High-intensity laser-solidinteractions generate relativistic electrons, as well as high- energy (multi-MeV) ions and X-rays. The directionality, spectra and total number of electrons that escape a target-foil is dependent on the absorption, transport and rear side sheath conditions. Measuring the electrons escaping the target will aid in improv- ing our understanding of these absorption processes and the rear-surface sheath fields that retard the escaping electrons and accelerate ions via the Target Normal Sheath Ac- celeration (TNSA) mechanism. A comprehensive Geant4 study was performed to help analyse measurements made with a wrap-around diagnostic that surrounds the target and uses differential filtering with a FUJI-film image plate detector. The contribution of secondary sources such as x-rays and protons to the measured signal have been taken into account to aid in the retrieval of the electron signal. Angular and spectral data from a high-intensity laser-solid interaction are presented and accompanied by simulations. The total number of emitted electrons has been measured as 2.6 × 10 13 with an estimated
The study of electron transport through dense plasmas is an active area of research of importance to many applications. Relativistic electrons generated in high-intensity laser–solidinteractions can pass through the critical surface, where the laser energy is mostly absorbed, and continue, free from the influence of the laser, through to the rear surface of the target. The propagation of this large current of electrons can be affected by self-generated electric and magnetic fields, as well as collisions. A better understanding of this transport is a key issue for the success of the fast ignitor approach to inertial confinement fusion [1, 2] and may lead to optimization of ion acceleration from laser-irradiated solid targets [3, 4].
The bremsstrahlung x-ray emission proﬁle from high intensity laser-solidinteractions provides valuable insight to the internal fast electron transport. Using penumbral imaging, we characterise the spatial proﬁle of this bremsstrahlung source as a function of laser intensity by incrementally increasing the laser focal spot size on target. The experimental data shows a dual-source structure; one from the central channel of electrons, the second a larger substrate source from the recirculating electron current. The results demonstrate than an order of magnitude improvement in the intensity contrast between the two x-ray sources is achieved with a large focal spot, indicating preferable conditions for applications in radiography. An analytical model is derived to describe the transport of suprathermal electron populations that contribute to substrate and central channel sources through a target. The model is in good agreement with the experimental results presented here and furthermore is applied to predict laser intensities for achieving optimum spatial contrast for a variety of target materials and thicknesses.
After the pulse compressor and diagnostic station three flat turning mirrors, A, B, and C in Fig. 4, were used to steer the 790 nm laser pulse within the target chamber towards the focusing optic. The first of these 共 A 兲 had a dielectric coating to facilitate transmission of a 532 nm beam used in a retro- focus arrangement described in Sec. III. The remaining two mirrors were gold coated. A f /3, 15.1 cm focal length off- axis, parabolic mirror was used to focus the beam onto tar- get. Due to the quantity of debris that can be generated in high-repetition rate laser–solidinteractions it was necessary to shield the gold coating on this optic with a transparent plastic pellicle. The pellicle is 10 m thick and therefore has a low B integral and does not effect the overall focus quality. To determine the size and quality of the laser focal spot a microscope objective was used to image it onto a change- coupled device 共 CCD 兲 camera. The spatial resolution of the system has been calibrated to measure the focal spot size. The beam energy was attenuated to facilitate this measure- ment. Figure 5 shows a typical image, where best focus cor- responded to a FWHM spot size of 2.5 m, which is almost diffraction limited. 41% of the total energy on target was determined to be contained within FWHM of this spot. The maximum energy on target was 225 mJ, as determined with a calorimeter positioned in the beam path just in front of the focusing optic. Using these pulse parameters, the focused intensity within FWHM of the main focal spot is calculated to be 3.2 ⫻ 10 19 W cm ⫺ 2 .
Relativistic electron beams can also be generated in the interactions of intense laser pulses with low-density gas or high-density solid targets. For gas targets, electrons can be accelerated to a GeV energy level with wakefields [11,12]. With such an electron beam, Leemans et al. have observed a ~0.3 μJ THz pulse through transition radiation . Strong THz radiation from laser-solidinteractions has also been demonstrated [14,15,16,17]. Compared with the cases of gas targets, electron beams from solid targets have much higher charge, up to nC-μC. Recently THz radiation with energies of >700 μJ has been reported from the rear surface of a foil target [18,19]. Since the THz yield is found to be correlated with the square of the proton number, it is attributed to a transient dipole-like charge distribution structure formed by target normal sheath acceleration (TNSA), referred to hereafter as TNSA radiation . On the other hand, in principle, THz radiation can be produced efficiently via the transition radiation process at the rear surface of a laser-irradiated thin solid foil , since both the short electron bunch duration and high bunch charge are ideal for this mechanism. Previous studies on the transition radiation from solid targets mainly concern in the optical region [21,22]. While the radiation in the THz regime has so far not yet been verified experimentally.
The bremsstrahlung x-ray emission pro ﬁ le from high intensity laser-solidinteractions provides valuable insight to the internal fast electron transport. Using penumbral imaging, we characterise the spatial pro ﬁ le of this bremsstrahlung source as a function of laser intensity by incrementally increasing the laser focal spot size on target. The experimental data shows a dual-source structure; one from the central channel of electrons, the second a larger substrate source from the recirculating electron current. The results demonstrate than an order of magnitude improvement in the intensity contrast between the two x-ray sources is achieved with a large focal spot, indicating preferable conditions for applications in radiography. An analytical model is derived to describe the transport of suprathermal electron populations that contribute to substrate and central channel sources through a target. The model is in good agreement with the experimental results presented here and furthermore is applied to predict laser intensities for achieving optimum spatial contrast for a variety of target materials and thicknesses.
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
A simple multichannel calorimeter system is designed and constructed, which consists of HRFZ-Si THz beam splitters, THz band pass filters and pyroelectric detectors. It provides a convenient single-shot solution to characterize the spectrum of broad-band THz radiation generated in an intense laser– plasma interaction. Spectral measurements of backward THz radiation are performed using the multichannel calorime- ter system in an intense picosecond laser–solid interaction experiment. The low-frequency component ( < 1 THz) is dominant for the BTR measured in the experiment. The BTR is attributed to coherent transition radiation by laser- accelerated fast electrons as they cross the plasma–vacuum surface. The dependence of BTR energy and spectrum on the target thickness indicates that CTR by refluxing fast electrons contributes to the BTR. The LMC mechanism starts to work with a large-scale pre-plasma, which enhances the intensity of the high-frequency components ( > 3 THz) of the signal.
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
surface. The experiment is performed at the Vulcan laser facility  of the Rutherford Appleton Laboratory, UK. A 1053 nm laser pulse with energy up to 100 J and duration 1 ps is best focused onto copper foil targets at an intensity of 10 20 W/cm 2 at an incident angle of 30 ◦ , as shown in Figure 3(a). THz emission generated from the front surface of the target is collected by a 50 mm diameter TPX lens placed at an angle of 60 ◦ from the target normal direction. The beam is reduced to 25 mm diameter using two off-axis parabolic mirrors with focal length 200 mm and 100 mm, respectively. The reduced beam is guided into the eight-channel calorimeter through a 50 mm TPX window. The transmission of the TPX lens and window is also measured with the FTIR spectrometer. In one of the eight channels, none filter was added so that the total energy of the THz radiation could be monitored. The remaining seven channels are equipped with band pass filters whose central frequencies are 0.5 THz, 1 THz, 1.5 THz, 3 THz, 6 THz, 10 THz and 20 THz, respectively. To avoid any disturbance of the calorimeters by pickup of electromagnetic pulses [34, 35] and high-energy ionizing radiation such as X-rays from the laser–plasma interaction, the calorimeter system is well shielded with a metal box and lead sheets.
in the front end of the laser, offering an intensity contrast ratio of 10 9 on the nanosecond timescale [ 20 ] . A double pulse was generated via the same module described in previous work, which investigated the optimisation of plasma mirror reﬂectivity , delivering a prepulse to main pulse energy of 1:10 and with variable delay of up to 100 ps. In total, 72±2 J of laser energy was delivered on target for the shots considered here, giving an average focused main pulse intensity of 4 ´ 10 W cm 18 - 2 . The accelerated proton beam spatial
), the laser very rapidly ionizes the target to form a plasma in which nonlinear quantum-electrodynamic (QED) effects play a crucial role [6 – 8]. Energetic electrons radiate MeV energy gamma-ray photons by nonlinear Compton scattering. The radiated gamma-ray photons can generate electron – positron pairs in the laser- ﬁ elds  which can radiate further photons. A cascade of pair production ensues, similar to that thought to occur in extreme astrophysical environments such as pulsar  and black hole  magnetospheres. Pair-plasmas more than eight orders of magnitude denser than currently achievable with ultra-high intensity lasers could be produced [12 – 14], enabling the study of collective behavior in relativistic pair-plasmas .
Electrons can be measured using FUJI-film image plate which is highly sensitive to ionising radiation . Hidding et al.  showed that interleaving multiple layers of image plate with filtering material in a stack behind a laser-solid interaction enables, the temperature and flux to be extracted from simple Monte Carlo modelling. Curving the image plate and also using multiple layers provides angular information on the flux and temperature [8,9]. In this investigation, the so called `wraparound' diagnostic is deployed on an experimental campaign at the PHELIX laser. During this campaign the laser was defocused by up to 400 µm, decreasing the incident intensity onto the target from 4x10 20 - 4x10 17
The injection and transport divergence properties of a high current beam of energetic electrons in metallic targets irradiated by ultraintense, picosecond laser pulses is investi- gated using simultaneous measurements of Ka fluorescence and proton acceleration, and a programme of 3-D hybrid-PIC simulations. The Ka fluorescence measurements, which are sensitive to the overall lateral extent of the electron beam, indicate that the effective transport half-angle is between 10 and 38 (17 and 31 if previous measurements with the same laser are included) as defined by the degree of uncertainty in the measurements (best fit 24 ), and this is supported by the simulation results. The simulations further reveal that the fast electron beam transport is strongly affected by self-generated magnetic fields, which in turn are sensitive to the average injection angle of the electrons at the front side of the target.
Producing a single pulse of compressed laser energy is an ideal that is usually unob- tainable in practice. In typical highly intense laser systems the chirped pulse arrives on target amidst a background or pedestal of optical noise termed Amplified Sponta- neous Emission (ASE). As the laser energy is been amplified within the gain media, some leakage occurs which propagates down the laser chain, through the compressor, and arrives on to the cold target some nanoseconds before the main pulse. Gating tech- niques involving Pockel Cells and saturable absorbers can help limit ASE. Although not as energetic as the main pulse, the ASE pedestal is typically nanoseconds long and can deliver sufficient energy to disrupt the target itself. As a figure of merit for intense laser systems, a value relating the intensity Contrast Ratio between the main pulse inten- sity and pedestal intensity is often used to describes the noise output. The higher the contrast the cleaner the pulse. A contrast of 10 8 measured at 3 ns before a 10 20 W/cm 2 pulse essentially means a 10 12 W/cm 2 pedestal will arrive on target 3 ns before the main
and increased sample throughput [5, 13-19] . LA-ICP-MS has been applied for bulk elemental and isotopic analyses of soils and sediments prepared in different forms such as pressed pellets or fused disks [20-30] . Most studies by LA-ICP-MS have been based on nanosecond laser (ns) systems (Nd:YAG and excimer lasers) [31-35] . Excimer lasers working at deep ultraviolet (DUV) have been reported to give controlled ablation in terms of crater geometry, particle size distributions and chemistry of the laser-produced aerosols. While Nd:YAG laser systems are less expensive and easier to maintain, the thermal characteristics of these systems can lead to elemental and isotopic fractionation, matrix effects, and less-representative sampling. Recently, femtosecond (fs) laser pulses have been suggested for chemical analysis by LA-ICP-MS to eliminate the problems associated with nanosecond laser systems [36-44] . Femtosecond laser pulses minimize the thermal effects generated during laser-solidinteractions and consequently have minimized fractionation and matrix effects. Compared to ns- LA-ICP-MS, fs-LA-ICP-MS has narrower particle size distributions, better transport efficiencies, and better vaporization, atomization and ionization of laser- produced aerosols in the ICP [36-44] . Because elemental analysis by SO-ICP-MS and LA-ICP-MS, like many other analytical techniques, necessitate comparisons to reference materials of known chemical compositions, it is important to have well-identified reference materials, which are not readily available for sediments.
The Garnache design was implemented in an MBE-grown InGaAs/GaAs VECSEL, which exhibited the power output characteristics shown in figure 3. Unlike the Kuznetsov device, this laser used no post-growth processing of any kind to reduce thermal impedance: the back surface of the intact substrate of the gain structure was made to contact a Peltier cooler using thermally conductive paste. The VECSEL was pumped with up to 1.5 W of 830-nm radiation from a fibre-coupled diode, imaged onto a 90-µm-diameter spot on the surface of the gain structure. Figure 3 shows the output power of this laser as a function of incident pump power for two temperature settings of the Peltier cooler. With the device running above ambient temperature, at a Peltier setting of 60˚C, the output power rolls over slowly and smoothly, passing through a maximum of >190 mW. The broad rollover characteristic indicates a device limited only by the intrinsic temperature dependence of the quantum well gain and not by thermal tuning of the longitudinal confinement factor. The laser was almost completely turned off for an incident pump power of 1.3 W, at which point the temperature of the active region reached an estimated 130˚C. With the Peltier cooler set to 0˚C, the laser reached a maximum output power of >400 mW, corresponding to an overall power conversion efficiency of ∼30%, enhanced by careful design of the DBR to back-reflect the unabsorbed pump light through the wells. The VECSEL operated in a TEM 00 mode up to the
Pengayun laser Ti:nilam telah dibangunkan berdasarkan teknik mod terkunci sendiri menggunakan rongga lipatan “ Z ” . Diode pam laser keadaan pepejal Verdi 5 telah digunakan sebagai sumber pengepaman dengan panjang gelombang asas 532 nm (sesuai untuk jalur penyerapan bagi hablur Ti:nilam). Rongga laser disusun atur melalui satu set cermin yang terdiri daripada cermin pantulan tinggi (99.8%) untuk memantulkan alur dalam julat 720 nm hingga 820 nm, dan pengganding keluaran dengan penghantaran 5%. Sepasang prisma untuk mengawal sebaran digunakan untuk menghasilkan denyut femtosaat. Denyut dicetuskan melalui gangguan luaran. Kestabilan laser dikekalkan dengan membekalkan sistem air penyejukan. Laser dioperasi dalam dua mod iaitu mod selanjar dan mod denyut dengan mekanisma mod terkunci. Kuasa keluaran maksimum laser selanjar Ti:nilam ialah 1.12 W sepadan dengan kuasa pengepaman 5.5 W dan kecekapan 26%. Kuasa purata optimum bagi laser Ti:nilam mod terkunci ialah 577 nm sepadan dengan kuasa pengepaman yang sama iaitu 5.5 W dengan kecekapan yang lebih rendah 18%. Frekuensi laser denyut mod terkunci ialah 96.43 MHz. Spektrum sinaran laser berpusat pada 806.74 nm dengan lebar jalur 22.37 nm pada lebar penuh separuh maksimum. Tempoh denyut bagi laser Ti:nilam mod terkunci ialah 30.53 femtosaat.
The collective response of plasma electrons in ultrathin foils to radiation pressure and the onset of RIT has been explored experimentally and by 3D simulations. For tar- gets which expand to densities close to the relativistically corrected critical density (l = 40 nm in the present study), for which radiation pressure is active for the duration of the interaction, the plasma electrons are swept from side to side in the plane of the linear polarization, resulting in an elliptical beam distribution, as first reported in Gray et al.  . New experimental results investigating the electron beam dynamics using two distinctive elliptical laser polarization with ∆θ = π /4 and ∆θ = −π /4 are presented. As in the linear polarization cases, when elliptically polarized light is used the accelerated plasma electrons exhibit a density distribution which is predominantly elliptical, with the major axis parallel to the major axis of the polarization direction. These results indicate a strong interaction between the electron and laser electric field for the laser conditions and this particular target thickness investigated.
optical communication in order to fabricate physical connections between two systems which have a line of sight arrangement, using serial communication for transfer of data. MAXIM Corporation’s IC MAX 232 is used, it needs only a single power supply of 5V (for TTL to RS-232 and vice- versa) level conversion and then an optical transmitter circuit is used to transmit data via fiber optic cable. There is a combination of laser and photodiode for transmitting and receiving the data respectively. We point the laser beam of transmitting module to fall on the photodiode of the receiving module connected to the other PC and vice-versa. The whole set up is kept in a dark box preferably black in color to avoid any reflection. The communication over the short distance of 2-3 m is possible using IR diodes. The range could be increased up to few meters, using the laser diode module in place of IR LEDs. The only need for fiber optic communication is felt because it is cheaper and corresponds to fewer losses.