M. Levin

Plasma structures for quasiphase matched high harmonic generation

A scheme for creation of periodic plasma structures by ablating a lithographic pattern is demonstrated. A proof of principle experiment was conducted, and plasma parameters were measured as a function of time with spatial resolution <10 and 100 μm periodicity. Several possible applications, in particular, quasiphase matching for high harmonic generation in plasma are considered.

Longitudinal profiles of plasma parameters in a laser-ignited capillary discharge and implications for laser wakefield accelerator applications

The evolution of longitudinal electron density and temperature profiles in plasma channel produced by a low-current Plexiglas capillary discharge with laser ignition was investigated by spectroscopic methods. The plasma was produced by an electric discharge using a 0.5 mm diameter, 15 mm long Plexiglas capillary. The electron density measured in near-outlet region was found to be lower by 30%. Simulations show that this variation of the plasma density near the entrance of the capillary can pose substantial difficulties for external injection of electrons for laser wakefield accelerator applications.

Guiding of 35 TW laser pulses in ablative capillary discharge waveguides

An ablatively driven capillary discharge plasma waveguide has been used to demonstrate guiding of 30 fs, 35 TW laser pulses over distances up to 3 cm with incident intensity in excess of 4×1018 W/cm2. The plasma density range over which good guiding was observed was 1–3×1018 cm−3. The quality of the laser spot at the exit mode was observed to be similar to that at the entrance and the transmitted energy was ∼25% at input powers of 35 TW. The transmitted laser spectrum typically showed blueshifting due to ionization of carbon and hydrogen atoms in the capillary plasma by the high intensity laser pulse. The low plasma density regime in which these capillaries operate makes these devices attractive for use in single stage electron accelerators to multi-GeV energies driven by petawatt class laser systems.

Low jitter capillary discharge channels

Generation of low jitter capillary discharge channels triggered by a relatively low-energy laser pulse is reported. For ablative discharges in polyethylene and boron nitride capillaries, the dependences of the breakdown delay and jitter on the incident laser intensity and discharge voltage are presented. Low jitter (5 ns and less) and the absence of electric noise associated with the discharge ignition make this method very useful for applications such as channel-guided laser-wakefield accelerators and x-ray lasers, where precise synchronization of the discharge with a laser and/or electron beam entering the channel is required.

Long plasma channels in segmented capillary discharges

Guided propagation of ultrashort (100fs) high intensity (1016Wcm−2) laser pulses over distances up to 12.6cm using optimized segmented capillary is reported. A new diagnostic technique is presented in which the transport of a guided laser pulse at different delay times from the initiation of the discharge is sampled on a single discharge shot. The current waveform was optimized to obtain a long lasting, deep radial profile. Radial profiles with the maximal electron density from 4×1017 to 2×1018cm−3 and up to 25% deep were obtained thereby, whereas longitudinal profiles were found to be remarkably uniform. The potential application of these long channels to the laser wake field accelerator is discussed.

Enhancement of a 24.77-nm line emitted by the plasma of a boron nitride capillary discharge irradiated by a high-intensity ultrashort laser pulse

Investigations of plasma produced by a boron nitride capillary discharge irradiated with a guided 20-TW Ti: sapphire laser pulse at a peak intensity of 4 x 10(18) W/cm2 are presented. The guided laser radiation in the plasma channel generated He-like ions that, subject to suitable plasma temperature, recombined into Li-like nitrogen ions. Intense radiation at a wavelength of 24.77 nm was observed, indicating possible lasing at the 3d(5/2) - 2p(3/2) transition in Li-like nitrogen.

Self-guided laser wakefield acceleration using ablated plasma targets

Laser wakefield acceleration (LWFA) was studied using ablated plasmas as the target medium. A low density laser-ablated plasma (carbon and fluorine) was produced by focusing a 100 mJ, 10 ns pulse from a Nd : YAG laser onto the surface of a plastic target to a peak intensity of 3 × 1010 W cm−2. A 30 fs interaction pulse from the HERCULES Ti : sapphire laser system with 30 TW laser power subsequently irradiated the plasma at a peak intensity of 3 × 1018 W cm−2. The plasma density profile was varied in situ by changing the time delay between the two laser pulses. It was observed that electron energies up to 120 MeV with monoenergetic features were observed. For larger delays, the electron beam charge increases while the transmittance of the interaction pulse decreases. This correlation suggests that pump depletion occurs due to wake excitation. The use of an ablated plasma target enables LWFA operation at much higher repetition rates due to the fast plasma dynamics and adds flexibility of plasma parameters such as temperature, charge state and ion composition.

High-current capillary discharge with prepulse ablative plasma

Generation of axisymmetric stable, long plasma channels with temperatures of 8 eV and electron densities ∼1019 cm−3 by a high-current evaporating-wall capillary discharge with prepulse ablative plasma is reported. Results of spectroscopic measurements of the temperature and electron density of plasma produced in a polyethylene capillary are presented. The discharge provides a convenient source of dense highly ionized plasmas for laser-plasma interaction studies.

Guiding and Ionization Blueshift in Ablative Capillary Waveguide Accelerators

Laser wakefield acceleration (LWFA) in plasmas has been demonstrated with gradients which are orders of magnitude greater than the limit on conventional Radio Frequency accelerators. However, the acceleration length is limited by two factors, the dephasing length and the Rayleigh range of the laser pulse. Dephasing length is the distance in which electrons overtake the laser pulse and can be increased by decreasing plasma density. Alternatively the interaction length can be extended by orders of magnitude by using ablative wall discharge capillary targets, in which a plasma is preformed with a transverse density profile capable of guiding the focused laser. We have demonstrated guiding of high intensity laser pulses from the HERCULES laser over 3 cm for powers up to 35 TW. The quality of the laser spot can be retained and the intensity remains high even at the exit of the capillary. The transmitted laser spectrum shows blueshifting due to field ionization by the laser pulse. This ionization might enhance ...

Laser wakefield acceleration experiments at the university of Michigan

Laser wakefield acceleration (LWFA) in a supersonic gas‐jet using a self‐guided laser pulse was studied by changing the laser power and electron density. The recently upgraded HERCULES laser facility equipped with wavefront correction enables a peak intensity of 8×1019 W/cm2 at laser power of 100 TW to be delivered to the gas‐jet using f/10 focusing optics. We found that electron beam charge was increased significantly with an increase of the laser power from 30 TW to 80 TW and showed density threshold behavior at a fixed laser power. Betatron motion of electrons was also observed depending on laser power and electron density.