C. Bargsten

Energy penetration into arrays of aligned nanowires irradiated with relativistic intensities: Scaling to terabar pressures

Nanowire arrays heated by laser pulses of relativistic intensity open a path to extreme energy densities and pressures. Ultrahigh-energy density (UHED) matter, characterized by energy densities >1 × 108 J cm−3 and pressures greater than a gigabar, is encountered in the center of stars and inertial confinement fusion capsules driven by the world’s largest lasers. Similar conditions can be obtained with compact, ultrahigh contrast, femtosecond lasers focused to relativistic intensities onto targets composed of aligned nanowire arrays. We report the measurement of the key physical process in determining the energy density deposited in high-aspect-ratio nanowire array plasmas: the energy penetration. By monitoring the x-ray emission from buried Co tracer segments in Ni nanowire arrays irradiated at an intensity of 4 × 1019 W cm−2, we demonstrate energy penetration depths of several micrometers, leading to UHED plasmas of that size. Relativistic three-dimensional particle-in-cell simulations, validated by these measurements, predict that irradiation of nanostructures at intensities of >1 × 1022 W cm−2 will lead to a virtually unexplored extreme UHED plasma regime characterized by energy densities in excess of 8 × 1010 J cm−3, equivalent to a pressure of 0.35 Tbar.

Efficient picosecond x-ray pulse generation from plasmas in the radiation dominated regime

The efficient conversion of optical laser light into bright ultrafast x-ray pulses in laser created plasmas is of high interest for dense plasma physics studies, material science, and other fields. However, the rapid hydrodynamic expansion that cools hot plasmas has limited the x-ray conversion efficiency (CE) to 1% or less. Here we demonstrate more than one order of magnitude increase in picosecond x-ray CE by tailoring near solid density plasmas to achieve a large radiative to hydrodynamic energy loss rate ratio, leading into a radiation loss dominated plasma regime. A record 20% CE into hν>1  keV photons was measured in arrays of large aspect ratio Au nanowires heated to keV temperatures with ultrahigh contrast femtosecond laser pulses of relativistic intensity. The potential of these bright ultrafast x-ray point sources for table-top imaging is illustrated with single shot flash radiographs obtained using low laser pulse energy. These results will enable the deployment of brighter laser driven x-ray sources at both compact and large laser facilities.

X-ray Generation From Ultra-High Energy Density Relativistic Plasmas by Ultrafast Laser Irradiation of Nanowire Arrays

We have demonstrated the volumetric heating of near-solid density plasmas to keV temperatures using ultra-high contrast femtosecond laser pulses of only 0.5 J energy to irradiate arrays of vertically aligned nanowires (Purvis et al. Nat Photonics 7:796–780, 2013). Our x-ray spectra and particle-in-cell (PIC) simulations show extremely highly ionized plasma volumes several micrometers in depth are generated by irradiation of Au and Ni nanowire arrays with femtosecond laser pulses of relativistic intensities. Arrays of vertically aligned Ni nanowires with an average density of 12 % solid were ionized to the He-like stage. The He-like line emission from the nanowire target exceeds the intensity of the Ni Kα line at this irradiation intensity. Similarly near-solid density Au nanowire arrays were ionized to the Co-like (Au52+). This volumetric plasma heating approach creates a new laboratory plasma regime in which extreme plasma parameters can be accessed with table-top lasers. Scaling to higher laser intensities promises to create plasmas with temperatures and pressures similar to those in the center of the sun. The increased hydrodynamic-to-radiative lifetime ratio is responsible for a dramatic increase in the x-ray emission with respect to polished solid targets. As highly efficient X-ray emitters and sources of extreme plasma conditions, these plasmas could play a role in the development of new ultra-short pulse soft x-ray lasers.