Top PDF Intense laser-plasma interactions

Intense laser-plasma interactions

Intense laser-plasma interactions

Research on the interaction between strong laser fields and matter first began in the 1970s and was largely driven by its uses for atomic and molecu- lar physics and laser-induced nuclear fusion. As the available intensities grew it became apparent that it also holds great potential for particle accelera- tion, which had been suggested and studied in earlier theoretical works, some even preceding the invention of the laser. At intensities above 10 16 W/cm 2 the laser field is able to rival the Coulomb field of an atom, leading to rapid ionization of a target, eventually creating a plasma. While conventional ac- celerators must avoid such plasma formation from occurring, high-intensity lasers do not rely on the cavity walls for field generation, allowing us to cir- cumnavigating these restrictions. Through the interaction of intense laser fields with a plasma it is possible to both create and sustain acceleration gra- dients of 100 GV/m and above, greatly surpassing the limits of conventional accelerators. This significantly reduces the required acceleration distances and opens up the possibility for more compact and thereby cheaper acceler- ators.
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Review of Energetic Particle Generation and Electromagnetic Radiation from Intense Laser-Plasma Interactions at the Institute of Physics, Chinese Academy of Sciences

Review of Energetic Particle Generation and Electromagnetic Radiation from Intense Laser-Plasma Interactions at the Institute of Physics, Chinese Academy of Sciences

This review covers recent developments in multi-hundred-TW femtosecond laser systems and progress in intense laser-plasma interactions at the Institute of Physics, Chinese Academy of Sciences. The peak power of the institute’s Xtreme Light III (XL-III) laser system has been upgraded from 350 to 725 TW, and the system used to observe the lateral transport of fast electrons due to magnetic and electrostatic fields at target surfaces. The surface fields cause fast-electron beams to be emitted along the front and rear target surfaces. Confinement and guiding of fast-electron propagation is demonstrated with wire- or wedge-shaped targets. Longitudinal transport is detected by optical and ion emission. Numerical simulations show that quasi-monoenergetic ion beams can be generated by collisionless electrostatic shock acceleration and phase-stable acceleration with a circularly polarized laser field. A new mechanism for high-power THz emission with plasmas as media is proposed; it can be used to obtain a single-cycle THz emission. The conversion efficiency from laser energy to K α X-ray emission in a laser-copper target interaction is increased to 10 − 4 using high-contrast laser pulses.
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Study of backward terahertz radiation from intense picosecond laser-solid interactions using a multichannel calorimeter system

Study of backward terahertz radiation from intense picosecond laser-solid interactions using a multichannel calorimeter system

It has been found that the high-frequency component ( > 10 THz) is dominant in backward THz radiation due to LMC [7] . However, the CTR or SFE mainly accounts for the BTR at low frequencies ( < 3 THz) [15, 19] . Therefore, spectral measurement of the emitted THz wave is necessary to determine the BTR generation mechanism. However, the existing THz spectrum measurement technology is not ideally applicable for THz sources based on intense laserplasma interactions (ILPI). This is because the repetition rate of many such sources is quite low at present, typically below 1 Hz. Also, the fluctuations between shots can be large in real experiments. As a result, multi-shot scanning-based methods are quite inefficient and potentially inaccurate when used to characterize ILPI-based sources, such as electro- optic sampling [21] and autocorrelation measurement based on a Michelson interferometer [22] . Moreover, the working bandwidth of general electro-optic crystals also limits the application of electro-optic sampling techniques. The band- width of BTR generated in intense laserplasma interactions can reach tens of THz [6, 7] . However, the effective bandwidth is only 5.3 THz for ZnTe and 7 THz for GaP [23] . In addition, the existing frequency-domain methods also share this dis- advantage such as THz spectrometers based on prism [24] and consecutive dispersion grating [25] .
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Study of backward terahertz radiation from intense picosecond laser-solid interactions using a multichannel calorimeter system

Study of backward terahertz radiation from intense picosecond laser-solid interactions using a multichannel calorimeter system

It has been found that the high-frequency component (>10 THz) is dominant in backward THz radiation due to LMC [7] . However, the CTR or SFE mainly accounts for the BTR at low frequencies (<3 THz) [15, 19] . Therefore, spectral measurement of the emitted THz wave is necessary to determine the BTR generation mechanism. However, the existing THz spectrum measurement technology is not ideally applicable for THz sources based on intense laserplasma interactions (ILPI). This is because the repetition rate of many such sources is quite low at present, typically below 1 Hz. Also, the fluctuations between shots can be large in real experiments. As a result, multi-shot scanning-based methods are quite inefficient and potentially inaccurate when used to characterize ILPI-based sources, such as electro- optic sampling [21] and autocorrelation measurement based on a Michelson interferometer [22] . Moreover, the working bandwidth of general electro-optic crystals also limits the application of electro-optic sampling techniques. The band- width of BTR generated in intense laserplasma interactions can reach tens of THz [6, 7] . However, the effective bandwidth is only 5.3 THz for ZnTe and 7 THz for GaP [23] . In addition, the existing frequency-domain methods also share this dis- advantage such as THz spectrometers based on prism [24] and consecutive dispersion grating [25] .
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Probing ultrafast dynamics of solid-density plasma generated by high-contrast intense laser pulses

Probing ultrafast dynamics of solid-density plasma generated by high-contrast intense laser pulses

cidence. The peak intensity was ∼ 4 × 10 18 W/cm 2 . A motorized high-precision stage was used to move the target so that every laser pulse interacted with a fresh position on the target. A small fraction of the main laser beam was extracted and up-converted to its second har- monic (400 nm) by β-barium borate (BBO) crystal and used as the probe pulse. A BG-39 filter was used after the BBO crystal to remove the residual 800 nm light in the probe. The probe was focused onto the plasma at normal incidence to a 120 µm focal spot (FWHM). The probe intensity was kept low ( ∼ 4 × 10 10 W/cm 2 ) so that it did not ionize the target. Spatial overlap between the pump and the probe beams was ensured by high res- olution imaging. The probe beam was time-delayed with respect to the pump beam using a high precision mo- torized retro-reflector delay stage. The reflected probe was collected by a lens and then split into two parts. One part went to a high-resolution (0.35˚ A) spectrome- ter (OOI, HR-2000) to record a spectrum at each time delay. The other part of the beam was fed to a photo- diode (PD) which measured probe reflectivity at each time delay. Temporal overlap between the pump and the probe beams was monitored by observing probe reflec- tivity as function of probe time-delay. When the probe arrives at the target surface before the pump (negative time delay), it is reflected from a cold target and gives high reflectivity. If the probe arrives simultaneously with pump beam, it is then reflected from plasma created by pump and there is a reduction in probe reflectivity due to plasma absorption. The delay at which there is a sudden transition in probe reflectivity is called time zero.
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Ionisation effects for laser plasma interactions by particle in cell code

Ionisation effects for laser plasma interactions by particle in cell code

The effect of ionisation in laser-plasma interactions is summarised by its three main mechanisms in Chapter 2; ionisation-induced defocussing, fast shuttering, and ion- isation injection. Simulations with field ionisation were performed for the first; the total divergence caused by the combined effects of Gaussian divergence and defocussing of the beam by ionisation was measured and found to be in good quantitative agreement with theory [70]. Fast shuttering was investigated in the context of plasma mirrors; due to the solid densities required both collisional and field ionisation were used in simu- lation. This was performed for both simple hydrogen mirrors and also glass; the laser profile from the Astra-Gemini laser at the Central Laser Facility so as to compare results with their published work [7]. Rapid switch-on of the mirror was observed for hydro- gen and glass; in the latter the contrast ratio produced was found to agree with the theoretical maximum when taking into consideration the cold reflectivity and neglecting collisions. The fast-shuttering effect was found to be relatively material independent. It was also found that the switch-on time increased with the inclusion of collisions; this was attributed to the increasing laser absorption as the electron plasma density rose to critical, which had the effect of lowering the ionisation rate. In the case of glass, it was found that the amount of laser absorption became unreasonably high ∼ 60% and the electrons exhibited excessive heating that appeared unrelated to the resolution of the Debye length, an instability discussed in § 1.3.5. This was found to occur in the absence of collisional ionisation, and through further testing the issue was found to be due to the collision module implemented in EPOCH; it is expected that the issue will become resolved following submission of this thesis.
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Intra-pulse transition between ion acceleration mechanisms in intense laser-foil interactions

Intra-pulse transition between ion acceleration mechanisms in intense laser-foil interactions

We acknowledge the support of Central Laser Facility staff and the use of the ARCHIE- WeST and ARCHER computers. This work is supported by EPSRC (grants: EP/J003832/1, EP/L001357/1, EP/K022415/1, EP/M018091/1 and EP/L000237/1), the US Air Force Office of Scientific Research (grant: FA8655-13-1-3008) and the European Unions Hori- zon 2020 research and innovation program (grant agreement No 654148 Laserlab-Europe). LCS acknowledges the EU-funded LA3NET consortium (grant: GA-ITN-2011-289191) and XHY acknowledges support from the National Basic Research Program of China (grant 2013CBA01502). EPOCH was developed under EPSRC grant EP/G054940/1. Data asso- ciated with research published in this paper is accessible at [insert DOI].
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Pulse Expansion and Doppler Shift of Ultrahigh Intense Short Pulse Laser by Slightly Overdense Plasma

Pulse Expansion and Doppler Shift of Ultrahigh Intense Short Pulse Laser by Slightly Overdense Plasma

The interactions between ultrahigh intense laser and overdense plasmas were investigated by the use of a 1-1 / 2 dimensional electromagnetic relativistic particle-in-cell code, EMPAC. When the e ff ective electron plasma frequency is reduced below the laser frequency by increasing the inertial electron mass due to the relativistic effect, the ultrahigh intense short pulse laser can penetrate the overdense plasma, but is completely reflected after propagating to a certain extent, except for a portion of the absorbed laser. The pulse length of the reflected laser is expanded more than that of the incident laser by a modulation due to the anomalous penetration, and the pulse expansion factor can be predicted by the schematic model. The frequency of the reflected laser can be calculated by the Doppler shift formula coupled with a relativistic dispersion relation, and is good agreement with the simulation result. The anomalously penetrating pulse shows soliton-like behaviors in the plasma after the incident laser has vanished.
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Role of magnetic field evolution on filamentary structure formation in intense laser-foil interactions

Role of magnetic field evolution on filamentary structure formation in intense laser-foil interactions

the temporal laser intensity profile. At the same time, the TNSA mechanism occurs, causing the rear surface proton population to expand away from the forming magnetic field structures. This results in increased filamentary behaviour in the proton beam for the fastest magnetic field formation and least degree of proton layer expansion. These proton density structures are observed in both simulation and experiment, and are seen to vary when the laser intensity profile in time is modified via the use of two temporally separated laser pulses of differing intensities. This enables additional pro- ton acceleration through RSIT mechanisms allowing these structures to be observed, while enabling the magnetic field formation and initial proton expansion to be varied at lower intensities. By understanding this interplay, we demonstrate fundamental insight into the sensitivity of laser-accelerated proton structures to the temporal laser intensity profile.
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Effects of relativistic electron temperature on parametric instabilities for intense laser propagation in underdense plasma

Effects of relativistic electron temperature on parametric instabilities for intense laser propagation in underdense plasma

The effects of relativistic plasma electron temperature on SRS and SBS instabilities have been investigated theoretically and numerically. The dispersion relations for these instabili- ties are obtained using the relativistic thermal electron fluid model, which uses the kinetic theory and assumes the J¨uttner- Synge distribution for relativistic hot electrons. The maxi- mum growth rates for these instabilities are obtained, which recover the cold plasma case at temperature close to zero. Ac- cording to the dispersion relations, it is found that the relativis- tic electron temperature can reduce the instability region and the growth rates for both SRS and SBS instabilities. More- over, it decreases the frequency of backscattering light.
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Laminar shocks in high power laser plasma interactions

Laminar shocks in high power laser plasma interactions

If the Sagdeev potential was the same downstream of the point where the potential reaches its maximum then we would just get a standard solitary wave solution, symmetric about this maximum. However, in the downstream region there is no reflected component and the second term in (1) is absent. This changes the Sagdeev potential and in the downstream region the notional particle motion which determines the solution is oscillation in a potential well. A composite solution can be obtained by starting at the maximum, with zero potential gradient and integrating upstream and downstream with the appropriate charge densities in (3). The result is shown in Figure 2, with the distance x in units of the ion thermal velocity divided by the ion plasma frequency.
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Short pulse laser interactions with high density plasma

Short pulse laser interactions with high density plasma

The constraints on particle-in-cell (PIC) simulations of short-pulse laser interactions with solid density targets severely limit the spatial and temporal scales which can be modelled routinely. Although recent advances in high performance computing (HPC) capabilities have rendered collisionless simulations at a scale and density directly applicable to experiments tractable, detailed modelling of the fast electron transport resulting from the laser interaction is often only possible by sampling the fast electron populations and passing this information to a separate, dedicated transport code. However, this approach potentially neglects phenomena which take place or are seeded near the transition between the two codes. Consequently there is a need to develop techniques capable of efficiently modelling fast electron transport in high density plasma without being subject to the usual grid-scale and time-step constraints. The approach employed must also be compatible with retaining the standard PIC model in the laser interaction regions in order to model laser absorption and charged particle acceleration processes. Such an approach, proposed by Cohen, Kemp and Divol [J. Comput. Phys., 229:4591, 2010], has been identified, adapted and implemented in EPOCH. The final algorithm, as implemented, is presented here. To demonstrate the ability of the adapted code to model high intensity laser-plasma interactions with peak densities at, and above, solid density, the results of simulations investigating filamentation of the fast electron population and heating of the bulk target, at high densities, are presented and compared with the results of recent experiments as well as other, similar codes.
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Measurement of the angle, temperature and flux of fast electrons emitted from intense laser-solid interactions

Measurement of the angle, temperature and flux of fast electrons emitted from intense laser-solid interactions

Hatchett, Stephen P., Brown, Curtis G., Cowan, Thomas E., Henry, Eugene A., Johnson, Joy S., Key, Michael H., Koch, Jeffrey A., Langdon, A. Bruce, Lasinski, Barbara F., Lee, Richard W., Mackinnon, Andrew J., Pennington, Deanna M., Perry, Michael D., Phillips, Thomas W., Roth, Markus, Sangster, T. Craig, Singh, Mike S., Snavely, Richard A., Stoyer, Mark A., Wilks, Scott C. & Yasuike, Kazuhito 2000 Electron, photon, and ion beams from the relativistic inter- action of Petawatt laser pulses with solid targets. Physics of Plasmas 7 (5), 2076–2082. Link, A, Freeman, R. R., Schumacher, D. W. & Van Woerkom, L. D. 2011 Effects of
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Effects of target pre-heating and expansion on terahertz radiation production from intense laser-solid interactions

Effects of target pre-heating and expansion on terahertz radiation production from intense laser-solid interactions

account of the diagnostic acceptance solid angle and the attenuations of the lens, window, and filters). The shot-to- shot reproducibility of the signal was checked over a series of laser shots on the same target type, and it showed a variation of <10%. The inset of Figure 2 shows a crude THz emission spectrum for the B7-only case. The central frequency for each point is the average of adjacent cutoff frequencies of the low-pass edge filters, and the horizontal error bars are obtained from their differentiation. The amplitude of the signal is plotted at the detector voltage per unit frequency (in THz). The vertical error bars reflect the degree of shot- to-shot fluctuation and account for systematic errors. The
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The effect of phase front deformation on the growth of the filamentation instability in laser–plasma interactions

The effect of phase front deformation on the growth of the filamentation instability in laser–plasma interactions

In fast ignition inertial fusion [23], one can choose between several options to deliver the high-intensity ignitor laser pulse securely to the compressed targets. Firstly, the ignitor pulse can be guided to the target core by a cone insert. Good results have been obtained through this method [24], but there are several important drawbacks. The cone material needs to have a large mass density, and high-Z materials are usually employed, but these lead to large energy losses via bremsstrahlung emission and there is the potential for contamination of the fusion fuel with high-Z matter ablated during the compression stage. An alternative method is to use a ∼ 100 ps relativistic intensity laser pulse to drill a channel through the outer layers of tenuous plasma surrounding the compressed core. Once a fairly clean, straight channel has been created this way [25], the ignitor pulse can be launched down this channel, and be guided straight to the compressed core. This removes the need to use high-Z components in the target completely, and also solves the issue of target alignment, as the channel is only drilled after the target has been positioned and the target itself can be spherically symmetric.
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Self-Guided Intense Laser Pulse Propagation in Air

Self-Guided Intense Laser Pulse Propagation in Air

We believe that we observe self-phase modulation under conditions when the self-focusing due to Kerr nonlinearity of gas molecules is compensated by that of plasma created by the intense laser pulse. The optimal focusing is aimed to maximize the product of intensity and interaction length. In this connection we also note that the initial wavelength is of great importance since the Rayleigh range of the focused beam is inversely proportional to the wavelength. In our experiments we detected only slight

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Neutron production by fast protons from ultraintense laser-plasma interactions

Neutron production by fast protons from ultraintense laser-plasma interactions

FIG. 3. Proton energy spectra measured at the front and back of a 20- ␮ m-thick aluminium target foil irradiated with a 230-J laser pulse. The spectra were deduced from observed proton-induced reactions in zinc samples and are in good agreement with proton energy spectra measured with other VULCAN petawatt laser shots and using nuclear activation of Cu foil stacks (see Refs. 9 and 20). The total conversion efficiency from laser- pulse energy to proton acceleration (in both directions) is ⬃ 13%. This was calculated by integrating the spectrum in the figure to determine the total energy in the ion beam and dividing this by the energy in the laser shot. The 13% value is typical for large, high-energy pulse laser (see Ref. 4).
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Plasma optical modulators for intense lasers

Plasma optical modulators for intense lasers

O ptical modulators are key components for manipulating optical signals, which are widely used in scientific and industrial applications. For example, high-speed compact electro-optic modulators (EOMs) are essential for data commu- nications 1–4 . EOMs can alter the fundamental characteristics (that is, amplitude, frequency, phase and polarization) of a light beam in a controllable manner, by making use of electro-optic effects to change the refractive index of a material when an external radio-frequency electric field driver is applied. Thanks to the rapid development of the field of radio-frequency photonics 5,6 together with advanced material and microfabrication techno- logies 3,4,7 , the modulation speed of EOMs has dramatically increased from megahertz to 100 gigahertz (refs 8–10) over the past decade. However, it is still very challenging to successfully achieve terahertz (THz) speed for EOMs using current technologies due to the speed limitation of the high-voltage driver 1 . On the other hand, the rather low optical damage threshold 11,12 and low bulk laser damage threshold of EOMs 13 severely limit their applications in the high-intensity regime. For example, a state-of-the-art commercially available magnesium-oxide-doped LiNbO 3 modulator can only handle
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Angle-dependent modulated spectral peaks of proton beams generated in ultrashort intense laser-solid interactions

Angle-dependent modulated spectral peaks of proton beams generated in ultrashort intense laser-solid interactions

The experiment was conducted on the Xtreme Light (XL) Ti: sapphire laser facility at the Institute of Physics, Chinese Academy of Sciences 16 . The schematic setup is shown in Fig 1. A p-polarized laser pulse of 100 fs pulse duration, 1~2.5 J energy and 800 nm central wavelength was focused using an f/1.67 off-axis parabola (OAP) mirror onto target foils at an incidence angle of 15 °. The laser energy on target after compression was from 1 to 2.5 J. The full width at half maximum (FWHM) of the laser focal spot was 8 μm, giving a maximum laser intensity of 5×10 19 W/cm 2 . The contrast ratio at 7 nanoseconds before
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Bremsstrahlung emission profile from intense laser-solid interactions as a function of laser focal spot size

Bremsstrahlung emission profile from intense laser-solid interactions as a function of laser focal spot size

The x-ray spatial profile from a high intensity laser-solid interaction results in a dual-source structure. The central channel dominated by fast electrons on their first pass through the target and a larger substrate source from recirculating electrons spreading laterally during their multiple passes through the target. Experimental results demonstrate a ×(10 ± 2) increase in the ratio of the central source to the substrate source, suggesting a way to increase the quality of x-ray radiographs by optimising this ratio and the flux. Through analytical modelling we are able to probe this rela- tionship as a ratio between escaping, attenuated, and recir- culating electrons and present optimum laser intensities for varying target parameters. The model provides good agree- ment with the experimental data, in terms of both the ratio between central and substrate x-ray flux and the total central x-ray flux produced.
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