Laser-driven generation of fast particles

Transcription

1 OPTO-ELECTRONICS REVIEW 15(1), 1 12 DOI: /s INVITED PAPER J. BADZIAK* Institute of Plasma Physics and Laser Microfusion, 23 Hery Str., Warsaw, Poland The great progress in high-peak-power laser technology has resulted recently in the production of ps and subps laser pulses of PW powers and relativistic intensities (up to 1021 W/cm2) and has laid the basis for the construction of multi-pw lasers generating ultrarelativistic laser intensities (above 1023 W/cm2). The laser pulses of such extreme parameters make it possible to produce highly collimated beams of electrons or ions of MeV to GeV energies, of short time durations (down to subps) and of enormous currents and current densities, unattainable with conventional accelerators. Such particle beams have a potential to be applied in numerous fields of scientific research as well as in medicine and technology development. This paper is focused on laser-driven generation of fast ion beams and reviews recent progress in this field. The basic concepts and achievements in the generation of intense beams of protons, light ions, and multiply charged heavy ions are presented. Prospects for applications of laser-driven ion beams are briefly discussed. Keywords: laser, plasma, ion beams. 1. Introduction Recent enormous progress in high-peak-power laser technology has resulted in the construction of lasers generating picosecond or subpicosecond pulses of power approaching 1 PW and of extremely high intensities exceeding 1020 W/cm2 [1 4]. The interaction of such a laser pulse with any material leads to the production of hot plasma, a highly energetic state of matter composed of electrons and ions. Huge electromagnetic fields induced by the laser pulse in the plasma can accelerate electrons and, possibly, ions to the velocities close to the speed of light and produce collimated beams of high-energy particles, like those produced by conventional accelerators. This laser-driven particle acceleration has some unusual features unattainable in conventional accelerators. In particular: accelerating field usually exceeds 1 GV/cm, so it is more than a thousand times higher than that in accelerators; as a result, electrons or ions can be accelerated to the energies from MeV to GeV at distances shorter than 1 cm, particle currents can exceed 1 MA near the source and tens of A in a far zone, and particle current densities can be well above 1 GA/cm2 (they are several orders higher than those in conventional accelerators), production of extremely short, picosecond or femtosecond pulses of both electrons and ions is possible (such short ion pulses are unattainable in conventional accelerators). These unique features of laser acceleration create the prospect of constructing compact and relatively inexpensive * badziak@ifpilm.waw.pl laser-based particle accelerators having potential to be used in various domains of science, technology, and medicine. The generation of fast particles in laser-produced plasma was observed as far back as in the sixties [5,6]. However, powers and intensities, IL, of the used lasers were low (IL < 1012W/cm2), so energies of produced particles were low (~kev) as well. The high-intensity experiments began in the seventies and up to the middle of the nineties they were performed almost exclusively with long nanosecond and subnanosecond laser pulses of intensities up to 1016 W/cm2 [7 10]. A real breakthrough in the field, especially in laser-driven generation of fast ion beams, took place, however, on the turn of the century, when short picosecond and subpicosecond laser pulses of powers from TW to PW level were employed in the experiments. Most of these short-pulse experiments were performed with laser intensities comparable to or higher than the, so-called, relativistic intensity defined as the intensity causing relativistic motion of electron in the laser field with the electron kinetic energy equal to the electron rest energy (Irel» /l 2L [W/cm2], ll [µm] is the laser wavelength). As a result, collimated beams of electrons as well as light and heavy ions of energies in the range from MeV to GeV were produced. This paper is a brief review of recent achievements in laser-driven generation of ion beams, so it covers only a part of the topic indicated in the title (the readers interested in laser-driven electron beam generation are encouraged to see, e.g., the review papers, Refs. [11 13]). The basic methods of light and heavy ion generation, the results of experiments (mostly the short-pulse ones) as well as the prospects for applications of laser-driven ion beams are discussed here. In Sec. 2, general ideas regarding ion beam generation driven by a laser are presented. In Sec. 3 and 4, J. Badziak

2 some results of the studies on generation of light ion beams (Sec. 3) and heavy ion beams (Sec. 4) are presented. The prospects for applications of ion beams produced with the use of lasers are considered in Sec. 5. The last section briefly summarizes the results presented in the paper. 2. Laser-driven generation of fast ions: general remarks and schemes Though fast ions can be produced by an intense laser pulse interacting with any medium (gaseous, solid, liquid), the most efficient generation of collimated ion beams is observed at the interaction of a laser pulse with a solid target. In general, two kinds of schemes are used for laser-driven generation of ion beams (Fig. 1). In the first one, which employs thick (³ 1 mm) targets, ions are emitted in the backward direction [against the laser beam, Fig. 1(a)]. Both light ions and heavy ions can be produced in this scheme. In the second scheme, which employs thin (~ µm) targets, ions are emitted mostly forward [Fig. 1(b)]. In such a scheme, usually light ions (mostly protons) are produced. In both schemes, a laser beam focused on a target produces hot highly ionised plasma. The plasma emits UV and X radiations, neutral particles as well as electrons and ions. A part of the absorbed laser energy is converted into directed plasma beam, that is the beam containing ions surrounded with an electrons cloud. As almost whole directed energy of the plasma beam is carried by ions, this beam is usually called the ion beam. The ions can be accelerated in laser-produced plasma by termokinetic (gasdynamic) forces or/and electromagnetic forces. The termokinetic ones are usually responsible for generation of low-energy ions, i.e., the ions of energies in the range kev. High-energy ions are accelerated by electromagnetic forces. Among these, the most important ones are electrostatic-like forces and ponderomotive forces [14]. The electrostatic forces are created due to a separation of charges in plasma induced by various non-linear processes, and particularly due to a partial separation of electrons and ions under the action of termokinetic or ponderomotive forces. The ponderomotive forces are induced by inhomogeneity of the laser field in plasma whose inhomogeneity is usually the result of inhomogeneous distribution of plasma density in space. In the simplest case, the ponderomotive forces are proportional to the gradient of electromagnetic energy in plasma [15]. Because of a great difference in masses of electron and ion, in practice, the ponderomotive force acts directly only on electrons. In spite of this, ions can be efficiently accelerated by this force indirectly, that is by the electrostatic field induced as a result of charge separation produced by the ponderomotive force in plasma (the electron cloud driven by the ponderomotive force pull ions). Characteristics of laser-driven ion beams can be measured using various methods. The most commonly used one is the time-of-flight method employing various types of ion collectors and electrostatic or magnetic ion analyzers [10,16]. Thomson mass spectrometers, solid-state track detectors and some nuclear activation methods are applied for these purposes as well. Using these methods, a large number of ion beam characteristic (e.g., the mean and maximum ion energies, the ion charge states, the ion current, the total ion charge, the angular distribution of the ion current density or the charge density etc.) can be determined. 3. Generation of light ion beams Among several laser methods of light ion beam s generation worked-out so far, two methods are specially successful and promising. The first one is referred to as target normal sheath acceleration (TNSA) [17,18]. In this method, ions are accelerated at the rear surface of a thin target. The second one is the, so-called, skin-layer ponderomotive acceleration (S-LPA) method [19,20]. In S-LPA, ions are accelerated in front of the target. Both these methods employ only short-pulse lasers (ps or fs ones) as the drivers. Though, basically, they can be used for production of both light and heavy (Sec. 4) ion beams, they are especially efficient in generating light ion beams, first of all the proton beams Target normal sheath acceleration Fig. 1. General schemes of laser-driven generation of fast ion beams. 2 In the TNSA method, an intense short ( 1ps) laser pulse interacting with the target front surface produces plasma and hot (fast) electrons (Fig. 2). At laser intensities ~ W/cm2, the temperature of the electrons can be 2007 SEP, Warsaw

3 Fig. 2. Simplified physical picture of ion beam generation by the TNSA mechanism. very high, Th ~10 kev 10 MeV [17,18]. The electrons penetrate through the target and at the target rear surface they form a Debye sheath playing the role of virtual cathode. The electric field induced by the cathode is equal to [17,18] e ac» Th el Dh, (1) where l Dh is the hot electrons Debye length. As, typically, l Dh ~1 µm, for Th ~10 kev 10 MeV we arrived at eac ~ V/µm = V/cm. So, high electric field efficiently ionizes atoms at the target rear surface and then accelerates the produced ions over the distance Lac ~10 µm to the energies Ei» zelaceac ~ MeV (z is the ion charge state). The ions are accelerated mostly along the rear surface normal, therefore the angular divergence of the ion beam is relatively small (~10 20 ). Generation of fast proton beams using the TNSA mechanism has been investigated experimentally by several research groups at both subrelativistic [21 25] and relativistic [2,18,26 33]) laser intensities. Collimated proton beams of maximum proton energies from hundreds of kev (at IL ~1017 W/cm2) [21 25] to tens of MeV (at IL ~1020 W/cm2) [2,18,33] have been observed. It was found that parameters of the proton beam essentially depend on the target thickness (an optimum target thickness exists) [22 24,34] as well as on the target structure [22,24,25]. In particular, it was shown that parameters of the proton beam can be significantly improved, when the double-layer target composed of high-z front layer and low-z hydrogen-rich (e.g., plastic) back layer is employed [22,24], instead of a commonly used single-layer target. At such target structure, the high-z layer enables increasing the hot electron s production efficiency and the hydrogen-rich layer ensures high density of H atoms at the target rear surface. As a result, both the proton energies and the proton current are higher [22,24] than in the case of a single-layer target, e.g., made of plastic. Though the current densities and beam intensities of proton beams produced by TNSA are usually fairly high, for further increase in these parameters a shaped target enabling ballistic focusing of the proton beam can be used, Fig. 3 [17,35]. The possibility of ion beam focusing results from the fact that the TNSA-produced ions are emitted mostly in the direction normal to the target rear surface. Thus, the ion beam can be focused by a proper curving of this surface, that is by the use, e.g., of a spherical target. However, contrary to the ion beams produced in conventional accelerators, whose focusability is limited by the Coulomb interaction between ions, in case of a laser-driven ion beam we focus actually a quasi-neutral plasma beam. Therefore the focal spot can be very small (potentially in the subm range, Ref. 17). The feasibility of focusing of a proton beam in this way was proved by both numerical simulations [17] and measurements [35]. In the experiment of Patel et al. [35], in which a spherical thin foil target was irradiated by 100 fs laser pulse of relativistic intensity, an increase in the proton current density and beam intensity by an order of magnitude (up to the values ~1 GA/cm2 and ~ W/cm2, respectively) was achieved. Fig. 3. Idea of ion beam focusing using the TNSA mechanism. A summary of recent achievements in generation of light ion beams with the use of the TNSA method is given in Table 1, where the highest parameters attained so far for proton beams [2,18,31,33] and light ion beams of 1 < Z < 10 are presented [36,37]. Fairly high proton energies reaching 50 MeV and great proton currents at the source surpassing 1 MA as well as very low transverse emittance of proton beams (much lower than that achieved in conventional accelerators) are worth underlining. However, a drawback of laser-driven ion beams currently produced is a broad energy spectrum of DE/E ~100%. We can expect that the parameters of light ion beams produced by TNSA will be significantly improved soon and, particularly, the generation of ion beams of a narrow energy spectrum with ion energies exceeding 1 GeV will be possible. 3 J. Badziak

4 Table 1. A summary of recent achievements in generation of light ion beams by TNSA at relativistic laser intensities. Ion beam parameter Maximum ion energy protons lights ions (1 < Z < 10) 50 MeV 100 MeV (5 6 MeV/amu) Total mumber 1013 of ions Ion current ³ 1 MA at the source Ion current density ³ 1 GA/cm2 at the source Angular divergence Transverse < 0.01 mm mrad emittance Longitudinal < 10 4 evs emittance 1011 ³ 10 ka ³ 10 MA/cm Skin-layer ponderomotive acceleration The second promising method of light ion beams generation is S-LPA. The S-LPA mechanism was actually discovered at the end of the nineties [38], but just recently this mechanism has been relatively well understood and proved by both numerical simulations and experiments [19,20, 39 42]. The S-LPA method (Fig. 4) employs strong ponderomotive forces induced at the skin-layer interaction of a short laser pulse with a thin preplasma layer (of Lpre << df) produced by the laser prepulse in front of a solid target (Lpre is the preplasma layer thickness, df is the laser focal spot diameter). The main short laser pulse interacts most intensely with the plasma in the skin layer near the surface of the critical electron density nec and the geometry of the interaction is almost planar (since Lpre << df). The high plasma density gradient in the interaction region induces two opposite ponderomotive forces which break the plasma and drive two thin ( ll) plasma blocks towards the vacuum and the plasma interior, respectively. As the density of the plasma blocks is high (the ion density ni» nec/z, where z is the ion charge state) even at moderate ion velocities vi ~108 cm/s, the ion current densities and beam intensities can be very high. The time duration tis of the ion flux flowing out of the interaction region (being the ion source) is approximately equal to the laser pulse duration tl and the block area Ss is close to the area of the laser focal spot Sf. Due to almost planar acceleration geometry, the angular divergence of the ion beam is small. The necessary condition to accomplish the S-LPA mechanism of ion acceleration is Lpre << df. In practice, it means that the intensity of the laser prepulse has to be sufficiently low (the contrast ratio of a laser pulse has to be sufficiently high). As this requirement is much harder attainable for relativistic laser intensities, the up-to-date S-LPA experiments have been performed for subrelativistic laser intensities (IL << Irel). For such intensities, the scaling laws for the ion energy Ei, the ion current density at the source (close to the target) js, and the ion beam intensity at the source Iis, can be expressed by the simple formulae [20,39] E i» azl 2L I L, [kev, W/cm2, µm], (2) 12 j s» 74 (az A)1 2 l -L1 I L, [A/cm2, µm, W/cm2], (3) 32 I is» ( z A)1 2 a 3 2 l L I L, [W/cm2, µm](4) where IL is the laser intensity in vacuum, A is the atomic mass number, a = S for the forward-accelerated ions or a = S 1 for the backward-accelerated ions and S is the dielectric swelling factor [14]. Taking as an example z/a = 1 (protons), a = 1, ll = 1 µm, IL = 1017 W/cm2, we obtain Ei» 9.3 kev, js» A/cm2, Iis» W/cm2. We can see that even at quite moderate laser intensity, the ion current density and the ion beam intensity of the plasma block generated by the ponderomotive force can be very high. The above scaling laws as well as ultrahigh ion current densities produced by S-LPA were confirmed in the experiments [20,39]. Some results of these experiments are shown in Fig. 5 and Table 2. In particular, Table 2 presents Fig. 4. Idea of production of ion beams by S-LPA SEP, Warsaw

5 the comparison of parameters of forward-accelerated proton beams produced from a thin foil target by S-LPA at subrelativistic intensities [39] with those achieved with TNSA at relativistic intensities [2,29 31]. From the results, presented in Table 2, the following conclusions can be drawn: proton beam intensities at the source generated within a fixed angle cone by subrelativistic S-LPA are comparable to those produced (within the same angle cone) by TNSA at relativistic laser intensities and much higher laser energies, proton current densities at the source generated within a fixed angle cone are significantly higher for S-LPA than those for TNSA, proton densities at the source produced by S-LPA are about a thousand times higher than those generated by TNSA within the same angle cone. High proton densities at the source, concluded from the above comparison, are the main reason for the observed very high proton current densities and beam intensities produced by S-LPA in spite of fairly moderate energies of generated protons. The other reason is quasi-planar geometry of the plasma block acceleration enabling generation of proton beams of low angular divergence. It should be noted that similarly to the case of TNSA, parameters of forward-emitted proton beams produced by S-LPA significantly depend on the thickness and structure of the target used [19,20,39,40]. In particular, using double-layer targets makes it possible to produce higher proton energies and proton currents [20,40] than in the case of single-layer targets. Though current densities of light ions produced by S-LPA at subrelativistic laser intensities can be very high, however, ion energies are relatively small, < 1 MeV/nucleon. For achieving MeV ion energies, relativistic laser intensities have to be used. The possibility of fast ion (proton) beam generation by S-LPA at relativistic intensities was investigated numerically using both hydrodynamic [42] and particle-in-cell (PIC) [43] computer codes. Fig. 5. Fast ion current density (a) and beam intensity (b) at the source of the ion beam produced by S-LPA as a function of laser intensity (after Ref. 39). It was shown that also at such laser intensities the S-LPA method is efficient and collimated ion beams of energies from MeV to GeV can be produced. It can be expected that within a few years these predictions will be experimentally confirmed and production of multi-mev multima proton beams of TA/cm2 current densities will be demonstrated. Table 2. A comparison of parameters of proton beams produced by S-LPA and TNSA. Method Laser beam Mean proton energy, MeV Proton current density at the source estimated for 10 angle cone, GAcm 2 Proton beam intensity at the source estimated for 10 angle cone, 1015 Wcm 2 Proton density at the source estimated for 10 angle cone, 1018 cm 3 Ref. S-LPA 0.5 J/1 ps 1017 W/cm [39,72] 30 J/0.35 ps 1019 W/cm2 LULI [31] 50 J/1 ps W/cm2 VULCAN [29,30] 500 J/0.5 ps W/cm2 PETAWATT [2] TNSA 5 J. Badziak

6 4. Generation of highly charged heavy ion beams High-peak-power lasers enable efficient generation not only of light ion beams but also intense fluxes of highly charged heavy ions (say of the atomic mass number A ³ 100). However, contrary to the laser production of light ions (protons, deuterons, etc.), in the laser-driven heavy ion generation, generally two problems are required to be solved: efficient ionization of atoms to high charge states, acceleration of produced ions to high energies without a significant loss of the charge state due to electron-ion recombination. The basic mechanisms of ionization of atoms at the interaction of intense laser light with a solid target are collisional ionization and photoionization. Collisional ionization is caused by inelastic collisions of free electrons accelerated in the electric field of the laser pulse with atoms (or ions). This kind of ionization is efficient if a medium is sufficiently dense. In particular, it dominates in the highdensity region of laser-produced plasma, i.e., in the region where plasma density is close to or higher than the critical plasma density nec. Production of highly charged ions by this mechanism can be efficient for both short and long laser pulses, even at moderate (< 1016 W/cm2) laser intensities. Photoionization of atoms (ions) by laser field can be accomplished through a multiphoton process or the tunnel effect. It can dominate in the underdense plasma (of ne < nec) provided that laser intensity is sufficiently high, i.e., > 1016 W/cm2. As such laser intensities are hardly attainable for long laser pulses, actually this mechanism can efficiently produce highly charged ions only in the case of short pulses. It should be mentioned, however, that the obtaining of highly charged ions in a far expansion zone is generally more difficult in the case of using short laser pulses. The reason is the short time of the laser-plasma interaction and the rapid adiabatic cooling of short-pulseproduced plasma, favourable electron-ion recombination. Two mechanisms of acceleration of highly charged heavy ions seem to be especially promising. The first one is the S-LPA mechanism, described in the previous section. The second one uses the, so-called, double-layer effect in expanding plasma corona [7,8]. The double-layer is formed on the front of expending plasma by a cloud of hot (fast) electrons escaping from the plasma. The negative charge of this cloud causes acceleration of nearby ions, first of all the ones of the highest charge state z (strictly, of the highest z/a ratio). These ions form a fast ion group, as opposed to the thermal, slower ion group accelerated by thermokinetic forces. The mechanism of acceleration using the double-layer in plasma corona dominates in the case of long driving pulses [7 10], but it can be also efficient in the case of short pulses of relatively low contrast (producing sufficiently thick underdense preplasma layer) [44,45]. Both the double-layer and S-LPA mechanisms were demonstrated to be efficient in generation backward-emitted heavy ion beams. 6 For forward acceleration of highly charged heavy ions the TNSA mechanism can be used, provided that the laser intensity is sufficiently high (>1019 W/cm2) [37]. This requirement results from the fact that the electric field produced by the hot electron cloud at the target rear surface (Sec. 3.1) has to be sufficiently high to ionize atoms at the rear surface to high charge states. As this field is lower than the laser field in front of the target, in general, achieving high ion charge states with this method is more difficult than in the case of two methods mentioned above. Properties of generation of multiply charged fast ions by S-LPA were studied in Refs using terawatt 1-ps Nd:glass laser as a driver [51]. Highly collimated heavy ion beams of the ion charge states up to z = 38 and the ion energy ³ 1 MeV were demonstrated at subrelativistic laser intensities [47]. The diagram showing the maximum charge state of ions measured by an electrostatic ion-energy analyzer [47] as a function of the atomic Z number of a target, is presented in Fig. 6. Fully stripped ions up to Z = 13 and highly charged heavy ions (Ag+29, Ta+38, Au+33) are visible in the diagram. Studies of laser-driven generation of highly charged ions with the use of long-pulse (ns or subns) lasers have a much longer history and have been performed by several research groups [7 10,52 58]. Some results obtained by the Czech-Polish group with the use of subns high-energy iodine lasers are presented in Table 3 [10]. It is worth noticing that heavy ions of z ~50 and energies in the MeV range are emitted in the beams of ion current densities in a far zone of tens ma/cm2. A summary of recent achievements in laser-driven generation of highly charged heavy ion beams is presented in Table 4, where maximum values of particular ion beam parameters attained with the use of both short-pulse and long-pulse lasers are shown. All these data concern back- Fig. 6. Maximum charge states of ions produced by S-LPA at subrelativistic laser intensities (dots) as a function of target Z number (after Ref. 47) SEP, Warsaw

7 Table 3. Parameters of ion beams produced by the long-pulse (subns) iodine laser at laser intensities of W/cm2. Element 47Ag Sn Ta W Pt Au Pb Bi 209 Maximum charge state Maximum ion energy, MeV Ion current density at 1 m, ma/cm ward-emitted ion beams [10,45,47,50,59], as the parameters of forward-emitted beams (produced by TNSA) measured up to now were lower [37]. We can note that the highest ion charge states were achieved with long-pulse lasers [10] in spite of the fact that the laser intensity produced by these lasers (~ W/cm2) was significantly lower than in the short-pulse case (up to W/cm2). On the other hand, the highest ion energies were observed when a short-pulse laser was used as an ion driver [45,59]. For both cases fairly high ion current densities and numbers of ions were recorded. 5. Prospects for applications of laser-driven ion beams The great progress in research on laser-driven fast ion generation taking place in recent years, in addition to the rapid development in short-pulse laser technology create a prospect for unique applications of laser-driven ion beams in various domains. They include: accelerator technology, inertial confinement nuclear fusion, nuclear and particle physics, laboratory astrophysics as well as medicine and material science and technology Accelerator technology Though the laser-driven ion source (LIS) can be considered itself as a separate, compact ion accelerator, it can also be used as an ion injector in large conventional accelerators [8,60,61]. The potential advantage of the laser ion injector is the high ion current, the small transverse and logitudinal emittances as well as the possibility of production of ions of arbitrary elements. However, a wide energy spectrum of laser accelerated ions is the problem waiting to be solved for successful application of LIS in the accelerator technology. Moreover, for the production of ion beams in the GeV energy range, the laser intensities ~1023 W/cm2 are necessary [43], and they seem to be attainable within a few years Inertial fusion One of the most important potential applications of laser-driven ion beams is inertial confinement (nuclear) fusion (ICF) in the so-called fast ignition scheme [62 64]. Fast ignition (FI) is an approach to ICF in which a deuterium-tritium (DT) fuel compressed by high-energy ns laser or X-ray beams is ignited by a separate energy source, e.g., by an ultra-intense particle beam. The FI approach has the potential for substantial advantages over conventional ICF: flexibility in compression drivers, lower susceptibility to hydrodynamic instabilities, higher energy gain and lower (~10-fold) driver energy (and cost) [63,64]. The price to be paid is the need for production and coupling to the compressed fuel a particle beam of extremely high parameters [65 67]. In the most studied FI scheme, the role of fast ignitor is played by a laser-driven relativistic electron beam [62,63,68,69]. In another concept, which promises higher beam-fuel coupling efficiency, a laser-driven MeV proton beam is proposed as the fast ignitor [39,70] (Fig. 7). It has been shown theoretically, that very high proton beam parameter required for DT ignition (e.g., tb ~10 ps, jb ~1013 A/cm2, Ib ~1020 W/cm2 [65,66]) which are not attainable with the use of even the biggest conventional accelerator can be achieved using strongly focused TNSA proton beams [70,71] or even unfocused (or slightly focused) proton beams produced by S-LPA at relativistic laser intensities [39,72]. Independent of the FI scheme to be used, the DT ignition producing a significant energy gain requires a multi-pw ps laser of energy approaching 100 kj [65 67]. Such a laser system is under design in USA [73] and Japan [69,74], and is also proposed to be built in Europe [75]. Table 4. A summary of recent achievements in laser-driven generation of highly charged heavy ion beams. Laser Short-pulse laser Maximum charge state Maximum ion enegry, MeV Ion current density at 1 m, ma/cm2 Total number of ions Long-pulse laser Subrelativistic IL Relativistic IL 38 (Ta) 46 (Pb) 55 (Ta) > > 20 ~ ³ 109 > J. Badziak

8 efficiency of the ion beam-target interaction. As a result, some transient and/or low-cross-section nuclear reactions, hardly observable with conventional accelerators, could be feasible to be studied. Using sub-gev and GeV protons (IL ~ W/cm2), efficient production of pions and muons as well as neutrino beams is possible [77]. Yet higher energies of laser produced ions (~100 GeV for protons or 1 GeV/nucleon for heavy ions) could potentially open up the opportunity to conduct new fundamental physical research that would range from studies of the strong force to the production of quark-gluon plasmas Material science and technology Fig. 7. Idea of fast ignition of inertial fusion by proton beams produced with the use of S-LPA. High-energy ns laser (or X-ray) beams compress DT fuel to ~1000 times of the solid density. PW ps laser beams produce short high-density proton beams. The proton beams heat a small part of the dense DT fuel core to ~10 kev igniting nuclear fusion (after Ref. 39) Laboratory astrophysics Rigorous scaling of the physical processes associated with many diverse astrophysical systems now permits viable laboratory analogues to be created. These provide strict tests of underlying theory and the numerical models of the phenomena, which would otherwise be under constrained by our limited observational database. One the possibilities to simulate conditions existing in some astrophysical objects is producing the, so-called, warm dense matter (WDM) by the isochoric heating (i.e., heating at constant volume) of matter with a short-pulse (ps/fs) beam of laser-driven ions [35,76]. WDM (the density ~1 10 of the solid density, the temperature ~ K), is quite common in the universe and, in particular, it may predominate in some layers of the sun and the cores of large planets, including (probably) Earth. It was demonstrated [35] that WDM can be produced by a ps proton beam driven by the laser of quite moderate intensity (~1019 W/cm2) and power (tens of TW). Using multi-pw laser as an ion beam driver, achieving much more extreme states of matter (energy density up to ~1 GJ/g, pressure ³ 1 Gbar, temperature ³ 107), matching conditions found only at the centre of a star, seems to be feasible Nuclear and particle physics on a tabletop Very short durations and extremely high current densities of laser-driven ion beams render nuclear physics studies possible in a completely new time scale and with unusual 8 An example of the use of laser accelerated ions in this domain is ion implantation [78 81]. It makes it possible to modify the surface of materials and to improve their properties significantly. The high current, high ion energies and the possibility of energy change in a wide range as well as the easiness of the selection of a kind of ions give laser ion source an advantage over those being used nowadays. Another example of the application of laser-driven ion beams is proton radiography [82,83]. The short duration and high quality of a laser produced proton beam enable it to be used successfully as a tool for high resolution (temporal and spatial) imaging of the structure of solids, high density matter (e.g., WDM), biological objects as well as strong transient electric fields in various media Medicine Among potential medical applications of laser-driven ions, the most often mentioned ones are proton cancer therapy and positron emission tomography (PET). At present, cyclotrons or synchrotrons are being used for these purposes. As these devices are large and very costly, the treatment with the use of these methods is within very few patients means. The use of laser accelerators seems to be very promising because of their compactness, moderate cost and additional capabilities of controlling the ion beam parameters. Proton beams have a number of advantages in comparison with other kinds of ionizing radiation used for the cancer therapy. First, a proton beam is insignificantly scattered by atomic electrons which reduces the irradiation of healthy tissues located on the side of the tumour. Second, the deceleration length of protons with a given energy is fixed, which allows one to avoid undesirable irradiation of healthy tissues behind the tumour. Third, the presence of a sharp maximum of proton energy losses in tissues (the Bragg peak) provides a substantial increase in the radiation dose in the vicinity of the beam stopping point [84]. The basic proton beam parameters required for the cancer therapy (a beam intensity of (1 5) 1010 protons/s and a maximum proton energy of MeV [85,86]) are attainable with the use of present laser technology. However, the other requirement, the beam must be highly monoenergetic, DE/E ~10 2 is a challenge demanding further extended re SEP, Warsaw

9 search. Moreover, high-repetition-rate (>> 1 Hz) laser systems are necessary for implementation of this technique in the clinic therapy. In the positron tomography, positrons coming from b+ disintegration of short-lived (2 100 min) radioactive isotopes such as 11C, 13N, 15O, 18F are used. They are produced by the bombardment of suitable elements with protons, deuterons, or helium nuclei of energies ~MeV. The possibility of creating such isotopes by means of beams of protons and deuterons produced by the laser was demonstrated, e.g., in Refs. 87, 88, and 89. The main problem here is the achievement of the appropriate amount of these isotopes in a short time, as a typical patient dose for PET is ~200 MBq. Basing on the up-to-date experiments, it was estimated [89] that producing GBq doses is possible with tabletop laser systems delivering 1-J pulses focused to 1020 W/cm2, and operating at 1 khz. The achievement of physical parameters of ion beams required for both medical applications seems to be realistic in the present decade. However, the important factor is the economical feasibility of the use of a laser accelerator. Although it is not easy now to determine what the real price of a medical laser accelerator will be, its use is likely to represent a significant simplification and lowering the cost of present equipment. 6. Conclusions Recent progress in high-peak-power laser technology has resulted in the production of short ( 1 ps) laser pulses of PW powers and relativistic intensities (up to 1021 W/cm2) and has laid the foundation for the construction of multipw lasers generating the intensities above 1023 W/cm2. The laser pulses of such extreme parameters make it possible to produce highly collimated beams of protons, light ions or multiply charged heavy ions of MeV GeV energies, of short time durations (down to subps) and of tremendous currents and current densities, unattainable with conventional accelerators. Such ion beams have a potential to be applied in numerous fields of scientific research (e.g., in nuclear and particle physics, inertial fusion, laboratory astrophysics, material science) as well as in medicine and technology development. Acknowledgements This work was supported in part by the State Committee for Scientific Research (KBN), Poland under Grant No 1 PO3B and by the International Atomic Energy Agency in Vienna under Contract No 11535/RO as well as by the European Communities under the Contract of Association between EURATOM and IPPLM. References 1. G. Mourou and D. Umstadter, Extreme light, Sci. Am. 286, No 5, (2002). 2. R.A. Snavely, M.H. Key, S.P. Hatchett, T.E. 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