Producing shock-ignition-like pressures by indirect drive

The shock ignition scheme is an alternative Inertial Confinement Fusion ignition scheme that offers higher gains and a robustness to hydrodynamic instabilities. A desirable aspect of shock ignition is that the required intensities are achievable on existing facilities. Conventional approaches to shock ignition have only considered the use of direct laser drive. This is in part due to concerns that achieving the rapid rise in drive pressure needed in the final pressure spike may not be feasible using the indirect drive approach. The primary advantage of being able to utilise a hohlraum drive for a shock ignition experiment is that experiments could be carried out at existing, or soon to be completed, Mega-Joule scale facilities. Furthermore, this could be done without the need for any major modification to the facility architecture, such as would be required for direct drive experiments. One and two-dimensional radiation hydrodynamic simulations have been performed using the codes HYADES and h2d. The simulations investigated the level of x-ray fluxes that could produce shock ignition scale pressures as well as the laser powers that would be required to generate those pressures in a NIF scale-1 hohlraum. The second aspect of this work was to investigate the x-ray flux rise times that would be necessary to create a large enough shock ignition spike pressure (200-300 Mbar). It was found that pressures of 230 Mbar could be achieved through indirect drive using a laser source with a peak power of 400 TW. In addition, the rate of pressure increase in the final pressure spike is similar to the expected requirements for directly-driven shock ignition

Novel Approaches to Indirect Drive Inertial Confinement Fusion

This thesis describes work that developed new techniques towards indirect drive inertial confinement fusion. The work predominantly used the 1-dimensional (1D) and 2-dimensional (2D) versions of the radiation hydrodynamics code HYADES. The scaling of ablation pressures produced by the irradiation of a material with soft X-rays was investigated. Materials with average atomic numbers between 3.5 and 22 were irradiated by X-ray sources with radiation temperatures ranging from 100 eV to 400 eV. For each material, pressure scaling laws were determined as a function of temperature and time. Additionally, the maximum drive temperature for subsonic ablation was found for all the materials. Materials with high atomic number tend to have weaker pressure scaling but higher maximum subsonic drive temperatures. The next study found the laser drive parameters required to produce shock-ignition-like pressures through indirect drive. First, 1D simulations found an X-ray drive profile that is capable of producing shock-ignition-like pressures in a beryllium target. From there, 2D simulations were carried out to simulate the laser to X-ray conversion in a hohlraum. A laser drive profile was found that was capable of producing the required X-ray intensity profile. The final piece of work developed a new technique for controlling the X-ray flux in- side hohlraums using burn-through barriers. Hohlraum designs that use multiple chambers separated by burn-through barriers were proposed. The burn-through barriers are used to modulate the spatial and temporal properties of the X-rays as they flow between the cham- bers. It is shown how a number of different barrier designs can be used to manipulate the properties of the X-rays in both time and space

Controlling X-Ray Flux in Hohlraums Using Burn-through Barriers

A technique for controlling X-ray flux in hohlraums is presented. In Indirect Drive Inertial Confinement Fusion (ICF) the soft X-rays arriving at the spherical fuel capsule are required to have a specific temporal profile and high spatial uniformity in order to adequately compress and ignite the fuel. Conventionally this is achieved by modifying the external driver, the hohlraum geometry, and the sites of interaction between the two. In this study a technique is demonstrated which may have utility in a number of scenarios, both related to ICF and otherwise, in which precise control over the X-ray flux and spatial uniformity are required. X-ray burn-through barriers situated within the hohlraum are shown to enable control of the flux flowing to an X-ray driven target. Control is achieved through the design of the barrier rather than by modification of the external driver. The concept is investigated using the one-dimensional (1-D) radiation hydrodynamics code HYADES in combination with a three-dimensional (3-D) time-dependent viewfactor code

Electron acceleration by a transient intense-laser-plasma electrode

Rapid strides in the technology of laser plasma-based acceleration of charged particles leading to high brightness, tunable, monochromatic energetic beams of electrons and ions has been driven by potential multidisciplinary applications in cancer therapy, isotope preparation, radiography and thermonuclear fusion. Hitherto laser plasma acceleration schemes were confined to large-scale facilities generating a few tens of terawatt to petawatt laser pulses at repetition rates of 10 Hz or less. However, the need to make viable systems using high-repetition-rate femtosecond lasers has impelled recent research into novel targetry [1,2]. Of contemporary importance is the generation of supra thermal electrons, beyond those predicted by the scaling relation, reflected in both theoretical and computational work [3,4]. In this work we present evidence of generation of relativistic electrons (temperature >200 keV, maximum energies >1 MeV) at intensities that are two orders of magnitude lower than the relativistic intensity threshold. The novel targets [6] are 15 micron sized crystals suspended as aerosols in a gas interacting with a kHz, few-mJ femtosecond laser focussed to intensities of 10 PW/cm2. A pre-pulse with 1-5% of the intensity of the main pulse, arriving 4 ns early, is critical for hot electron generation. In addition to this unprecedented energy enhancement, we also characterize the dependence of X-ray spectra on the background gas of the aerosol. Intriguingly, easier the gas is to ionise, greater is the number of hot electrons observed, while the electron temperature remains the same. 2-D Radiation hydrodynamics and Particle-in-cell (PIC) simulations explain both the experimentally observed electron emission and the role of the low-density plasma in yield enhancement. We observe a two-temperature electron spectrum with about 50 and 240 keV temperatures consistent with the measurements made in the experiments. The simulations show that the following features contribute to the high-energy electron emission. The pre-pulse generates a hemispherical plasma-profile that enhances the coupling of the laser light. Overdense plasma is generated about the hemispherical cavity on the particle due to the main pulse interaction. The gradient in the plasma density in and around the cavity serves as a reservoir of low energy electrons to be injected into the particle potential and enables the hot electron generation observed in the experiments. Higher energy electron emission is dominantly from the edges of the hemispherical cavitation. The increase in total X-ray yield observed in the experiments scales with the number of electrons generated in the low density neighborhood surrounding the particle. In a simple-man picture, the laser interacts with the particle and ejects electrons from the particle. The particle acquires a strong positive potential that can only be brought down by ion expansion that occurs over 10's of picoseconds. The particle with strong positive potential acts as an 'accelerating electrode' for the electrons ionized in the low-density gas neighborhood. These results assume importance in the context of applications such as fast fuel ignition [6] or in medical applications of laser plasmas [7] where high irradiance of energetic electrons is of consequence. 1. D. Gustas et al., Phys. Rev. Accel. Beams, 21, 013401 (2018). 2. S. Feister et al , Opt. Express, 25, 18736 (2017). 3. B. S. Paradkar, S. I. Krasheninnikov, and F. N. Beg, Physics of Plasmas, 19, 060703 (2012). 4. A. P. L. Robinson, A. V. Areev, and D. Neely, Phys. Rev. Lett., 111, 065002 (2013). 5. R. Gopal, et al., Review of Scientific Instruments, 88, 023301 (2017). 6. M. Tabak et al., Physics of Plasmas, 1, 1626 (1994). 7. A. Sjogren, M. Harbst, C.-G. Wahlstrom, S. Svanberg, and C. Olsson, Review of Scientific Instruments, 74, 2300 (2003)

Femtosecond, two-dimensional spatial Doppler mapping of ultraintense laser-solid target interaction

We present measurements of the spatio-temporal evolution of a hot-dense plasma generated by the interaction of an intense 25 femtosecond laser pulse with a solid target, using pump-probe two-dimensional Doppler spectrometry. Measuring the time-dependent Doppler shifts at different positions across the probe beam, we achieve velocity mapping at hundreds of femtoseconds time resolution simultaneously with a few micrometer spatial resolution across the transverse length of the plasma. Simulations of the interaction using a combination of 2D particle-in-cell (PIC) and 2D radiation hydrodynamics codes agree well with the experiment

Tailored mesoscopic plasma accelerates electrons exploiting parametric instability

Laser plasma electron acceleration from the interaction of an intense femtosecond laser pulse with an isolated microparticle surrounded by a low-density gas is studied here. Experiments presented here show that optimized plasma tailoring by introducing a pre-pulse boosts parametric instabilities to produce MeV electron energies and generates electron temperatures as large as 200 keV with the total charge being as high as 350 fC/shot/sr, even at a laser intensity of a few times 1016 Wcm−2. Corroborated by particle-in-cell simulations, these measurements reveal that two plasmon decay in the vicinity of the microparticle is the main contributor to hot electron generation

Tailored mesoscopic plasma accelerates electrons exploiting parametric instability

Laser plasma electron acceleration from the interaction of an intense femtosecond laser pulse with an isolated microparticle surrounded by a low-density gas is studied here. Experiments presented here show that optimized plasma tailoring by introducing a pre-pulse boosts parametric instabilities to produce MeV electron energies and generates electron temperatures as large as 200 keV with the total charge being as high as 350 fC/shot/sr, even at a laser intensity of a few times 1016 Wcm−2. Corroborated by particle-in-cell simulations, these measurements reveal that two plasmon decay in the vicinity of the microparticle is the main contributor to hot electron generation

Shaped liquid drops generate MeV temperature electron beams with millijoule class laser

MeV temperature electrons are typically generated at laser intensities of 1018 W cm−2. Their generation at non-relativistic intensities (~1016 W cm−2) with high repetition rate lasers is cardinal for the realization of compact, ultra-fast electron sources. Here we report a technique of dynamic target structuring of micro-droplets using a 1 kHz, 25 fs, millijoule class laser, that uses two collinear laser pulses; the first to create a concave surface in the liquid drop and the second, to dynamically-drive electrostatic plasma waves that accelerate electrons to MeV energies. The acceleration mechanism, identified as two plasmon decay instability, is shown to generate two beams of electrons with hot electron temperature components of 200 keV and 1 MeV, respectively, at an intensity of 4 × 1016 Wcm−2, only. The electron beams are demonstrated to be ideal for single shot high resolution (tens of μm) electron radiography

Laser structured micro-targets generate MeV electron temperature at 4 x10^16 W/cm^2

Relativistic temperature electrons higher than 0.5 MeV are generated typically with laser intensities of about 10^18 W/cm^2. Their generation with high repetition rate lasers that operate at non-relativistic intensities (~10^16W/cm^2) is cardinal for the realization of compact, ultra-short, bench-top electron sources. New strategies, capable of exploiting different aspects of laser-plasma interaction, are necessary for reducing the required intensity. We report here, a novel technique of dynamic target structuring of microdroplets, capable of generating 200 keV and 1 MeV electron temperatures at 1/100th of the intensity required by ponderomotive scaling(10^18 W/cm^2) to generate relativistic electron temperature. Combining the concepts of pre-plasma tailoring, optimized scale length and micro-optics, this method achieves two-plasmon decay boosted electron acceleration with "non-ideal" ultrashort (25 fs) pulses at 4 x10^16 W/cm^2 only. With shot repeatability at kHz, this precise in-situ targetry produces directed, imaging quality beam-like electron emission up to 6 MeV with milli-joule class lasers, that can be transformational for time-resolved, microscopic studies in all fields of science

Shaped liquid drops generate MeV temperature electron beams with millijoule class laser

MeV temperature electrons are typically generated at laser intensities of 1018 W cm−2. Their generation at non-relativistic intensities (~1016 W cm−2) with high repetition rate lasers is cardinal for the realization of compact, ultra-fast electron sources. Here we report a technique of dynamic target structuring of micro-droplets using a 1 kHz, 25 fs, millijoule class laser, that uses two collinear laser pulses; the first to create a concave surface in the liquid drop and the second, to dynamically-drive electrostatic plasma waves that accelerate electrons to MeV energies. The acceleration mechanism, identified as two plasmon decay instability, is shown to generate two beams of electrons with hot electron temperature components of 200 keV and 1 MeV, respectively, at an intensity of 4 × 1016 Wcm−2, only. The electron beams are demonstrated to be ideal for single shot high resolution (tens of μm) electron radiography