R. P. Fischer

Plasma wakefield generation and electron acceleration in a self-modulated laser wakefield accelerator experiment

A self-modulated laser wakefield accelerator (SM-LWFA) experiment was performed at the Naval Research Laboratory. Large amplitude plasma wakefields produced by a sub-picosecond, high intensity laser pulse (7×1018 W/cm2) in an underdense plasma (ne≈1019 cm−3) were measured with a pump–probe coherent Thomson scattering (CTS) technique to last for less than 5 ps, consistent with the decay of large amplitude plasma waves due to the modulational instability. A plasma channel was observed to form in the wake of the pump laser pulse, and its evolution was measured with the pump–probe CTS diagnostic. The trailing probe laser pulse was observed to be guided by this channel for about 20 Rayleigh lengths. High energy electrons (up to 30 MeV) have been measured using an electro-magnetic spectrometer, with the energy spectra and divergence of lower energy (up to 4 MeV) electrons obtained using photographic films. Highly nonlinear plasma waves were also detected using forward Raman scattering diagnostics and were obser...

Incoherent Combining and Atmospheric Propagation of High-Power Fiber Lasers for Directed-Energy Applications

High-power fiber lasers can be incoherently combined to form the basis of a directed high-energy laser system which is highly efficient, compact, robust, low-maintenance and has a long operating lifetime. This approach has a number of advantages over other beam combining methods. We present results of the first field demonstration of incoherent beam combining using kilowatt-class, single-mode fiber lasers. The experiment combined four fiber lasers using a beam director consisting of individually controlled steering mirrors. Propagation efficiencies of ~90%, at a range of 1.2 km, with transmitted continious-wave power levels of 3 kW were demonstrated in moderate atmospheric turbulence. We analyze the propagation of combined single-mode and multimode beams in atmospheric turbulence and find good agreement between theory, simulations and experiments.

Initial operation of a higher-power quasi-optical gyrotron

Results from the initial operation of a high-power quasi-optical gyrotron based on the 90-kV, 50-A Varian VUW-8144 electron gun are reported. The output power and efficiency have been measured for a resonator mirror separation of 19.4 cm with a magnetic field of 4.95 T, corresponding to resonator output coupling of 1.9%, and for a resonator mirror separation of 21.4 cm with a magnetic field of 4.7 T, corresponding to a resonator output coupling of 3.1%. Operation was multimoded with 3-6 modes excited in the range of 125-130 GHz for the 4.95-T magnetic field. A peak efficiency of 15% at an output power of 161 kW was obtained for a gun voltage of 93 kV and a current of 12 A. A peak-output power of 364 kW at an efficiency of 10% was obtained at a voltage of 95.6 kV and 37.5 A. >

High-power lasers for directed-energy applications.

In this article, we review and discuss the research programs at the Naval Research Laboratory (NRL) on high-power lasers for directed-energy (DE) applications in the atmosphere. Physical processes affecting propagation include absorption/scattering, turbulence, and thermal blooming. The power levels needed for DE applications require combining a number of lasers. In atmospheric turbulence, there is a maximum intensity that can be placed on a target that is independent of the initial beam spot size and laser beam quality. By combining a number of kW-class fiber lasers, scientists at the NRL have successfully demonstrated high-power laser propagation in a turbulent atmosphere and wireless recharging. In the NRL experiments, four incoherently combined fiber lasers having a total power of 5 kW were propagated to a target 3.2 km away. These successful high-power experiments in a realistic atmosphere formed the basis of the Navys Laser Weapon System. We compare the propagation characteristics of coherently and incoherently combined beams without adaptive optics. There is little difference in the energy on target between coherently and incoherently combined laser beams for multi-km propagation ranges and moderate to high levels of turbulence. Unlike incoherent combining, coherent combining places severe constraints on the individual lasers. These include the requirement of narrow power spectral linewidths in order to have long coherence times as well as polarization alignment of all the lasers. These requirements are extremely difficult for high-power lasers.

A study of millimeter-wave sintering of fine-grained alumina compacts

A number of high-frequency microwave sintering studies of alumina have reported that sintering proceeds much faster in microwave furnaces when compared to conventional furnaces, and that densification can occur at lower temperatures. These differences have motivated the search for a nonthermal microwave enhancement effect such as the time-averaged microwave field-induced mass transport effect proposed by Rybakov and Semenov (1994). To assess the difference between microwave and conventional sintering, and the presence of a nonthermal effect in microwave sintering, a study of millimeter-wave (mm-wave) (35 GHz) sintering has been conducted at the Naval Research Laboratory (NRL) using a well-studied fine-grained (submicron) commercial alumina with reproducibly manufactured properties, Sumitomo AKP-50. This paper reports our results, which generally indicate no large differences in the required temperatures for densification of conventionally and microwave sintered compacts, or between the resulting microstructures. The nonthermal effect proposed by Ryakov and Semenov was evaluated for fine-grained alumina and found to be small compared to the surface energy driving force for sintering.

Conductivity Measurements of Femtosecond Laser–Plasma Filaments

Experiments are performed to characterize the electrical properties of plasma filaments that are generated by self- guided femtosecond laser pulses propagating in air. A single plasma filament passes through a high-voltage sphere pulsed at -100 kV to a grounded electrode, which serves as a current monitor. The experiments utilize moderate electric fields to probe the filament conductivity, thereby avoiding the strong perturbations caused by electric discharges. The measured filament current decreases as ~1/R2 as the separation R between the electrodes is increased up to 1.5 m. The pulselength of the filament current signal is 2 ns (full-width at half-maximum), but the time resolution is limited by the bandwidth of the oscilloscope. The typical plasma density in the conducting filament is 9 times 1015 cm-3, which is inferred from the conductivity measurements and the size of the optical filaments. Comparisons are made with mobility values derived from electron swarm data, where the mobility depends upon the applied electric field. The conductivity of the filament is measured as the laser pulselength is varied from 50 fs to 1.5 ps. We find that relatively long laser pulses (1 ps) produce filaments with the largest conductivity. A model is used to predict the longitudinal position where the plasma filament forms and is in reasonably good agreement with measurements.

Observation of 20 eV x‐ray generation in a proof‐of‐principle laser synchrotron source experiment

A laser synchrotron source (LSS) [P. Sprangle, A. Ting, E. Esarey, and A. Fisher, J. Appl. Phys. 72, 5032 (1992)] was proposed to generate short‐pulsed, tunable x rays by Thomson scattering of laser photons from a relativistic electron beam. A proof‐of‐principal (p.o.p.) experiment on this LSS configuration is performed. An intense laser pulse (λ0=1.053 μm) is Thomson backscattered from a focused relativistic electron beam. Time integrated x‐ray signals from a photocathode/electron multiplier, at an electron beam energy of 650 keV and an x‐ray photon energy of 20 eV, indicate an increase in the x‐ray signals above the baseline by an amount comparable to the theoretically predicted value.

Operating characteristics of a continuous-wave-relevant quasioptical gyrotron with variable mirror separation

Results from a quasioptical gyrotron experiment with a 20–28 cm mirror separation are presented, showing operation at powers up to 150 kW and efficiencies up to 12%. The output coupling would be varied from 0.4%–3% by changing the mirror separation and operating frequency. Operation was obtained over frequencies ranging from 95–130 GHz by changing the axial magnetic field, limited on the low end by waveguide cutoff in the diagnostics and at the high end by the maximum magnetic field achievable. The output power varied by approximately a factor of 2 over this range. Frequency variation of 4% was achieved by varying only the electron gun voltage; however, the output power also varied substantially due to the fact that the electron beam power was changing dramatically. Efficiency optimization by variation of output coupling and tapering of the magnetic field has been demonstrated. Regions of single‐mode operation at powers up to 125 kW have been characterized and compared to recently developed theory.

Measurements of intense femtosecond laser pulse propagation in air

The nonlinear self-focusing of an intense femtosecond pulse propagating in air can be balanced by the plasma defocusing as the laser intensity is increased above the threshold for multiphoton ionization. The resultant laser∕plasma filament can extend many meters, suitable for many applications such as remote atmospheric breakdown, laser induced electrical discharge, and femtosecond laser material interactions. Direct (bore-sight) measurements of filament size and fluence over 4 m showed a preservation of the total energy in the filament during propagation. This indicates the energy lost in creating the central plasma column through multiphoton ionization was continuously being replenished from the surrounding radiation. Electrical measurement of the filament conductivity estimated the plasma density to be 1×1016cm−3 and electrical discharges triggered by a femtosecond laser filament were found to occur at substantially reduced breakdown fields.

New Results and Applications for the Quasioptical Gyrotron

The quasioptical gyrotron (QOG), which features an open resonator formed by a pair of spherical mirrors instead of the conventional gyrotron waveguide cavity, has been under development at the U. S. Naval Research Laboratory as a tunable high power millimeter‐wave source for tokamak plasma heating, advanced radars, and power beaming. Results have recently been obtained for a quasioptical gyroklystron (QOGK) realized by the addition of an open‐mirror prebunching resonator driven by an 85 GHz, 1.5 kW extended interaction oscillator. Efficiency enhancement by mode priming has been investigated, and efficiencies up to 19% have been obtained by increasing the frequency detuning of the operating mode. An overall efficiency of 30% was obtained by the addition of a simple depressed collector. Phase‐locked operation was demonstrated at a power of 57 kW and efficiency of 16%. The high circulating power in the QOG resonator is currently being considered for use as an electromagnetic wiggler for compact infrared free‐electron lasers. The QOG is also promising as a source for an active sensor of upper atmosphere trace impurities.