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2021
Volume: 6 Issue 4
8 Article(s)
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Fundamental Physics at Extreme Light
Investigation of magnetic inhibition effect on ion acceleration at high laser intensities
H. Huang, Z. M. Zhang, B. Zhang, W. Hong... and W. M. Zhou|Show fewer author(s)
The irradiation of a target with high laser intensity can lead to self-generation of an intense magnetic field (B-field) on the target surface. It has therefore been suggested that the sheath-driven acceleration of high-energy protons would be significantly hampered by the magnetization effect of this self-generated B-The irradiation of a target with high laser intensity can lead to self-generation of an intense magnetic field (B-field) on the target surface. It has therefore been suggested that the sheath-driven acceleration of high-energy protons would be significantly hampered by the magnetization effect of this self-generated B-field at high enough laser intensities. In this paper, particle-in-cell simulations are used to study this magnetization effect on sheath-driven proton acceleration. It is shown that the inhibitory effect of the B-field on ion acceleration is not as significant as previously thought. Moreover, it is shown that the magnetization effect plays a relatively limited role in high-energy proton acceleration, even at high laser intensities when the mutual coupling and competition between self-generated electric (E-) and B-fields are considered in a realistic sheath acceleration scenario. A theoretical model including the v × B force is presented and confirms that the rate of reduction in proton energy depends on the strength ratio between B- and E-fields rather than on the strength of the B-field alone, and that only a small percentage of the proton energy is affected by the self-generated B-field. Finally, it is shown that the degraded scaling of proton energy at high laser intensities can be explained by the decrease in acceleration time caused by the increased sheath fields at high laser intensities rather than by the magnetic inhibitory effect, because of the longer growth time scale of the latter. This understanding of the magnetization effect may pave the way to the generation of high-energy protons by sheath-driven acceleration at high laser intensities..
Matter and Radiation at Extremes
- Publication Date: Jul. 28, 2021
- Vol. 6, Issue 4, 044401 (2021)
High Pressure Physics and Materials Science
Bright betatron radiation from direct-laser-accelerated electrons at moderate relativistic laser intensity
O. N. Rosmej, X. F. Shen, A. Pukhov, L. Antonelli... and N. E. Andreev|Show fewer author(s)
Direct laser acceleration (DLA) of electrons in a plasma of near-critical electron density (NCD) and the associated synchrotron-like radiation are discussed for moderate relativistic laser intensity (normalized laser amplitude a0 ≤ 4.3) and ps length pulse. This regime is typical of kJ PW-class laser facilities designeDirect laser acceleration (DLA) of electrons in a plasma of near-critical electron density (NCD) and the associated synchrotron-like radiation are discussed for moderate relativistic laser intensity (normalized laser amplitude a0 ≤ 4.3) and ps length pulse. This regime is typical of kJ PW-class laser facilities designed for high-energy-density (HED) research. In experiments at the PHELIX facility, it has been demonstrated that interaction of a 1019 W/cm2 sub-ps laser pulse with a sub-mm length NCD plasma results in the generation of high-current well-directed super-ponderomotive electrons with an effective temperature ten times higher than the ponderomotive potential [Rosmej et al., Plasma Phys. Controlled Fusion 62 , 115024 (2020)]. Three-dimensional particle-in-cell simulations provide good agreement with the measured electron energy distribution and are used in the current work to study synchrotron radiation from the DLA-accelerated electrons. The resulting x-ray spectrum with a critical energy of 5 keV reveals an ultrahigh photon number of 7 × 1011 in the 1–30 keV photon energy range at the focused laser energy of 20 J. Numerical simulations of betatron x-ray phase contrast imaging based on the DLA process for the parameters of a PHELIX laser are presented. The results are of interest for applications in HED experiments, which require a ps x-ray pulse and a high photon flux..
Matter and Radiation at Extremes
- Publication Date: Jul. 28, 2021
- Vol. 6, Issue 4, 048401 (2021)
Inertial Confinement Fusion Physics
First results from a 760-GW linear transformer driver module for Z-pinch research
Lin Chen, Wenkang Zou, Jihao Jiang, Liangji Zhou... and Yuanjun Zhang|Show fewer author(s)
In this paper, the results of tests on a 0.76-TW linear transformer driver (LTD) module for Z-pinch research are presented for the first time. Ten LTD cavities, each generating a 1-MA/90-kV pulse on a matched load, were connected in series with a magnetically insulated voltage adder to drive the e-beam diode. Three innIn this paper, the results of tests on a 0.76-TW linear transformer driver (LTD) module for Z-pinch research are presented for the first time. Ten LTD cavities, each generating a 1-MA/90-kV pulse on a matched load, were connected in series with a magnetically insulated voltage adder to drive the e-beam diode. Three inner stalks with different radii were tested, and the results indicate that the output parameters of the ten cavities are sensitive to the cathode radii. As an intermediate step, a high-current pulse with 832 kV/912 kA/130 ns was obtained on the e-beam diode. To date, this is the maximum power generated directly by a fast LTD with mega-ampere current output..
Matter and Radiation at Extremes
- Publication Date: Jul. 28, 2021
- Vol. 6, Issue 4, 045901 (2021)
Beam wavefront retrieval by convoluted spatial spectral benchmark
Xuewei Deng, Xiaoxia Huang, Deen Wang, Ying Yang... and Dongxia Hu|Show fewer author(s)
We propose a method for retrieving a beam wavefront from its near-field intensity distribution after a 4f system by simply inserting a benchmark at the Fourier plane. Through a convolution of the mark-blocked spatial frequency component and the original optical field with the 4f system, the separation between the focusWe propose a method for retrieving a beam wavefront from its near-field intensity distribution after a 4f system by simply inserting a benchmark at the Fourier plane. Through a convolution of the mark-blocked spatial frequency component and the original optical field with the 4f system, the separation between the focus of any sub-aperture and the benchmark can be determined to reconstruct the beam wavefront. Theoretical and experimental studies demonstrate the validity of this method, which has potential applications in real-time wavefront sensing..
Matter and Radiation at Extremes
- Publication Date: Jul. 28, 2021
- Vol. 6, Issue 4, 045902 (2021)
Radiation and Hydrodynamics
Generation of strong magnetic fields for magnetized plasma experiments at the 1-MA pulsed power machine
V. V. Ivanov, A. V. Maximov, R. Betti, L. S. Leal... and N. A. Huerta|Show fewer author(s)
Pulsed power technology provides a platform for investigating plasmas in strong magnetic fields using a university-scale machine. Presented here are methods for generating and measuring the 1–4-MG magnetic fields developed for the 1-MA Zebra pulsed power generator at the University of Nevada, Reno. A laser coupled withPulsed power technology provides a platform for investigating plasmas in strong magnetic fields using a university-scale machine. Presented here are methods for generating and measuring the 1–4-MG magnetic fields developed for the 1-MA Zebra pulsed power generator at the University of Nevada, Reno. A laser coupled with the Zebra generator produces a magnetized plasma, and experiments investigate how a megagauss magnetic field affects the two-plasmon decay and the expansion of the laser-produced plasma in both transverse and longitudinal magnetic fields..
Matter and Radiation at Extremes
- Publication Date: Jul. 28, 2021
- Vol. 6, Issue 4, 046901 (2021)
Free-surface velocity measurements of opaque materials in laser-driven shock-wave experiments using photonic Doppler velocimetry
N. Nissim, E. Greenberg, M. Werdiger, Y. Horowitz... and S. Eliezer|Show fewer author(s)
We present a novel photonic Doppler velocimetry (PDV) design for laser-driven shock-wave experiments. This PDV design is intended to provide the capability of measuring the free-surface velocity of shocked opaque materials in the terapascal range. We present measurements of the free-surface velocity of gold for as longWe present a novel photonic Doppler velocimetry (PDV) design for laser-driven shock-wave experiments. This PDV design is intended to provide the capability of measuring the free-surface velocity of shocked opaque materials in the terapascal range. We present measurements of the free-surface velocity of gold for as long as ∼2 ns from the shock breakout, at pressures of up to ∼7 Mbar and a free-surface velocity of 7.3 km/s with an error of ∼1.5%. Such laboratory pressure conditions are achieved predominantly at high-intensity laser facilities where the only velocity diagnostic is usually line-imaging velocity interferometry for any reflector. However, that diagnostic is limited by the lower dynamic range of the streak camera (at a temporal resolution relevant to laser shock experiments) to measure the free-surface velocity of opaque materials up to pressures of only ∼1 Mbar. We expect the proposed PDV design to allow the free-surface velocity of opaque materials to be measured at much higher pressures..
Matter and Radiation at Extremes
- Publication Date: Jul. 28, 2021
- Vol. 6, Issue 4, 046902 (2021)
Fabrication of ZnO-nanowire-coated thin-foil targets for ultra-high intensity laser interaction experiments
D. Calestani, M. Villani, G. Cristoforetti, F. Brandi... and L. A. Gizzi|Show fewer author(s)
The coupling of ultra-intense, ultra-short laser pulses with solid targets is heavily dependent on the properties of the vacuum–solid interface and is usually quite low. However, laser absorption can be enhanced via micro or nanopatterning of the target surface. Depending on the laser features and target geometry, condThe coupling of ultra-intense, ultra-short laser pulses with solid targets is heavily dependent on the properties of the vacuum–solid interface and is usually quite low. However, laser absorption can be enhanced via micro or nanopatterning of the target surface. Depending on the laser features and target geometry, conditions can be optimized for the generation of hot dense matter, which can be used to produce high-brightness radiation sources or even to accelerate particles to relativistic energies. In this context, ZnO nanowires were grown on metallic, thin-foil targets. The use of a thin-foil substrate was dictated by the need to achieve proton acceleration via target normal sheath acceleration at the rear side. The chemical process parameters were studied in-depth to provide control over the nanowire size, shape, and distribution. Moreover, the manufacturing process was optimized to provide accurate reproducibility of key parameters in the widest possible range and good homogeneity across the entire foil area..
Matter and Radiation at Extremes
- Publication Date: Jul. 28, 2021
- Vol. 6, Issue 4, 046903 (2021)
Proton radiography in background magnetic fields
C. Arran, C. P. Ridgers, and N. C. Woolsey
Proton radiography has proved increasingly successful as a diagnostic for electric and magnetic fields in high-energy-density physics experiments. Most experiments use target-normal sheath acceleration sources with a wide energy range in the proton beam, since the velocity spread can help differentiate between electricProton radiography has proved increasingly successful as a diagnostic for electric and magnetic fields in high-energy-density physics experiments. Most experiments use target-normal sheath acceleration sources with a wide energy range in the proton beam, since the velocity spread can help differentiate between electric and magnetic fields and provide time histories in a single shot. However, in magnetized plasma experiments with strong background fields, the broadband proton spectrum leads to velocity-spread-dependent displacement of the beam and significant blurring of the radiograph. We describe the origins of this blurring and show how it can be removed from experimental measurements, and we outline the conditions under which such deconvolutions are successful. As an example, we apply this method to a magnetized plasma experiment that used a background magnetic field of 3 T and in which the strong displacement and energy spread of the proton beam reduced the spatial resolution from tens of micrometers to a few millimeters. Application of the deconvolution procedure accurately recovers radiographs with resolutions better than 100 µm, enabling the recovery of more accurate estimates of the path-integrated magnetic field. This work extends accurate proton radiography to a class of experiments with significant background magnetic fields, particularly those experiments with an applied external magnetic field..
Matter and Radiation at Extremes
- Publication Date: Jul. 28, 2021
- Vol. 6, Issue 4, 046904 (2021)
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