INTRODUCTION
Experiments performed up to now proved that the nonlinear processes, accompanying intense-laser interactions with preformed plasma, are significant additional (or even leading) mechanisms at a production of ions with very high characteristics (Láska et al., Reference Láska, Jungwirth, Krása, Pfeifer, Rohlena, Ullschmied, Badziak, Parys, Wolowski, Gammino, Torrisi and Boody2005). The plasma, created by a slowly increasing front edge of a long (>100 ps) laser pulse, with which the main part of the pulse continues to interact, can be regarded as a kind of such preformed plasma. The characteristics of the produced ions differ, more or less, in dependence on various factors (Láska et al., Reference Láska, Badziak, Boody, Gammino, Jungwirth, Krása, Krouský, Parys, Pfeifer, Rohlena, Ryć, Skála, Torrisi, Ullschmied and WoLowski2007). Among them, very important is the laser-beam focus position (FP) with respect to the target surface, which determines not only the nominal laser intensity, but also the length (duration) of the laser beam interaction with such a self-created plasma. Another important factor is the laser beam incidence angle onto the target (Láska et al., Reference Láska, Jungwirth, Krása, Krouský, Pfeifer, Rohlena, Velyhan, Ullschmied, Gammino, Torrisi, Badziak, Parys, Rosinski, Ryć and WoLowski2008a). Third, target thickness determines the balance (possibility) between the backward direction of ion emission (from the front side of the target, against the laser beam) and the forward direction of ion emission (from the rear side, following the laser beam) (Badziak, Reference Badziak2007). With the intensity of laser radiation higher than ~1 × 1014 W/cm2, the charge states and the energy of the generated ions, as well as their amount, increase due to the participation of ponderomotive, relativistic, or magnetic self-focusing (channeling, filamentation) of the laser beam, or due to other nonlinear effects occurring at such intensities (Hora, Reference Hora1969, Reference Hora1975; Hora et al., Reference Hora, Kane and Hughes1978; Hauser et al., Reference Hauser, Scheid and Hora1992; Pukhov & Meyer-ter-Vehn, Reference Pukhov and Meyer-ter-Vehn1996; Borghesi et al., Reference Borghesi, MacKinnon, Barringer, Gaillard, Gizzi, Meyer, Willi, Pukhov and Meyer-ter-Vehn1997, Reference Borghesi, MacKinnon, Gaillard, Willi, Pukhov and Meyer-ter-Vehn1998; Láska et al., Reference Láska, Jungwirth, Krása, Krouský, Pfeifer, Rohlena, Ullschmied, Badziak, Parys, Wolowski, Gammino, Torrisi and Boody2006). Experiments on the hydrodynamics of intense laser-produced plasmas, performed at the PALS laboratory, have been presented recently (Batani et al., Reference Batani, Dezulian, Redaelli, Benocci, Stabile, Canova, Desai, Lucchini, Krousky, Masek, Pfeifer, Skala, Dudzak, Rus, Ullschmied, Malka, Faure, Koenig, Limpouch, Nazarov, Pepler, Nagai, Norimatsu and Nishimura2007).
This contribution is directed to the experimental investigation of the generation of ions with extreme characteristics, obtainable with the 1 kJ/300 ps iodine laser system PALS. Main attention was devoted to a determination of the highest attainable kinetic energy of ions and of the conditions at which they are produced. Simultaneously, the emission of hard X-rays was recorded and correlated with the generation of fast ions. An additional goal was to compare the ion emission and ion characteristics, when changing the target thickness. The reported results serve to identify and to discuss the mechanisms, participating in the generation (acceleration) of highly charged ions with extremely high kinetic energy, which have not yet been unambiguously identified. The conclusions should be useful for an optimization of the laser ion sources for pre-injectors of large heavy-ion accelerators, as well as for other scientific and/or technological purposes. Demands for collimated high currents of high-energy (fast) protons for inertial fusion experiments, and/or for (light) ions for medical use, make such investigations at present a real topic.
EXPERIMENTAL ARRANGEMENT
Our studies were performed using the PALS iodine high-power laser system in Prague (Jungwirth, Reference Jungwirth2005; Jungwirth et al., Reference Jungwirth, Cejnarová, Juha, Králiková, Krása, Krouský, Krupicková, Láska, Mašek, Mocek, Pfeifer, Präg, Renner, Rohlena, Rus, Skála, Straka and Ullschmied2001) with the fundamental (λ = 1.315 µm) and the 3rd harmonic (λ = 0.438 µm) frequencies, with laser energies up to ~400 J in a pulse < 300 ps long (full width in half maximum (FWHM)), and at variable focus positions (FP) with regard to the target surface. The convention used is that FP = 0 when the minimum focal spot coincides with the target surface, while FP < 0 means that it is located in front of the target surface, and FP >0 means that it is behind the target. The laser beam was focused onto the target with an aspherical lens of focal length f = 627 mm for 1ω and f = 600 mm for 3ω. The minimum diameter of the focal spot of ~80 µm results in the attainable laser intensity of up to 5 × 1016 W/cm2. While the intensity contrasts for the fundamental harmonics, 1ω is about 106 at ~1 ns prior to the laser pulse maximum, the energy ratio is only 102 at −0.5 ns. It means that, when taking into account, the integrated beam precursor during several hundreds of ps before the main part of the pulse, about 5 J of laser energy is deposited on the target to produce a pre-plasma (Ullschmied, Reference Ullschmied2006; Láska et al., Reference Láska, Jungwirth, Krása, Krouský, Margarone, Pfeifer, Rohlena, Ryć, Skála, Torrisi, Ullschmied and Velyhan2008b).
When irradiating the target perpendicularly, ions were detected at an angle of 30° with respect to the target normal, due to the target chamber construction (the angle between the input and output windows is 30°). The experimental chamber is sketched in Figure 1 (Torrisi et al., Reference Torrisi, Margarone, Láska, Krása, Velyhan, Pfeifer, Ullschmied and Ryc2008a). Changing the target tilt angle (laser beam incidence angle), the detection angle is automatically changed, too. Using thick (>1 mm) targets (slabs), the ions are emitted in the backward direction only (against the laser beam). When employing thinner targets and/or increasing significantly the laser intensity, the shape of produced craters changes (Torrisi et al., Reference Torrisi, Gammino, Mezzasalma, Gentile, Krása, Láska, Rohlena, Badziak, Parys, Woryna and Wolowski2003; Margarone et al., Reference Margarone, Láska, Torrisi, Gammino, Krása, Krouský, Parys, Pfeifer, Rohlena, Rosinski, Ryc, Skála, Ullschmied, Velyhan and WoLowski2008), the target starts to burn through and ions are emitted also (or mostly) forward (in the laser beam direction) (Maksimchuk et al., Reference Maksimchuk, Gu, Flippo, Umstadter and Bychenkov2000; Mackinnon et al., Reference Mackinnon, Borghesi, Hatchett, Key, Patel, Campbell, Schiavi, Snavely, Wilks and Willi2001). To study this transition, altogether three thicknesses of the Au target (500 µm, 200 µm, 80 µm) were tested.
Fig. 1. View of the target chamber (scheme).
Two ion collectors of different constructions and a cylindrical electrostatic ion energy analyzer (IEA), based on the time-of-flight effect, were used for the ion diagnostics (Láska et al., Reference Láska, Krása, Mašek, Pfeifer, Králiková, Mocek, Skála, Straka, Trenda, Rohlena, Woryna, Farny, Parys, Wolowski, Mroz, Shumshurov, Sharkov, Collier, Langbein and Haseroth1996; Woryna et al., Reference Woryna, Parys, Wolowski and Mróz1996). Measurements were performed in the far expansion zone (>1 m), where the ion charge-states are “frozen.” The standard circular (flat) ion collector (IC3) with a variable entrance window (diameter 2.25 cm in this case) was the first one, the second one, a ring ion collector (ICR, ∅1 = 3.8 cm, ∅2 = 5.0 cm), made possible the IC measurements close to the IEA axis. The grid transparency was 0.58. Both the collectors were negatively biased (~−100V). The IEA has a bending radius R 0 = 10 cm, the deflection angle Ψ = 90° and the gap between the cylindrical electrodes of the analyzing capacitor is ΔR = 0.5 cm. Using the ±10 kV voltage as a maximum on the electrodes of IEA, ions with energy-to-charge ratio up to E i/z = 200 keV can be detected. The path of flight L was 258 cm for IEA, 180 cm for ICR, and 165 cm for IC3, if the backward emitted ions were measured; it was 136 cm for ICR and for the forward emitted ions. For a better comparison, some of the ion collector signals were recalculated using relations j = U/RST(1 + γ) and j 1/j2 = L 23/L 13 to the current densities at the distance of 100 cm. U is the output IC voltage, the load resistance is R = 25Ω, S is the IC surface, T is the IC grid transmission, γ is the coefficient of secondary electron emission induced by the impact of ions on the IC electrode, and z is the charge-state of the ions.
The IC measures the time-resolved, but charge-integrated signal, which can be expressed as U(t) = R i(t) = R dQ/dt = R d(N(t)v < z(t) > e)/dt, where N(t) = ΣN z(t) is the total number of ions involving all charge-states z at the instant t, < z(t) > =< N z(t)z(t) >/< N z(t) > is the averaged charge-state of ions, e is the elementary charge. Since the measured ion current is a sum of partial currents due to various ionized species (which, moreover, may be produced by different mechanisms), both the magnitudes N and <z> can be determined only with the use of some kind of additional ion diagnostics (IEA, Thomson parabola spectrograph TPS (Woryna et al., Reference Woryna, Parys, Wolowski and Mróz1996)), or by completing it with a numerical deconvolution of the IC signal, containing a higher or lower number of peaks or humps, into a certain number of peaks (Krása et al., Reference Krása, Jungwirth, Krouský, Láska, Rohlena, Pfeifer, Ullschmied and Velyhan2007).
The IEA gives the possibility to identify the ion species produced; it determines the relative amount of single ions, having the same value of energy-to-charge state ratio E/z (at a fixed IEA voltage), their energy and abundance. The energy of ion species E i (passing through the IEA) can be determined by using the relation E i = 2zeκU/2, where e is the elementary charge, κ is a device constant (10 in our case) and ± U/2 are the declination voltages on IEA electrodes. Since our measurements were performed mostly with U/2 = 1500 V, a corresponding ion energy of ~1.75 MeV was measured for the charge state of 58 +. The maximum detectable E i depends on the maximum IEA voltage admitted.
Semiconductor photodiodes (Ryc et al., 2003), thermoluminescent detectors TLD (Krása et al., Reference Krása, Cejnarová, Juha, Ryć, Scholz and Kubeš2002), and CVD diamond detectors (Torrisi et al., Reference Torrisi, Margarone, Láska, Verona-Rinati, Milani, Cavallaro, Ryc, Krása, Rohlena and Ul lschmied2008b), all screened with various filters, monitored the X-ray radiation produced. For the detection of hard X-rays, a “FLM” type photodiode from ITE, Warsaw, was employed, the active-layer thickness of which was 380 µm, and a 307 µm Al foil as a filter. The hard radiation detector was sensitive in the range of 9.9 – 28 keV. For detection of the soft X-ray radiation, a “BPY03” type photodiode, also from ITE, Warsaw, was employed (the thickness of its active layer was about 2 µm) with the 5 µm Al-foil filter. This detector has two ranges of sensitivity: 0.8–1.6 keV and 2.1–6.3 keV. TLDs were screened with various X-ray filters to detect both the soft and hard X-rays.
RESULTS AND DISCUSSION
Experiments were performed first with a target thickness of 500 µm. Figure 2 shows an example of the IEA spectrum of Au ions, recorded at the third laser harmonic frequency 3ω, at the laser pulse energy E L~170 J, at the focus position FP = −200 µm, and for ions with the energy-to-charge ratio E i/z = 30 keV. The highest z max = 58 was found in the FP region from FP = −300 µm to FP = 0. Figure 3 demonstrates very illustratively the combined dependence of the detected charge states of ions on the angle of target irradiation (incidence) and on the angle of detection, recorded at 3ω, at E L = 170 J, at FP = −100 µm, and for ions of E i/z = 30 keV again. Generally, it is accepted that the main amount of the produced (fast) ions is emitted normally to the target (Ehler, Reference Ehler1975), up to 30° independently of the target incidence angle (Tallents et al., Reference Tallents, Luther-Davies and Horsburgh1986), with a narrow angular distribution, depending on the mass of the target (Buttini et al., Reference Buttini, Thum-Jager and Rohr1998), appearing even as a plasma jet long after the extinguishing of the heating laser pulse at some experimental conditions (Kasperczuk et al., Reference Kasperczuk, Pisarczyk, Borodziuk, Ullschmied, Krouský, Mašek, Rohlena, Skála and Hora2006, Reference Kasperczuk, Pisarczyk, Borodziuk, Ullschmied, Krouský, Mašek, Pfeifer, Rohlena, Skála and Pisarczyk2007, Reference Kasperczuk, Pisarczyk, Kalal, Martinkova, Ullschmied, Krousky, Masek, Pfeifer, Rohlena, Skala and Pisarczyk2008). Other observed directions of the emitted ion groups can be explained only by admitting the presence of nonlinear processes in preformed plasma (Láska et al., Reference Láska, Jungwirth, Krása, Krouský, Pfeifer, Rohlena, Velyhan, Ullschmied, Gammino, Torrisi, Badziak, Parys, Rosinski, Ryć and WoLowski2008a). Several ion groups are clearly seen in Figure 3, differing from each other in their charge state (they contain charges up to ~15 + , up to ~30 + , and up to 58+), and in the detection (incidence) angle. The record amount of ions with the highest charge state was recorded neither at the incidence angle Φ = 0° (irradiation perpendicularly to the target, the nominal laser intensity is maximal, but the angular distribution is narrower than the detection angle), nor at the incidence angle Φ = 30° (detection normally to the target, the laser intensity is lower according to cos Φ), but at Φ = ~20° (Θ~10°). Only a smaller secondary maximum was recorded at Θ = 0°. This fact likely implicates the presence of nonlinear processes, namely the acceleration (and declination) of ions due to the indirect action of ponderomotive forces (which affect primarily the electrons).
Fig. 2. IEA spectrum of Au ions obtained at 3ω, target thickness Δ = 500 µm, target tilt angle Φ = 30°, laser energy E L= 172 J, focus position FP = −200 µm, E i/z = 30 keV.
Fig. 3. (Color online) Au ion charge states recorded in dependence on the target tilt angle Φ (3ω, Δ = 500 µm, EL= 170 J, FP = −200 µm, Ei/z = 30 keV).
Generation of highly charged ions is strongly influenced by the focus position (by the corresponding value of the focus spot area). Generally, ions with the lower charge state (up to about 15 +) and lower kinetic energy are produced at lower laser intensities, and their production maxima are located more or less symmetrically on both sides from the exact focus FP = 0. This fact is attributed, similarly as the measured maxima of the soft X-rays, to larger focal spots and, therefore, to larger volumes of the plasma produced (Tallents et al., Reference Tallents, Luther-Davies and Horsburgh1986). With higher (increasing) laser intensities, a single pronounced maximum close to FP = 0 is typical for charge states from about 15 +. For charge states above 30 + , one or even more maxima appear, the FP distance between them is about hundred μm. The highest current peak is usually that observed close to FP = 0, but it is not a rule. At optimum focus position (FP < 0), more ions with the highest charge states and energy are produced than at FP > 0. It is ascribed to a more favorable focus location with regard to the expanding plasma plume and, consequently, to a more effective self-focusing process.
An example of values of the generated ion charge states in dependence on the focus position from FP =−600 µm to FP = +500 µm (at 1ω, E L = 370 J, E i/z = 30 keV), is presented in Figure 4. A small current of ions with z max~55+ was recorded at FP = +200 µm in that case. The absolutely largest amount of highly charged ions with the charge states from ~40+ to 50+ (peak signal at FP =−450 µm), in addition to several smaller maxima, are again supposed to be due to the presence of nonlinear processes. The distances between those maxima copy (qualitatively, at least) similar distances of hard X-rays maxima, measured simultaneously (Láska et al., Reference Láska, Cavallaro, Jungwirth, Krása, Krouský, Margarone, Mezzasalma, Pfeifer, Rohlena, Ryć, Skála, Torrisi, Ullschmied, Velyhan and Verona-Rinati2008c). For a thorough quantitative comparison, it would be necessary to measure the charge-energy distribution for each FP. The kinetic energy of ions with the charge state 50+ is 1.5 MeV only, which is not the highest one observed.
Fig. 4. (Color online) Au ion charge states recorded in dependence on the focus position FP (1ω, Δ = 500 µm, Φ = 0°, EL= 170 J, E i/z = 30 keV).
Another possibility to determine E i by using the IEA (considering limitations mentioned in Experimental Arrangement) means to estimate (calculate) it from the IC signal. The IC signal consists of a response to the X-ray radiation, hitting the IC first (a narrow, at some conditions line-like photo-peak, which determines the beginning of the IC signal), and of a more or less narrow, exponentially decreasing signal, with or without some (small) peaks superimposed on it, as it is seen in Figure 5 at IC signal recorded for 1ω at Φ = 0° (10 V~100 mA/cm2 at L = 1 m). However, the shape of the curve may look quite differently at different experimental conditions; such signals pass through a minimum (usually at short times, < 1 µs in the far expansion zone >1 m), followed by smaller or larger humps and maxima, depending on the relevant ion production and acceleration mechanisms (as it is seen in Fig. 5 for IC signal at 3ω and Φ = 30°). A careful analysis of the initial part of the IC signals (variable course of that decreasing part in dependence on the laser intensity and on the focus position, significant maxima distinguishable even at the time delay around 0.1 µs from the photo-peak) suggests that this part corresponds to the ions with the highest velocity. According to the relation E i [keV] = 5.18A v2 (108 cm/s) (A is the atom mass), the time-of-flight peak at 0.2 µs means the ion velocity 9 × 108 cm/s, and the energy per mass unit E i/u = 419 keV/u. This value corresponds, in fact, to the energy of fast protons (hydrogen present from impurities), but also to the energy E i = 82.6 MeV of the Au ions. The IC signals, acceptable even at time positions shorter than 0.1 µs, indicate the velocities above 1.8 ×109 cm/s, which represent a kinetic energy of Au ions above E i~330 MeV. It is in agreement with the similar results in Haseroth and Hora (Reference Haseroth and Hora1996) (A = 181 is presented there as Au). Haseroth and Hora (Reference Haseroth and Hora1996) and Hora et al. (Reference Hora, Kane and Hughes1978) have predicted such huge values of heavy ion energies, due to relativistic self-focusing. Experimentally they were observed and reported by Clark et al. (Reference Clark, Krushelnik, Zepf, Beg, Tatarakis, Machacek, Santala, Watts, Noreys and Dangor2000). Figure 6 demonstrates the changes of the shape of IC signals in dependence on the focus positions at 3ω, Φ = 30°, and E L = 170 J. The largest amount of fast (v > 1 × 108cm/s) and the most energetic Au ions were produced at FP = −150 µm (i.e., in front of the target), while the lowest emission was found at FP = 0. Worth noticing are the surprisingly large changes, observed for short FP steps of 50 µm only.
Fig. 5. (Color online) Ion collector signals recorded at 1ω, Φ = 0°, E L= 370 J and at 3ω, Φ = 30°, E L= 170 J (Au, Δ = 500 µm, FP = −150 µm).
Fig. 6. (Color online) Ion collector signals recorded in dependence on the focus position FP (Au, Δ = 500 µm, 3ω, Φ = 30°, E L= 170 J).
The influence of the target thickness Δ on the ion production was examined by using 200 µm and 80 µm thick Au foils, which burn through at our experimental conditions. Ion collectors were located on both the front and the rear sides of the target, in the target normal direction. The amplitudes of IC signals were recalculated to the current densities j at a distance of 1 m. The recalculated currents for the target of 200 µm thickness, obtained at 1ω, at Φ = 30°, at E L = 390 J, and at two focus positions (FP =−100 µm and –200 µm), are presented in Figure 7. The total amount of ions produced from the rear side (forward emission) is much smaller than that from the front side (backward emission), in particular for the fastest ions. The peak ion current density of the fast forward Au ions, generated at FP = +50 µm (in Fig. 8), is j max~30 mA/cm2, while the backward ion emission is about three times higher for the same focus position. The energy per mass unit of ions at this peak is 29.4 keV/u. In the part corresponding to the ions with the energy around 1 MeV/u, this ratio increases to 10. It means that the main differences in backward and forward ion emission at these experimental conditions are significantly larger amount and energy of backward emitted fast ions. This relation between both the currents persist, in principle, even if the focusing is varied from the focus position FP = +200 µm to FP =−500 µm. The forward emission is maximal at FP =−100 µm (see Fig. 9), proving that the burn-through is the fastest in this case, but the forward emission is not as effective as the backward one.
Fig. 7. (Color online) Ion collector signals of backward and forward emissions in dependence on the focus position FP (Au, Δ = 200 µm, 1ω, Φ = 30°, E L= 390 J).
Fig. 8. (Color online) Comparison of ion collector signals of backward and forward emissions (Au, Δ = 200 µm, 1ω, Φ = 30°, E L= 390 J, FP = +50 µm).
Fig. 9. (Color online) Ion collector signals of forward emission in dependence on the focus position FP (Au, Δ = 200 µm, 1ω, Φ = 30°, E L= 390 J).
The situation for 80 µm foil looks a little bit different. The signals shown in Figure 10, with maximum current density of backward emission~100 mA/cm2, were recorded at 1ω, at Φ = 30°, at E L = 416 J, and at FP = +50 µm. At these experimental conditions, the burn-through is much faster, and the emission from the rear side seems to be almost comparable with that from the front side, even for the focus located behind the target (FP > 0). As for the front (backward) emission, a strong dependence on the FP was found (see Fig. 11). With the FP increasing in the minus direction (FP < 0), the amount of the fast ions strongly decreases, and the fastest ions disappear (in contrast to the focus position dependence shown in Fig. 6). At FP <−300 µm, most of the Au ions are produced with the mean ion energy of about 10 keV only. This can be explained by the fact that the conditions for self-focusing are not fulfilled due to the fast burn-through of the target. Then a large hole is created and forward emission prevails.
Fig. 10. (Color online) Comparison of ion collector signals of backward and forward emissions (Au, Δ = 80 µm, 1ω, Φ = 30°, E L= 416 J, FP = +50 µm).
Fig. 11. (Color online) Ion collector signals of backward emission in dependence on the focus position FP (Au, Δ = 80 µm, 1ω, Φ = 30°, E L= 390 J).
Ions in laser-produced plasma can be accelerated by various mechanisms, all of which are associated primarily with the generation, properties, and behavior of the electrons. The thermokinetic (gas-dynamic) and electrostatic forces are usually responsible for generation of the low-energy ions (<100 keV). The high-energy ions are accelerated due to the electromagnetic (electrostatic-like and ponderomotive) forces (Badziak, Reference Badziak2007). The electrostatic forces are created because of separation of electrons and ions in the plasma due to the action of thermokinetic forces, or induced by various nonlinear processes. Since the electric field may attain values of TV/m (Malka et al., Reference Malka, Faure, Fritzler, Manclossi, Guemnie-Tafo, d'Humieres, Lefebvre and Batani2008), it means that the charge-separated ions can be accelerated even to an energy ~100 MeV along the Debye-length distance (~ 1 µm). The ponderomotive forces are induced by inhomogeneity of the laser field in the plasma, usually because of inhomogeneous space distribution of plasma density. In the simplest case, the ponderomotive forces are proportional to the gradient of electromagnetic energy in the plasma (Badziak, Reference Badziak2007). Because of a great difference in the electron and ion masses, the ponderomotive force acts, in reality, directly on electrons only. Ions are then efficiently accelerated indirectly by the electric field created because of charge separation due to that force.
The laser power threshold for ponderomotive self-focusing is ~1 MW. If a Nd-laser beam of intensity exceeding the value I = 3 × 1015 W/cm2 (Iλ2~const. for other wavelengths) is incident on a target, the relativistic self-focusing occurs and the laser beam diameter shrinks down to about λ within the length L SF comparable with the initial laser beam diameter (Hora et al., Reference Hora, Kane and Hughes1978). A short-range shrinking of a laser beam generates intensities I = P/λ2, which accelerate highly charged ions above MeV energies. A decreasing laser beam diameter results in a significant increase in the radial components of both ponderomotive and thermokinetic forces in the plasma.
Huge electromagnetic fields induced in the plasma by the laser pulse can accelerate electrons to relativistic velocities. The oscillatory energy of electrons in the laser field (depending on the laser intensity) induces an acceleration of z-times ionized ions leading to the kinetic energy given by E i (eV) = 3.1 × 10−6 × ZxP(W), independent of the laser wavelength (the relation is valid for subrelativistic ion energies only, the validity limit is v max = c/2) (Hora et al., Reference Hora, Kane and Hughes1978). When using this relation and considering conditions of our experiment, the calculated ion energies of Au ions should be around 175 MeV, and of the fast protons~3 MeV, in a full correspondence with the recorded ones.
CONCLUSIONS
Comparing different conditions of our experiment, the results can be summarized in the following way. Highly charged Au ions with charge states above 50+ and with the kinetic energy above 100 MeV, can be generated using both the 1ω and 3ω harmonics of iodine laser. Due to the precursor of the laser pulse at the fundamental frequency, better conditions for laser interactions with the self-created pre-plasma are established. The critical density is lower, more fast electrons are produced, and the threshold power density for the relativistic self-focusing is lower, too. For the third harmonic frequency, the precursor is removed, the plasma is denser, and collision dominated. Moreover, the power density threshold for the self-focusing goes up by an order of magnitude. However, focusing with the blue light is tighter. Since the generation of the fast electrons is less likely for the nominal laser intensity at 3ω, we also attribute their generation to the onset of self-focusing.
The laser pulse precursor and the illumination geometry (incidence angle, focus position, extraction angle) play a key role in the processes of generation and detection of ions with the extreme characteristics. Au ions with the kinetic energy in a region of ~300 MeV and with the maximum charge-states 58+ were recorded. The current density may reach the values higher than ~100 mA/cm2 at a 1 m distance. The front side (backward) emission of the fastest ions is the strongest for a focus position of about 100 µm in front of the target, and for a target thickness of 200 µm.
ACKNOWLEDGMENT
This research was kindly supported by the Grant Agency of the ASCR, v.v.i. (grant No. IAA 100100715).
