The objective of this work is to investigate the creation of elongated plasma channels to reach low loss propagation of highly intense and ultra short pulses used for the development of soft X-Ray laser sources. Intellectual Merit: The proposed research will shed light on the complex physics of laser beam propagation through plasma and plasma waveguide formation. This study will combine theoretical simulations, experimental design and time and spatial-resolution interferometry for plasma diagnostics and include external collaboration with other leaders in the field. Broader Impact: The investigation will elucidate issues of intense laser beam propagation in areas such as soft X-Ray laser sources, laser particle accelerators and laser propagation in atmosphere. In addition the research provides an excellent platform for the teaching, training and dissemination of advanced plasma physics. Graduate students supported under this grant will gain experience in theoretical and experimental physics and gain from the collaboration with other leading research teams.
1. Designing & Proceeding-Experiment General considerations: We are in the process of setting up the experiment for demonstration of lasing action at 3.4 nm using our recent computer modeling results, experience with generation lasing action in transition to ground state of Li III ions, and a new approach to create narrow, high aspect ratio plasma channels. We do not yet have a full simulation of lasing at 4.0 nm in He-like CV ions to ground state, preliminary results indicate, when lasing action takes place at 3.4 nm, its ground state is rapidly populated leading to a high rate for 3-body recombination from CVI to CV and population inversion in CV singlet transition to ground state (21P1 –11S0). Therefore, by creating the condition for lasing action at 3.4 nm in CVI, in a short time sequence lasing action at 4.0 nm is also feasible. For preparation of lasing conditions in CVI ions at 3.4nm and in CV at 4.0 nm, as for soft-X ray region in general, it is important to create a small diameter channel (high aspect ratio of diameter to the length of the channel) in such medium for high intensity pumping beam propagation. Experiment: To observe the creation of very narrow plasma channel and laser beam propagation in it requires high resolution in space and time interferometer. Fig.1 shows a schematic of the interferometer, and an interferogram for plasma channel with aspect ratio ~ 1000 is shown on Fig.2a; radial electron density profile for area indicated by red is shown in Fig.2b. The plasma channel was created in a supersonic gas jet with a pulsed gas valve and using 2 pre-pulses from Ti/Sapphire laser at wavelength 800nm. Pre-pulses are referred to as pre-pulse1 and 2, where pre-pulse 1 is "short" (100 fs pre-pulse) and pre-pulse 2 is "long" (200ps). The pre-pulse1 was focused by conical lens (axicon) and the pre-pulse 2 was focused by spherical lens or off-axis parabola mirror through the hole in axicon. This very effective technique to create plasma channels for high intensity laser beam propagation was developed by H.Milchberg and his group at University of Md. A rectangular orifice, 6 x 0.5 mm2, allowed a change to the effective depth of the gas target by changing the orientation angle of the orifice with respect to the laser propagation axis. The density of particles in the gas jet was controlled by changing the backing pressure on the valve or its opening timing. 2. Demonstration Large Aspect Ratio Narrow Plasma Channel Fig.3a shows a schematic of the experimental arrangement with a Ti/Sapphire laser intensity of 1019 Wcm-2 in 50 or 100 fs. Details of setup for creating a plasma channel using two pre-pulses is shown in Fig 3b, as well as the photograph of the spark (plasma channel) obtained by means of focusing 200 ps ("long") pre-pulse with the axicon into the N2 gas jet (just for testing the axicon’s ability to create an elongated plasma column with low intensity laser beam). Fig.3b shows the setup of the interferometric measurement using a 400 nm, 200 fs beam. The interferometer, the optics for two laser pre-pulses and for "main" laser pulse (high intensity pumping pulse), which are located in a vacuum chamber are shown in Fig.3b. Incorporated into the system is a grazing incidence soft X-spectrometer; the role of the line intensity measurement is more clearly presented in Fig.3a. We will proceed the continuation of the investigation of very high intensity (~ 1018 -1019 W/cm2 in 50 or 100 fs) laser beam propagations (pumping laser beam propagations) in very narrow plasma channels which was recently successfully created (Figs.1, 2). The time evolution of the channels can be seen in Fig.4, which was obtained from high resolution interferograms using Abel inversion. We will measure the propagation efficiency and uniformity of laser beam diameter in ethane pre-plasma using a high resolution interferometer. However, initial experiments on propagation laser beams will be conducted with intensities not exceeding 1018 W/cm2 (energies below 150 mJ) for testing of the optics, observing reflections and beam "hot spots" in order to avoid damaging the optics. It will be followed by the optimization of channeling for plasma densities of ~ 1020/cm3 when hydrogen is added to the ethane (as "prescribed" by our computer gain modeling) and the measurements of the propagation efficiency and uniformity of laser beam diameter for intensities up to 1019 W/cm2 (energy ~1.5 J in 50 and 100 fs) for plasma densities in channels ~ 1 – 2*1020/cm3 (ethane with hydrogen added). These last results will be compared with very advanced computer modeling provided by P, Sprangle and his group at NRL. This computer modeling has been used during the earlier stage of plasma channel formations and was very helpful in better understanding the propagation problems and in designing the present experiments.
