Irradiance scaling of fast ion expansion for CO2 laser produced plasmas

Church, P.; Martín, F.; Pépin, H.; Decoste, R.

doi:10.1063/1.330554

Cited by 11 publications

(3 citation statements)

References 17 publications

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“…Also two other experiments were published with measured angular distributions, namely, those of Church, Martin, and Pepin [14] and Ehler [16]. But since in these cases the electron density was not measured, we cannot compare our theory with these data.…”

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confidence: 93%

Theory of ions emitted from a plasma by relativistic self-focusing of laser beams

1992

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Fast ions are emitted from the focus of a high-intensity laser beam irradiating a plasma. The selffocusing of the laser beam is caused by the dependence of the index of refraction on the relativistic mass of the electrons, which again depends on the electric-field strength. The ions are accelerated out of the laser focus due to the combined action of nonlinear forces and double layers. Results for the maximal energy and angular distribution of the ions are presented and compared with experimental data. PACS number(s): 52.40.Nk, 29.25. Cy, 52.25.Tx, 52.60.+h 4me n, CO ymHere, m, is the rest mass of the electron and n, the electron density. The relativistic Lorentz factor y depends on the electric-field strength E [5]. For example, the factor y is given for a circularly polarized wave of frequency co as 2 E2 1/2(2)The electric field of the very intense laser beam can be calculated by solving the nonlinear wave equation hE+ k E=O, derived from the Maxwell equations in certain approximations, where k =k (E)=n to /c and n is the refractive index of a noncollisional plasma depending on the plasma frequency given in Eq. (1):Analytical calculations of the self-focusing of a laser field penetrating a plasma were carried out by Hauser, Scheid, and Hora [6]. In these calculations we neglected the effects of collisions. Collisions were only found to have an inAuence near the low-intensity threshold and when the electron density is very close to the critical density Self-focusing of electromagnetic waves in a medium can arise if the index of refraction depends on the electric-field strength. In this case the effective radius of the laser beam decreases along the beam direction until it reaches a value of about a half wavelength of the incoming light. The theory of self-focusing was treated, for example, in the articles of Akhmanov, Sukhorukov, and Khokhlov [1], Svelto [2], Spatschek [3], and Hora [4].If a very intense laser beam (e.g., 10' W/cm for a Nd-glass laser with A, =1.06 pm) penetrates a plasma, relativistic self-focusing of the beam arises because of the relativistic velocity of the electrons which enters the relativistic mass of the electrons in the plasma frequency [5] e FNL= --, ' VE = -V(m, c y) . m, yco [4,5]. Since we assume that the threshold has been exceeded in the experiments considered in this Brief Report, we simply neglect the collisional implications in our treatment. An important consequence of relativistic self-focusing is the increase of the energy density of the laser field up to extremely high values in the region around the focal point. That means that nonlinear forces and double layers are developing which drive electrons and ions out of the focus region [4,7].The ions emitted from the focus can be accelerated up to energies of 10 MeV/nucleon depending on the laser intensity [8,9]. Therefore this effect can in principle be applied for the acceleration of particles by intense laser beams and has already been discussed under the name "laser focus accelerator" by Sessler [10], Hora et al. [11], and ...

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confidence: 93%

Theory of ions emitted from a plasma by relativistic self-focusing of laser beams

1992

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“…The ion collectors [13,[20][21][22][23][24][25] are widely used to measure and reconstruct spatial distribution of such ion flow parameters as the average velocity or kinetic energy of expansion, total number of particles or total charge in a solid angle, the current density, the energy spectrum. However, for absolute ion energy spectra measurements it is necessary to determine average charge state number of the registered ions and the coefficient of secondary emission from the collector surface.…”

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confidence: 99%

Study of angular dependences of ion component parameters in CO₂-laser-produced plasma

Stepanov

Satov

Makarov

et al. 2003

Plasma Phys. Control. Fusion

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CO 2 -laser-produced plasma ion component parameters were studied for aluminium and lead targets at laser intensity of P = 4×10 13 W cm −2 and pulse duration of τ = 15 ns experimentally and numerically. Angular dependences of ion number density for different charge states, average velocity and its spread were measured by time-of-flight method. Ion charge state distribution shows high-charge and low-charge state groups at normal expansion direction. Ions in these groups have different average expansion velocity and longitudinal velocity spread. Angular distribution of high-charge states is narrower than that of the low-charge state ion group, maximum yield of low-charge states occur at some angle from normal. For Al target results show similar trends as for Pb target, but simulations have indicated that the effect of laser ponderomotive force is more pronounced in this case.

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“…The fraction of laser energy carried inward is now ~15%, in agreement with the fact that the electrons return directly back towards the target losing less energy to the ions. 17 10" The energy-deposition pattern across the target surface is obtained from measurements involving both axial and lateral resolution. Figure 4(a) shows the hot-electron energy deposited within a given radius either below 11 jum of plastic or on the target surface (0.1 /xm CH) at 4xl0 13 W cm" 2 .…”

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confidence: 99%

Hot-Electron Energy Deposition in CO2-Laser-Irradiated Targets Consistent with Magnetic-Field-Induced Surface Transport

et al. 1983

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Hard-x-ray continuum spectra from high-Z tracers are used to obtain lateral and axial energy-deposition profiles by hot electrons in C0 2 -laser-irradiated slab targets. The interpretation of the results emphasizes a local direction of incidence of the hot electrons on the target and an energy-deposition pattern which are consistent with self-generated magnetic fields. PACS numbers: 52.25.Fi, 52.25.Ps, 52.50.Jm At sufficiently high intensities the interaction of a long-wavelength laser beam with a target is completely dominated by suprathermal electrons, generated mainly through resonance absorption at the critical density, and predominantly emitted in the corona. The hot electrons which are drawn back into the target by space-charge electric fields transport inward a fraction of the incident laser energy. Thus the hot-electron absorption in a thick shell of low-Z material could in principle be used as a driving mechanism for the shell compression of laser fusion targets. Features of the transport process have been addressed at least qualitatively in several experiments. 1 " 10 Recently, the trajectories of hot electrons in self-generated magnetic fields have been computed and surface transport has been shown to be dominated by these magnetic fields. 11 In this paper we present a consistent set of quantitative data on electron transport in C0 2 -laserirradiated slab targets which demonstrates the effects of self-generated magnetic fields on the hot-electron trajectories and on the energy-deposition pattern.Lateral and axial hot-electron energy-deposition profiles are inferred from the local hard-xray continuum emitted by hot electrons decelerated in the dense target material. High-Z tracingmaterial techniques are used to obtain the local emission. Besides, angular distribution of x rays yields direct information on the electron direction of incidence on the target surface. At high irradiances, we find that the electrons deposit most of their energy at oblique angles on the target surface and that the axial energy-deposition profile is in good agreement with a planar semiisotropic source of incident electrons penetrating the target surface. At low irradiances, the incident electrons appear to be normal to the target surface. A consistent pattern of local energy deposition at high irradiance is obtained from the lateral transport measurements. Near the focal spot, the hot electrons penetrate the target at near-normal incidence, are less energetic, and transport within 1 mm diam only 10% of the total deposited energy. Away from the focal spot, over a very large area (^ 1 cm 2 ), the hot electrons are increasingly energetic and incident at oblique angles. All these results are consistent with transport dominated by self-generated magnetic fields.The study was carried out with a C0 2 laser facility giving a 50-J, 1.2-nsec (full width at half maximum) pulse. The experiments described were made at energy levels up to 15 J with a laser beam incident at 28°, focused onto the target by a//1.5, 15-cm focal length, off-ax...

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Irradiance scaling of fast ion expansion for CO2 laser produced plasmas

Cited by 11 publications

References 17 publications

Theory of ions emitted from a plasma by relativistic self-focusing of laser beams

Theory of ions emitted from a plasma by relativistic self-focusing of laser beams

Study of angular dependences of ion component parameters in CO₂-laser-produced plasma

Hot-Electron Energy Deposition in CO2-Laser-Irradiated Targets Consistent with Magnetic-Field-Induced Surface Transport

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Irradiance scaling of fast ion expansion for CO2 laser produced plasmas

Cited by 11 publications

References 17 publications

Theory of ions emitted from a plasma by relativistic self-focusing of laser beams

Theory of ions emitted from a plasma by relativistic self-focusing of laser beams

Study of angular dependences of ion component parameters in CO2-laser-produced plasma

Hot-Electron Energy Deposition in CO2-Laser-Irradiated Targets Consistent with Magnetic-Field-Induced Surface Transport

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Study of angular dependences of ion component parameters in CO₂-laser-produced plasma