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We propose a new method of estimating the mean diameter and dispersion of clusters formed in a cooling gas, right after the nucleation stage. Using a moment model developed by Friedlander [S.K. Friedlander, Ann. N.Y. Acad. Sci. 354 (1983)], we derive an analytic relationship for both cluster diameter and diameter dispersion as a function of two of the characteristic times of the system - the cooling time and primary constituents collision time. These formulas can be used to predict diameter and dispersion variation with process parameters such as the initial monomer pressure or cooling rate. It is also possible to use them as an input to the coagulation stage, without the need to compute complex cluster generation during the nucleation burst. We compared our results with a nodal code and got excellent agreement., Comment: 10 pages, 11 figures

It is shown that ion acoustic waves in plasmas with E0×B0 electron drift become unstable due to the closure of plasma current in the chamber wall. Such unstable modes may enhance both near-wall conductivity and turbulent electron transport in plasma devices with E0×B0 electron drift and unmagnetized ions. It is shown that the instability is sensitive to the wall material: a high value of the dielectric permittivity (such as in metal walls) reduces the mode growth rate by an order of magnitude but does not eliminate the instability completely.

A condition for sheath instability due to secondary electron emission (SEE) is derived for low collisionality plasmas. When the SEE coefficient of the electrons bordering the depleted loss cone in energy space exceeds unity, the sheath potential is unstable to a negative perturbation. This result explains three different instability phenomena observed in Hall thruster simulations including a newly found state with spontaneous ∼20 MHz oscillations. When instabilities occur, the SEE propagating between the walls becomes the dominant contribution to the particle flux, energy loss and axial transport.

Background plasma can be used as an effective neutralization scheme to transport and compress intense charged particle beam pulses and is used in many applications involving the transport of fast particles in plasmas, including astrophysics [1], accelerator applications [2], and inertial fusion, in particular, fast ignition [3] and heavy ion fusion [4]. The application of a solenoidal magnetic field allows additional control and focusing of the beam pulse. A strong magnetic lens with a magnetic field up to a few Tesla can effectively focus beam pulses in short distances of order a few tens of centimeters. However, the magnetic field can also affect the degree of charge and current neutralization. In this Letter, we show that even a small solenoidal magnetic field, less than 100 G, strongly changes the self-fields in the beam pulse propagating in a background plasma. Such values of magnetic field can be present over distances of a few meters from the strong solenoid, and affect the focusing of the beam pulse. Moreover, a small solenoidal magnetic field can be applied to optimize beam propagation through a background plasma over long distances. In Refs. [5], the response of a magnetized plasma to intense ion-beam injection was studied while neglecting electron inertia effects, which corresponded to magnetic fields of a few Tesla in ion ring devices. In the present Letter, we analyze the opposite limit, corresponding to small values of magnetic field. In the collisionless limit and without an applied solenoidal magnetic field, the return current is driven by an inductive electric field which is balanced by electron inertia effects [6]. Taking electron inertia effects into account allows us to determine the conditions under which the applied solenoidal magnetic field begins to affect the return current in the plasma, and to reveal the range of magnetic field values that strongly affect the self-electric and self-magnetic fields of a beam pulse propagating in a background plasma. In a previous study, we developed reduced nonlinear models that describe the stationary plasma disturbance (in the beam frame) excited by the intense ion-beam pulse [6]. The model predicts very good charge neutralization during quasi-steady-state propagation, provided the beam is nonrelativistic and the beam pulse duration is much longer than the electron plasma period, i.e., � b!pe � 2� . Here, !pe �� 4�e 2 np=m� 1=2 is the electron plasma frequency, and np is the background plasma density. A high solenoidal magnetic field inhibits radial electron transport, and the electrons move primarily along the mag

In a recent Letter, Cheng et al. calculated/predicted several new effects: that (a) fraction of the short laser pulse momentum can be imparted to plasma electrons via collisional damping of the laser, thereby exciting a long-lived (longer than an oscillation period) plasma wave, which (b) gives rise to a spatially uniform dipole moment of a plasma, which (c) emits far-field narrow-band radiation at the plasma frequency omega subscript ''p'' over the recombination time of the plasma. We claim that the calculation of the effect (a) is in error and the predicted effects (b,c) do not occur as described. In fact, predicted narrow-band emission at omega subscript ''p'' would not occur even if the momentum transfer and the dipole excitation were calculated correctly.