rotation frequency, and to the relative concentration of each ion species, in quantitative agreement with analytic theory. These shifts are measured at small excitation amplitudes on the typical center-of-mass m = 1 mode, and also on cyclotron modes with m = 0 and m = 2 azimuthal dependence. The frequency spacing between these modes is proportional to the rotation frequency of the ion cloud, which is controlled and measured using a “rotating wall” and laser-induced fluorescence. These cylindrical ion plasmas consist of Mg+ isotopes, with H3O+ and O2+ impurities. It is observed that the shift in the m = 1 cyclotron frequency is larger for the minority species 25Mg+ and 26Mg+, than for the majority species 24Mg+. A simple center-of-mass model is presented, which is in quantitative agreement with these results. It is also shown that this model interprets and expands the intensity dependent calibration equation, (M/q) = A/f + B/f2 + CI/f2. Graphical Abstractᅟ

Measurements of long-range enhanced collisional velocity drag through plasma wave damping

We present damping measurements of axial plasma waves in magnetized, multispecies ion plasmas. At high temperatures T≳10−2 eV, collisionless Landau damping dominates, whereas, at lower temperatures T≲10−2 eV, the damping arises from interspecies collisional drag, which is dependent on the plasma composition and scales roughly as T−3/2. This drag damping is proportional to the rate of parallel collisional slowing, and is found to exceed classical predictions of collisional drag damping by as much as an order of magnitude, but agrees with a new collision theory that includes long-range collisions. Centrifugal mass separation and collisional locking of the species occur at ultra-low temperatures T≲10−3 eV, which reduce the drag damping from the T−3/2 collisional scaling. These mechanisms are investigated by measuring the damping of higher frequency axial modes, and by measuring the damping in plasmas with a non-equilibrium species profile.