Direct path from microscopic mechanics to Debye shielding, Landau damping and wave-particle interaction
The derivation of Debye shielding and Landau damping from the N-body description of plasmas is performed directly by using Newton’s second law for the N-body system. This is done in a few steps with elementary calculations using standard tools of calculus and no probabilistic setting. Unexpectedly, Debye shielding is encountered together with Landau damping. This approach is shown to be justified in the one-dimensional case when the number of particles in a Debye sphere becomes large. The theory is extended to accommodate a correct description of trapping and chaos due to Langmuir waves. On top of their well-known production of collisional transport, the repulsive deflections of electrons are shown to produce shielding, in such a way that each particle is shielded by all other ones, while keeping in uninterrupted motion.
On the validity of quasilinear theory
whereζm is the complex envelope for wave m, with wavenumber km, phase velocityvm= ωm/km and pulsationωm = ωp √ 1+3k2 mλ 2 D. The couplingε = √ 2η/(1+η) is determined by the ratio η = nb/np of beam densitynb to plasma density np. Let ∆vm = |vm+1−vm−1|/2,u0 = min(vm), u1 = max(vm) ; ∆vspec= u1−u0 is the phase velocity width of the wave spectrum. The saturation of the beam–plasma system was first predicted theoretically [6, 4] by considering the wave–particle interaction as perturbative and ne glecting all mode couplings in (1)-(2), except for their effect on the space averaged distribution f unction f̄ . This leads to the quasilinear (QL) equations couplinḡf (t,v) and the waves power spectrum ψ(t,v)
Spectroscopy of high rate events during active interrogation
Abstract Pulsed accelerators are being used for radiographic inspections in homeland security applications. Adding spectroscopic detection capability enables additional information to be extracted but challenges signal processing hardware. This work compares two approaches to cost-effective digital spectroscopy systems designed to handle these high rate scenarios. In the first system, the signal from a PMT is directly digitized and then deconvolved on a PC using standard techniques. Bypassing the preamplifier, this system can handle high count rates (>10 6  cps) from an LaCl 3 detector with no dead time and an energy resolution of 5% at 1.33 MeV. Spectra acquired from fission product gamma rays between interrogation pulses are shown. At higher rates, this approach is limited by the DAQ transfer rate to the PC. Instead, we tested a fast ADC (125 MSPS) and an FPGA that performed onboard, real-time digital signal processing (DSP). The energy spectrum is generated on the FPGA board during the measurement and transferred later. This system was tested in nuclear resonance fluorescence (NRF) experiments. Resonance peaks from NRF reactions are shown.