An explanation for the energetic ions observed in the PetaWatt experiments is presented. In solid target experiments with focused intensities exceeding 1020 W/cm2, high-energy electron generation, hard bremsstrahlung, and energetic protons have been observed on the backside of the target. In this report, an attempt is made to explain the physical process present that will explain the presence of these energetic protons, as well as explain the number, energy, and angular spread of the protons observed in experiment. In particular, we hypothesize that hot electrons produced on the front of the target are sent through to the back off the target, where they ionize the hydrogen layer there. These ions are then accelerated by the hot electron cloud, to tens of MeV energies in distances of order tens of μm, whereupon they end up being detected in the radiographic and spectrographic detectors.

Energetic Proton Generation in Ultra-Intense Laser-solid Interactions

Experimental data [F. N. Beg et al., Phys. Plasmas 4, 447 (1997)] indicate that for intense short-pulse laser-solid interactions at intensities up to 5 10 Wcm 2 the hot-electron temperature /ðI 2Þ1=3. A fully relativistic analytic model based on energy and momentum conservation laws for the laser interaction with an overdense plasma is presented here. A general formula for the hot-electron temperature is found that closely agrees with the experimental scaling over the relevant intensity range. This scaling is much lower than ponderomotive scaling. Examination of the electron forward displacement compared to the collisionless skin depth shows that electrons experience only a fraction of a laser-light period before being accelerated forward beyond the laser light’s penetration region. Inclusion of backscattered light in a modified model indicates that light absorption approaches 80%–90% for intensity >10 Wcm .

The hot electron temperature and laser light absorption in fast ignition

Ion beam therapy for cancer has proven to be a successful clinical approach, affording as good a cure as surgery and a higher quality of life. However, the ion beam therapy installation is large and expensive, limiting its availability for public benefit. One of the hurdles is to make the accelerator more compact on the basis of conventional technology. Laser acceleration of ions represents a rapidly developing young field. The prevailing acceleration mechanism (known as target normal sheath acceleration, TNSA), however, shows severe limitations in some key elements. We now witness that a new regime of coherent acceleration of ions by laser (CAIL) has been studied to overcome many of these problems and accelerate protons and carbon ions to high energies with higher efficiencies. Emerging scaling laws indicate possible realization of an ion therapy facility with compact, cost-efficient lasers. Furthermore, dense particle bunches may allow the use of much higher collective fields, reducing the size of beam ...

Laser Acceleration of Ions for Radiation Therapy

Powerful terahertz (THz) radiation is observed from large-scale underdense preplasmas in front of a solid target irradiated obliquely with picosecond relativistic intense laser pulses. The radiation covers an extremely broad spectrum with about 70% of its energy located in the high frequency regime over 10 THz. The pulse energy of the radiation is found to be above $100\text{ }\text{ }\ensuremath{\mu}\mathrm{J}$ per steradian in the laser specular direction at an optimal preplasma scale length around $40--50\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. Particle-in-cell simulations indicate that the radiation is mainly produced by linear mode conversion from electron plasma waves, which are excited successively via stimulated Raman scattering instability and self-modulated laser wakefields during the laser propagation in the preplasma. This radiation can be used not only as a powerful source for applications, but also as a unique diagnostic of parametric instabilities of laser propagation in plasmas.

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Bursts of Terahertz Radiation from Large-Scale Plasmas Irradiated by Relativistic Picosecond Laser Pulses.

A theory for ion acceleration by ultrashort laser pulses is presented to evaluate the maximum ion energy in the interaction of ultrahigh contrast (UHC) intense laser pulses with a nanometer-scale foil. In this regime, the ion energy may be directly related to the laser intensity and subsequent electron dynamics. This leads to a simple analytical expression for the ion energy gain under the laser irradiation of thin targets. Significantly higher energies for thin targets than for thicker targets are predicted. The theory is concretized with a view to compare with the results and their details of recent experiments.

/pdf/theory-of-laser-ion-acceleration-from-a-foil-target-of-165177m2q2.pdf

Theory of laser ion acceleration from a foil target of nanometer thickness

Terahertz (THz) radiation has attracted much attention due to its wide potential applications. Though radiation can be generated with various ways, it is still a big challenge to obtain strong tabletop sources. Plasma, with the advantage of no damage limit, is a promising medium to generate strong THz radiation. This review reports recent advances on strong THz radiation generation from low-density gases and high-density solid targets at different laser intensities.

https://iopscience.iop.org/article/10.1088/1674-1056/21/9/095203/pdf

High power terahertz pulses generated in intense laser—plasma interactions

With the advent of ultrashort high intensity laser pulses, laser absorption during the laser?solid interactions has received significant attention over the last two decades since it is related to a variety of applications of high intensity lasers, including the hot electron production for fast ignition of fusion targets, table-top bright X-ray and gamma-ray sources, ion acceleration, compact neutron sources, and generally the creation of high energy density matters. Normally, some absorption mechanisms found for nanosecond long laser pulses also appear for ultrashort laser pulses. The peculiar aspects with ultrashort laser pulses are that their absorption depends significantly on the preplasma condition and the initial target structures. Meanwhile, relativistic nonlinearity and ponderomotive force associated with the laser pulses lead to new mechanisms or phenomena, which are usually not found with nanosecond long pulses. In this paper, we present an overview of the recent progress on the major absorption mechanisms in intense laser?solid interactions, where emphasis is paid to our related theory and simulation studies.

https://iopscience.iop.org/article/10.1088/1674-1056/24/1/015201/pdf

Absorption of ultrashort intense lasers in laser–solid interactions*

Acceleration of ions from ultrathin foils irradiated by intense circularly polarized laser pulses is investigated using a one-dimensional particle-in-cell code. As a circularly polarized laser wave heats the electrons much less efficiently than the wave of linear polarization, the ion can be synchronously accelerated and bunched by the electrostatic field, thus a monoenergetic and high intensity proton beam can be generated.

http://iopscience.iop.org/article/10.1088/0256-307X/25/9/063/pdf

Mono-Energetic Proton Beam Acceleration in Laser Foil-Plasma Interactions

Acceleration of protons by the radiation pressure of a circularly polarized laser pulse with the intensity up to 1021 W/cm2 from a double-layer or multi-ion-mixed thin foil is investigated by two-dimensional particle-in-cell simulations. The double-layer foil is composed of a heavy ion layer and a proton layer. It is found that the radiation pressure acceleration can be classified into three regimes according to the laser intensity due to the different critical intensities for laser transparency with different ion species. When the laser intensity is moderately high, the laser pushes the electrons neither so slowly nor so quickly that the protons can catch up with the electrons, while the heavy ions cannot. Therefore, the protons can be accelerated efficiently. The proton beam generated from the double-layer foil is of better quality and higher energy than that from a pure proton foil with the same areal electron density. When the laser intensity is relatively low, both the protons and heavy ions are accelerated together, which is not favorable to the proton acceleration. When the laser intensity is relatively high, neither the heavy ions nor the protons can be accelerated efficiently due to the laser transparency through the target.

Acceleration of Protons from a Double-Layer or Multi-Ion-Mixed Foil Irradiated by Ultraintense Lasers

Recent experiments have observed magnetic reconnection in laser-produced high-energy-density (HED) plasma bubbles. We perform two-dimensional (2-D) particle-in-cell (PIC) simulations to investigate magnetic reconnection between two approaching HED plasma bubbles. It is found that the expanding velocity of the bubbles has a great influence on the process of magnetic reconnection. When the expanding velocity is small, a single X line reconnection is formed. However, when the expanding velocity is sufficiently large, we can observe a plasmoid in the vicinity of the X line. At the same time, the structures of the electromagnetic field in HED plasma reconnection are similar to that in Harris current sheet reconnection.

http://iopscience.iop.org/article/10.1088/0256-307X/30/4/045201/pdf

Sheng Zheng-Ming

Papers

High power terahertz pulses generated in intense laser—plasma interactions

Absorption of ultrashort intense lasers in laser–solid interactions*

Mono-Energetic Proton Beam Acceleration in Laser Foil-Plasma Interactions

Acceleration of Protons from a Double-Layer or Multi-Ion-Mixed Foil Irradiated by Ultraintense Lasers

Particle-in-Cell Simulations of Fast Magnetic Reconnection in Laser-Plasma Interaction