Self-Similar Structure and Experimental Signatures of Suprathermal Ion Distribution in Inertial Confinement Fusion Implosions.

The distribution function of suprathermal ions is found to be self-similar under conditions relevant to inertial confinement fusion hot spots. By utilizing this feature, interference between the hydrodynamic instabilities and kinetic effects is for the first time assessed quantitatively to find that the instabilities substantially aggravate the fusion reactivity reduction. The ion tail depletion is also shown to lower the experimentally inferred ion temperature, a novel kinetic effect that may explain the discrepancy between the exploding pusher experiments and rad-hydro simulations and contribute to the observation that temperature inferred from DD reaction products is lower than from DT at the National Ignition Facility.

Turbulence-driven bootstrap current in low-collisionality tokamaks.

Neoclassical bootstrap current is expected to provide a significant fraction of the equilibrium plasma current in tokamak reactors. Here we report a novel mechanism through which a bootstrap current may be driven even in a collisionless plasma. In analogy with the neoclassical mechanism, in which the collisional equilibrium established between trapped and passing electrons produces a steady state current, we show that resonant scattering of electrons by drift wave microturbulence provides an additional means of determining the equilibrium between trapped and passing electrons and thus driving a bootstrap current. Employing a linearized Fokker-Planck collision operator, the plasma current in the presence of both collisions and resonant electron scattering is computed, allowing for the relative strength of these two mechanisms to be quantified as a function of collisionality and fluctuation amplitude.

Edge temperature gradient as intrinsic rotation drive in Alcator C-Mod tokamak plasmas.

Intrinsic rotation has been observed in I-mode plasmas from the C-Mod tokamak, and is found to be similar to that in H mode, both in its edge origin and in the scaling with global pressure. Since both plasmas have similar edge ∇T, but completely different edge ∇n, it may be concluded that the drive of the intrinsic rotation is the edge ∇T rather than ∇P. Evidence suggests that the connection between gradients and rotation is the residual stress, and a scaling for the rotation from conversion of free energy to macroscopic flow is calculated.

Toroidal rotation driven by the polarization drift.

Starting from a phase space conserving gyrokinetic formulation, a systematic derivation of parallel momentum conservation uncovers a novel mechanism by which microturbulence may drive intrinsic rotation. This mechanism, which appears in the gyrokinetic formulation through the parallel nonlinearity, emerges due to charge separation induced by the polarization drift. The derivation and physical discussion of this mechanism will be pursued throughout this Letter.

Relationships between simultaneously recorded Purkinje cells and nuclear neurons.

In decerebrate, unanesthetized cats, the activity of a Purkinje cell and a paired neuron in the interposed nuclei (subsequently referred to as a Purkinje cell-nuclear cell pair) were simultaneously recorded to compare their spontaneous discharge characteristics and responses to a peripheral input. Microstimulation techniques were used to determine if the two neurons were in related regions of the cerebellar cortex and interposed nuclei. First, the electrode used to record the interposed nuclear neuron was used to antidromically activate the Purkinje cell. Second, inhibitory responses with the appropriate time course were evoked in the interposed neurons with the Purkinje cell recording electrode. Both the spontaneous discharge of each pair as well as the cells' responses to a mechanically generated forepaw displacement were evaluated. A total of 35 Purkinje cell-nuclear cell pairs satisfying the microstimulation criteria were studied. During spontaneous activity there was no consistent relationship between the Purkinje cell simple spike activity and the nuclear neuron activity based on cross-correlation techniques. Auto-correlograms of both cells exhibited positive correlation for a brief time period. The variability of the interspike interval of the Purkinje cell discharge was greater than that for nuclear neurons, but there was no difference in the mean interspike interval for the pairs. The simple spike activity of Purkinje cells in response to sinusoidal displacements of the forepaw was weakly modulated. The modulation was limited to a small frequency range (2-7 Hz). In contrast the nuclear neurons exhibited a greater depth of modulation than Purkinje cells and responded to a wider range of frequencies (2-15 Hz). For many pairs of cells the relationship between the Purkinje cell and nuclear neuron discharge was not reciprocal. The responses of Purkinje cell-nuclear cell pairs to forepaw displacement were more often reciprocal to square wave than to sinusoidal stimuli. Using a technique which estimated the reciprocity of Purkinje cell and nuclear cell responses, 49% of the responses were reciprocal while 51% were not. These findings suggest that a nuclear neuron's discharge characteristics are not dominated by inputs from a single Purkinje cell and the firing relationship between a Purkinje cell and a related nuclear cell need not be reciprocal.

Changes in the responses of cerebellar nuclear neurons associated with the climbing fiber response of Purkinje cells.

Previous studies demonstrated that climbing fiber activity produces a short-term increase in the responsiveness of Purkinje cells to mossy fiber inputs. This led to the hypothesis that there are concomitant alterations in the discharge of cerebellar nuclear neurons. This series of experiments was initiated to test this hypothesis in simultaneously recorded Purkinje cell-nuclear cell pairs in related regions of the cerebellar cortex and nuclei. In decerebrate cats 50 pairs of Purkinje cells and nuclear neurons were identified and simultaneously recorded during spontaneous activity and during peripheral inputs. Auto-correlograms of nuclear cell activity and cross-correlograms of the simple spike and nuclear cell activity triggered on the occurrence of spontaneous complex spikes demonstrated little correlation between these events and the discharge of nuclear neurons. To examine the effect of evoked climbing fiber inputs on the Purkinje cell simple spike and nuclear cell responses, square wave mechanical stimuli which modulated the discharge of both cells of a pair were applied to the forepaw. A separation technique was used to construct one histogram illustrating the responses of the nuclear neuron and Purkinje cell in trials in which the peripheral stimulus evoked a climbing fiber input to the Purkinje cell and another histogram showing their responses in trials in which no climbing fiber input was activated. Using this separation technique it was shown that the amplitude of most Purkinje cell responses increased by 120-1200% in trials in which climbing fiber inputs were activated. The response amplitude of 68% of the nuclear cells was modified for these pairs. Most changes in nuclear cell responses were increases ranging from 120-220%. These changes were felt to reflect the action of many Purkinje cells converging on the isolated nuclear neuron. The modulation of the nuclear neuron was not due only to the effect of the related Purkinje cell, since the gain change of the Purkinje cell and nuclear cell of each pair was not correlated (r = 0.01). The discussion of these findings emphasizes that the increased responses of the nuclear cell are most likely produced by the intracortical action of the climbing fiber system on the responsiveness of Purkinje cells to mossy fiber inputs. Climbing fiber collateral input to nuclear neurons also may contribute to the changes in the nuclear cell responses observed in these experiments.

The changes in Purkinje cell simple spike activity following spontaneous climbing fiber inputs.

The purpose of these experiments was to systematically examine the characteristics of the excitability change occurring after the inactivation period evoked by the climbing fiber input to Purkinje cells. Ninety-eight Purkinje cells were isolated extracellularly in unanesthetized decerebrate cats. Simple spikes and complex spikes were discriminated separately. Post-stimulus time histograms were constructed from 100 consecutive trials triggered by the occurrence of spontaneous complex spikes. Seventeen Purkinje cells exhibited a reduction of simple spike discharge rate following the inactivation period. However, 14 cells showed no change in simple spike activity, and in 67 cells the discharge rate increased. These changes in excitability following a spontaneous complex spike were independent of the tonic simple spike activity of the Purkinje cell. Single traces of spike train data from Purkinje cells showed that the change in discharge rate was variable, some complex spikes being followed by an increase and others by a decrease in activity. The basis for these observations and the differences between these data and those from studies in which the climbing fiber input was evoked by electrical olivary stimulation are discussed.