Mapping the self-generated magnetic fields due to thermal Weibel instability.

The origin of the seed magnetic field that is amplified by the galactic dynamo is an open question in plasma astrophysics. Aside from primordial sources and the Biermann battery mechanism, plasma instabilities have also been proposed as a possible source of seed magnetic fields. Among them, thermal Weibel instability driven by temperature anisotropy has attracted broad interests due to its ubiquity in both laboratory and astrophysical plasmas. However, this instability has been challenging to measure in a stationary terrestrial plasma because of the difficulty in preparing such a velocity distribution. Here, we use picosecond laser ionization of hydrogen gas to initialize such an electron distribution function. We record the 2D evolution of the magnetic field associated with the Weibel instability by imaging the deflections of a relativistic electron beam with a picosecond temporal duration and show that the measured [Formula: see text]-resolved growth rates of the instability validate kinetic theory. Concurrently, self-organization of microscopic plasma currents is observed to amplify the current modulation magnitude that converts up to ~1% of the plasma thermal energy into magnetic energy, thus supporting the notion that the magnetic field induced by the Weibel instability may be able to provide a seed for the galactic dynamo.

Post-compression of long-wave infrared 2 picosecond sub-terawatt pulses in bulk materials.

We have experimentally demonstrated the post-compression of a long-wave infrared (9.2 µm) 150 GW peak power pulse from 2 ps to less than 500 fs using a sequence of two bulk materials with negative group velocity dispersion (GVD). The compression resulted in up to 1.6-fold increase of the peak power and up to 2.8-fold increase of the intensity in the center of a quasi-Gaussian beam. The partial decoupling of the self-phase modulation and chirp compensation stages by using two materials with significantly different ratios of nonlinear refractive index to GVD provides accurate optimization of the compression mechanism and promises a viable path to scaling peak powers to supra-terawatt levels. During the preparatory study, we measured, for the first time to our knowledge, the nonlinear refractive indices of NaCl, KCl, and BaF2 for picosecond pulses in the long-wave infrared region.

Single-shot measurement of the nonlinear refractive index of air at 9.2 µm with a picosecond terawatt CO2 laser.

We developed a simple, accurate single-shot method to determine the nonlinear refractive index of air by measuring the evolution of the spatial shape of a laser beam propagating through the atmosphere. A distinctive feature of this new method, which relies on a modified Fresnel propagation model for data analysis, is the use of a hard aperture for producing a well-defined, high-quality beam from a comparatively non-uniform quasi-flat-top beam, which is typical for high-peak-power lasers. The nonlinear refractive index of air for a very short (2 ps) long-wave infrared (LWIR) laser pulse was measured for the first time, to the best of our knowledge, yielding n2=3.0×10-23m2/W at 9.2 µm. This result is 40% lower than a corresponding measurement with longer (200 ps) LWIR pulses at a similar wavelength.

Plasma emission characteristics in laser-induced breakdown spectroscopy of silicon with mid-infrared, multi-millijoule, nanosecond laser pulses from a Ho:YLF excitation source.

We characterized the plasma emission produced by the interaction of multi-millijoule, 40 ns duration, mid-infrared laser pulses with a silicon surface. The laser pulses were produced by a Q-switched Ho:YLF master oscillator power amplifier system. Using spectral measurements and a framing camera, we observed a spatial separation of the plasma plume, increased emission signal with low white-light generation, and a drop in the time- and space-averaged apparent plasma density with increasing pump energy. Our results can be explained by continuous heating of the plasma by the pump pulse due to the more efficient inverse bremsstrahlung absorption at longer wavelengths.

Enhanced sensitivity in H photofragment detection by two-color reduced-Doppler ion imaging.

Two-color reduced-Doppler (TCRD) and one-color velocity map imaging (VMI) were used for probing H atom photofragments resulting from the ~243.1 nm photodissociation of pyrrole. The velocity components of the H photofragments were probed by employing two counterpropagating beams at close and fixed wavelengths of 243.15 and 243.12 nm in TCRD and a single beam at ~243.1 nm, scanned across the Doppler profile in VMI. The TCRD imaging enabled probing of the entire velocity distribution in a single pulse, resulting in enhanced ionization efficiency, as well as improved sensitivity and signal-to-noise ratio. These advantages were utilized for studying the pyrrole photodissociation at ~243.1 and 225 nm, where the latter wavelength provided only a slight increase in the H yield over the self-signal from the probe beams. The TCRD imaging enabled obtaining high quality H(+) images, even for the low H photofragment yields formed in the 225 nm photolysis process, and allowed determining the velocity distributions and anisotropy parameters and getting insight into pyrrole photodissociation.

Computational modeling of laser-plasma interactions: pulse self-modulation and energy transfer between intersecting laser pulses.

The nonlinear interaction of intense femtosecond laser pulses with a self-induced plasma channel in air and the energy transfer between two intersecting laser pulses were simulated using the finite-difference time-domain particle-in-cell method. Implementation of a simple numerical code enabled modeling of various phenomena, including pulse self-modulation in the spatiotemporal and spectral domains, conical emission, and energy transfer between two intersecting laser beams. The mechanism for energy transfer was found to be related to a plasma waveguide array induced by Moiré patterns of the interfering electric fields. The simulation results provide a persuasive replication and explanation of previous experimental results, when carried out under comparable physical conditions, and lead to prediction of others. This approach allows us to further examine the effect of the laser and plasma parameters on the simulation results and to investigate the underlying physics.