Alex Rockwood

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.

High Average Power Table-Top Soft X-Ray Lasers Using Diode-Pumped Laser Drivers

Soft X-ray lasers (SXRLs) produce the highest energy pulses of coherent ultrashort wavelength radiation. Their large number of photons per pulse allow us to perform single-shot imaging of nanoscale objects, to develop material composition sensitive nanoprobes, and to conduct interferometric diagnostics of bright dense plasmas. However, until recently, with the exception of capillary discharge lasers at 46.9 nm, their average power was limited by the low repetition rate of the high energy optical pump lasers required to drive them and by the relatively low pumping efficiency. To overcome this limitation, we have developed a diode-pumped, picosecond Yb:YAG CPA laser driver that allowed us to demonstrate the first table-top SXRL capable of 100 Hz repetition rate gain saturated operation. These new pump lasers combined with efficient plasma heating techniques enable the operation of SXRLs at four orders of magnitude higher repetition rate than the first plasma-based collisional SXRLs (Fig. 2.1). Laser operation at 100 Hz repetition rate generated an average power of 0.2 mW at 18.9 nm (Ni-like Mo) and 0.1 mW at λ = 13.9 nm (Ni-like Ag). We have recently improved upon this diode-pumped laser technology, demonstrating a record 0.5 kHz repetition rate, 1 J, picosecond laser with good beam quality and high stability. We will discuss the results of the first demonstration of a compact table-top soft X-ray laser at repetition rates up to 400 Hz using this new pump laser.

High Repetition Rate Soft X-Ray Lasers and Bright Table-top X-Ray Sources from Nanostructured Target Plasmas Irradiated at Relativistic Intensities

We report the first operation of table-top soft x-ray lasers at repetition rates > 100 Hz and the generation of bright ultrashort hard X-ray pulses from nanostructured targets irradiated with femtosecond pulses of relativistic intensity.