Detailed numerical calculations for the nighttime ionosphere are performed to evaluate the quantitative importance of nighttime radiation fields. Models for the intensities of night sky H I 1216, H I 1026, He I 584, and He II 304‐Å radiation are presented, and the production rates of ion species are calculated. The intensity of the Lyman α and Lyman β radiation fields from terrestrial and extraterrestrial sources is sufficient to maintain the nighttime lower ionosphere at observed electron density levels. Analysis of ion composition data indicates that on winter anomalous days of D region ionospheric absorption the E region is substantially depleted of NO molecules.

Quasimonoenergetic electron beams with maximum energy >0.5 GeV and 2 mrad divergence have been generated in pure nitrogen gas via wakefield acceleration with 80 TW, 30 fs laser pulses. Long low energy tail features were typically observed due to continuous ionization injection. The measured peak electron energy decreased with the plasma density, agreeing with the predicted scaling for electrons. The experiments showed a threshold electron density of 3×1018cm-3 for self-trapping. Our experiments suggest that pure Nitrogen is a potential candidate gas to achieve GeV monoenergetic electrons using the ionization induced injection scheme for laser wakefield acceleration.

We show through experiments that a transition from laser wakefield acceleration (LWFA) regime to a plasma wakefield acceleration (PWFA) regime can drive electrons up to energies close to the GeV level. Initially, the acceleration mechanism is dominated by the bubble created by the laser in the nonlinear regime of LWFA, leading to an injection of a large number of electrons. After propagation beyond the depletion length, leading to a depletion of the laser pulse, whose transverse ponderomotive force is not able to sustain the bubble anymore, the high energy dense bunch of electrons propagating inside bubble will drive its own wakefield by a PWFA regime. This wakefield will be able to trap and accelerate a population of electrons up to the GeV level during this second stage. Three dimensional (3D) particle-in-cell (PIC) simulations support this analysis, and confirm the scenario.

A novel method for remote optical diagnostics of the atmosphere at heights 30-60 km is proposed. TI• method relies on exciting atoms and molecules of minority species by electron impact during and following an ionizing microwave pulse injected from a focused ground-based transmitter. Free electrons produced in the breakdown region are the exciting agents for the atmospheric target molecules. The mixing ratio of the minority species can then be measured by either detecting the direct emission from allowed transitions or by utilizing lidar techniques to measure the excitation level of metastable states. Computer simulations of the intensity of the expectexl emission, based on kinetic theory of air breakdown, are presented. It is shown that mixing ratios below particle per trillion can be detecte• using microwave heaters with state of the art effective radiation power and modern detection technology. 1. Introduction Accurate, simultaneous, and continuous measurements of the atmospheric mixing ratios of active species are critical in determining quantitative links of increased greenhouse gases and climate change. Despite significant effort the database on which predictions and assessments of climate change are based is still insufficient. There are, in general, two types of data acquisition techniques: in situ and remote. The in situ measurements rely on rockets and stratospheric balloons. These are single-shot measurements that c0nnot be applied to study systematically the temporal dynamics of the concentration of the relevant species. Furthermore, they are hindered by altitude limitations and local contamination problems. The remote techniques rely on ground-or space-based lidars. Despite significant progress, propagation issues, signal to noise ratio limitations, and the high cost and calibration problems of space-based systems limit the extent of passive spectroscopy and lidar-based techniques. The objective of this paper is to propose a novel concept that allows for continuous monitoring of the strato-1Also at ARCO Power Technologies, Incorporated,

Laser wakefield acceleration of 500 MeV to 1 GeV electron bunches has been demonstrated using ionization injection in mixtures of 4% to 10% of CO2 in He. 80 TW laser pulses were propagated through 5 mm gas jet targets at electron densities of 0.4−1.5×1019cm−3. Ionization injection led to lower density thresholds, a higher total electron charge, and an increased probability of producing electrons above 500 MeV in energy compared to self-injection in He gas alone. Electrons with GeV energies were also observed on a few shots and indicative of an additional energy enhancement mechanism.