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Emission characteristics and stability of laser ion sources
Vacuum 85 (2010) 617e621
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Emission characteristics and stability of laser ion sources Josef Krása a, *, Andriy Velyhan a, Eduard Krouský a, Leos Láska a, Karel Rohlena a, Karel Jungwirth a, Jirí Ullschmied b, Antonella Lorusso c, Luciano Velardi c, Vincenzo Nassisi c, Agata Czarnecka d, Leszek Ry c d, Piotr Parys d, Jerzy Wo1owski d a
Institute of Physics, ASCR, v.v.i., Na Slovance 2, 182 21 Prague, Czech Republic Institute of Plasma Physics, ASCR, v.v.i., Za Slovankou 3, 182 20 Prague, Czech Republic c Department of Physics, University of Salento, Laboratorio di Elettronica Applicata e Strumentazione, L.E.A.S. I.N.F.N. sez. di Lecce, Via Provinciale Lecce-Arnesano, CP 193, 73100 Lecce, Italy d Institute of Plasma Physics and Laser Microfusion, 00-908 Warsaw, Poland b
a b s t r a c t Keywords: Laser ion sources Ion emission reproducibility Thermal and fast ions Ion temperature Centre-of-mass velocity
A new classification of laser ion sources concerning their pulse-to-pulse reproducibility in the ion emission is proposed. In particular, we distinguish between plasmas according to the electron distribution and changing its characteristics at a laser intensity threshold of 1014 W/cm2. Well reproducible continuous pulsed ion currents are typical for the intensity below the threshold. In contrast to this plasma the “twotemperature” plasma arising for the intensity above this threshold shows not only a separation of charges in space and time during the expansion but it also shows outbursts of ions similar to a self-pulsing instability leading to a chaos. The sequence of fast ion outbursts visible on the time-resolved ion currents is sensitive to the details of non-linear interaction of the laser beam with the generated plasma. Analysis of ion sources based on a time-of-flight signal function is applied to quantify their emission characteristics as the hidden partial currents of the ionized species constituting, the total measured current, the centre-of-mass velocity of individual charge states, their abundance and the ion temperature. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The laser-produced plasma expanding into the vacuum can be used as a source of multiply and highly charged ions. The emitted ions expand with energies in the eV [1] up to multiply-MeV [2] range depending on characteristics of a focused laser beam. The influence of external electromagnetic field, e.g. strong laser field or intrinsic weak electrostatic and magnetic fields, on the generated plasma and its expansion has become a problem of topical interest. For specific application purposes, such as sources of MeV ions, a sufficient reproducibility in time-of-flight spectra of pulsed ion currents is needed. The pulse-to-pulse fluctuations can be significantly suppressed by employing a laser exhibiting well reproducible pulse properties and by stable target surface properties obtained by careful target preparation. Nevertheless, if the laser beam is realigned, then the maximum ion energy and ion current amplitude can show other values for the same target and laser
* Corresponding author. Fax: þ420 286 890 265. E-mail address: [email protected] (J. Krása). 0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.08.021
parameters [3]. Since this effect corresponds to an imperfect realignment, the focus of the laser beam (i.e. the minimum focus spot) must be positioned with regard to the target surface with accuracy better than 50 mm [4]. The accuracy in the focus setting is demanded mainly due to the process of optimum absorption of the laser radiation by the created plasma [5]. The laser radiation penetrates into the plasma plume only as far as the electron density is lower than the critical density ncr. Since the laser-produced plasma is inhomogeneous due to the decreasing electron density from ncr at the core to the outer expanding boundary of the plasma plume, the absorption of laser radiation is the strongest near the critical density. If the laser beam is exactly focused onto the surface of a thick metal target (the focus position FP ¼ 0), it was observed that the absorption is less due to its higher reflectivity [5]. The electron temperature increases with the energy deposited per unit volume and, thus, the heating is most effective if the plasma region with the maximum absorption coefficient coincides with the spot position of the region of the highest intensity of the focused laser beam. It was also experimentally demonstrated that the maximum ion energy, the highest charge states of ions, and the highest intensity of emitted hard X-rays corresponding to the maximum electron
J. Krása et al. / Vacuum 85 (2010) 617e621
2. Experimental arrangement Three different lasers were employed in our experiments. The first one was the high-power iodine laser system at the PALS Research Centre ASCR in Prague (sFWHM z 300 ps; l0 ¼ 1315 nm;
1.5 Fe target Nd:YAG laser
1.0 Inpurities
temperature are reached when the laser beam focus is positioned in front of the surface of a thick metallic foil target. This phenomenon appears if the focused laser intensity exceeds a threshold value of about 3 1014 W/cm2 [6]. For the laser intensity above the threshold, the focus-position dependence of plasma parameters shows a quasi-periodic modulation, e.g. in the ion current amplitude, the maximum charge state as well as in the hard x-ray emission accompanying the generation of fast electrons [7]. Moreover, outbursts of fast ions were identified under these conditions [8]. In the search for a phenomenon which could lead to the creation of outbursts in the ion emission from laser-produced plasma, a variety of phenomena in the underdense expanding plasma responsible for seeding instabilities can be considered. For example, the formation of hot spots with the radiation intensity many times higher than the average laser beam intensity can be induced by the filamentation which may be due to both the thermal and the ponderomotive mechanisms above a threshold ranging from 7.9 1013 W/cm2 to 4.5 1014 W/cm2 [9]. Also, the repeated bursts of back-reflected laser emission at 1u0 and its harmonics from the expanding laser-produced plasma, which disappear below 3 1014 W/cm2, should be mentioned [10,11]. Evidently, the seeding of instabilities in the transient laserproduced plasma at intensity above the threshold can result into significant pulse-to-pulse fluctuations in the ion emission. The reproducibility of ion beams generation requires stabilized laser pulse parameters including a very high contrast ratio of w107. Also in this case there is a pre-pulse at 1e2 ns ahead of the main high-power pulse which can deposit on the target an intensity above 1012 W/cm2, which is high enough to create a plasma. The main high-power pulse will therefore always interact with a plasma and never with a solid target as a result of the finite contrast ratio [12]. Then it is not surprising that the observed pulseto-pulse reproducibility of ion emission can be, for example, w50% for MeV C5þ ion bunches produced by an ultrahigh-intensity [13]. For practical laser generation and acceleration of ions, the plasma should be a highly stable medium. It cannot be generated if the laser pulse propagation in the plasma is unstable due to the instabilities. Apparently, there are two ways how to adjust the laser ion source stability. The first one is to employ a laser delivering very low intensities just above the threshold of ablation which avoids the onset of instabilities. Then an ion beam can be extracted and modified applying external electric and magnetic fields [1,14]. On the other hand, some of the state-of-the-art lasers can deliver ultraintense, ultrashort laser pulses with a very high contrast ratio which could significantly minimize the plasma formation by the pre-pulse [15,16]. Nevertheless, fluctuations in the ion production remain large [13]. Since various responses of ion generation to the laser pre-pulses should be generally anticipated in each experiment directed on the production of high-energy ions as it can be deduced from Ref. [17], the pulse-to-pulse reproducibility remains a topical problem for these laser ion sources. The goal of our contribution is to give an account of reproducibility of the ion emission from a laser-produced plasma regarding the focused laser intensity below and above a threshold value of about 3 1014 W/cm2 for nanosecond and sub-nanosecond laser pulses, respectively. The laser ion sources will be studied with respect to the stability of the laser pulse propagation in the generated plasma. Ion collector signals will be analyzed to characterize the pulse-to-pulse fluctuations in the ion emission.
IIC [mA]
618
0.5
0.0
0
10
20 30 40 Time-of-flightL=1m [µs]
50
60
Fig. 1. Pulse-to-pulse fluctuations in the emission of ions emitted from a Fe plasma generated by a Nd:YAG laser.
minimum focal spot diameter of 80 mm) [18]. The laser energy EL ranged up to 500 J. The laser beam was focused with an aspherical lens onto a target at 30 with respect to its surface normal. The second laser employed was the KrF laser at INFN, Lecce, Italy operated at l ¼ 248 nm with sFWHM ¼ 23 ns and EL ¼ 600 mJ. Laser pulses were focused onto a target at 70 with respect to the normal to the target surface. The third laser was a Nd:YAG laser at IPPLM, Warsaw delivering laser pulses of energy EL ¼ 0.8 J at l ¼ 1064 nm and sFWHM ¼ 3.5 ns. The laser beam illuminated the target surface at an angle of 45 with respect to the target normal. Ion currents, i.e. time-of-flight (TOF) spectra, were detected with the use of ion collectors (IC) positioned along the normal to the target surface. The mass spectra of ionized species were measured employing a cylindrical electrostatic ion analyzer. Both the time-of-flight and current scales were rescaled to 1-m distance applying the similarity relations: tL1 ¼ (L1/L2) tL2 and JL1 ¼ (L2/ L1)3 JL2, where JL is related to the 1-cm2 surface of IC active area. 3. Results and discussion 3.1. Laser intensity below threshold of 1014 W/cm2 Fig. 1 shows an example of pulse-to-pulse fluctuations of 8 TOF spectra of ions produced by the Nd:YAG glass laser with a fluence of 122 J/cm2 (intensity of 3.5 1010 W/cm2) focused onto a Fe target. The ion current emitted from the Fe plasma is a sum of partial currents of all the ionized species, as primary Feqþ (1 q 4) ions and the ionized surface impurities of Cqþ (1 q 3), protons e Hþ and Oqþ (1 q 3):
jIC ðL; tÞ ¼
X
jFe q ðL; tÞ þ
X
jCq ðL; tÞ þ jH ðL; tÞ þ
X
jO q ðL; tÞ:
(1)
The ionized impurities compose fast small peaks detected at the leading edge of the ion current (1e6 ms). The central peak of the TOF spectra is largely composed of the current of Fe2þ and Fe3þ ions, and, thus, the pulse-to-pulse fluctuations in the ion emission is mainly due to fluctuations in generation of the Fe2þ and Fe3þ ions and recombination processes during the plasma free expansion. The recovery of the hidden partial currents of ionized species constituting the expanding plasma is shown in Fig. 2. The partial currents of the ionized species constituting the expanded plasma were retrieved by the deconvolution of a measured detector’s signal by fitting a detector’s signal function, which was derived from a three-dimensional velocity distribution [8,19]. The deconvolution revealed that the highest partial current of z1 mA/cm2 at a distance of 1 m from the target was exhibited by Fe3þ ions. The deconvolution also gave the abundance of ionized species shown in Table 1. The signal function used
J. Krása et al. / Vacuum 85 (2010) 617e621
619
3
uCM [10 m/s]
100
50
0
1
2
3
4
q Fig. 3. Charge-state dependence of centre-of-mass velocity of Feqþ (1 q 4) ions generated by a Nd:YAG laser. The applied fluence was 30.6 J/cm2, see Fig. 2.
Fig. 2. Comparison of partial ion currents uncovered by deconvolution of a collector signal UIC (bottom curve e observed), with cylindrical electrostatic energy analyzer signal, UCEA (uppercurve), induced by ions of Fe plasma having kinetic energy per charge state of 840 eV. The time scale of the CEA signal was rescaled for the IC distance of 1 m from the target. The applied fluence was 30.6 J/cm2.
fIC ðL; tÞfL2 t 5 exp½bðL=t uCM Þ;
(2)
where b ¼ m=2kT, makes the determination of the temperature T of ions and their centre-of-mass kinetic energy ECM ¼ m u2CM =2 possible. The average value of the temperature of Feqþ (1 q 4) ions was fitted to be z150 eV. The charge-state dependence of the centre-of-mass velocity uCM(q) of Feqþ ions is shown in Fig. 3. The linear increase in the velocity uCM fq can be explained by the balance between the ambipolar electric field E and the collisions between the ion species expressed by the rate f for all Feqþ ions generated and, consequently, by the term eE/f in Eq. (3) for the relative velocity, ur, between the ion species
q2 q eE 1 : m2 m1 f
ur ¼ u2 u1 ¼
(3)
3.2. Laser intensity above threshold of 1014 W/cm2 Increasing the intensity above a threshold of 1014 W/cm2, a multigroup distribution of hot electrons is created [6,20]. The laser-produced plasma is characterized by both the temperature of hot electrons and by the temperature of the background cold plasma electrons; a two-temperature model elucidates this observable fact [20]. The hot electrons generate, e.g. highly charged
25 20 IIC [µA]
The measured dependence indicates in the first approximation that Feqþ ions are accelerated with the same reduced electric field (eE/f ¼ const) for all the ion charge states (1 q 4) of the main ion group. Similar observations were reported in Ref. [8,19]. As for the impurities, C3þ and O2þ ions exhibited the highest peaks in the mass spectra obtained with the use of the cylindrical electrostatic ion mass analyzer. The total abundance of impurities was estimated to be 2%. The deconvolution of the IC signal shown in imp y100 T imp . The temperature Timp of the Fig. 2 gave the value of ECM ionized impurities ranged from z20 eV to z30 eV. The effective electric field Eimp accelerating the impurities was about 2 times higher then E accelerating the group of Fe4þ ions. The centre-ofmass energy of Feqþ ions (1 q 4) increases non-linearly with q due to the collisions between the ion species, as Eq. (3) shows. The charge-state dependence of the ratio of the centre-of-mass energy to the temperature ECM/T obtained for the Feqþ ions confirms this
effect, as presented in Table 1. The value of the temperature fitted as a free parameter was 135 23 eV. The abundances of impurities and of Feqþ (1 q 4) ions are also presented in Table 1. Fluctuations of time-resolved ion currents can be quantified as the fluctuations of the total charge carried by expanding ion flux if the shape of the TOF spectrum does not show any significant pulseto-pulse variation. The determined standard error of the total charge generated by a single laser shot is 0.62%. This value indicates a good reproducibility of the ion emission. As one might expect, the impurities exhibit more substantial plus-to-pulse fluctuations in contrast to the Feqþ (1 q 4) ions due to their coincident absorption on the target surface. Similar low pulse-to-pulse fluctuations were observed employing a KrF laser for a generation of Al, Cu, Mo and Au ions. Higher fluctuations were exhibited by Ni, Nb, W, Ta and finally by Sn and Pb, as Fig. 4 illustrates for the ion emission from a Pb plasma. The applied laser fluence was 2.4 J/cm2 (I z 1 108 W/cm2, l ¼ 248 nm). The corresponding standard error of the total charge carried by the Pb ions is 2%. The corresponding error of the peak current is 3.2%. The shape of the TOF spectra shown in Figs.1 and 4 is characteristic of so-called thermal ions. Although the total ion current is composed of partial ion currents, the TOF spectrum of thermal ions generally shows only a single peak with no visible evidence of separated maxima corresponding to the partial ion currents, as Eq. (1) indicates.
Table 1 Abundance and ratio of centre-of-mass kinetic energy and ion temperature, ECM/T, of Feqþ (1 q 4) ions generated by the Nd:YAG laser, which was operated at l ¼ 1064 nm and delivered a fluence of 30.6 J/cm2. The abundance of ionized impurities was deduced from the first peak of the ion collector signal at TOF ¼ 4.5 ms. þ
2þ
Ionized species
Fe
Fe
Abundance [%] ECM/T
54 5e4
24 1.9
3þ
Fe
17 4.3
3 21
Fe
4þ
Impurities 2 z100
Pb target KrF laser
15 10 5 0
0
50
100 150 200 Time-of-flightL=1m [µs]
250
300
Fig. 4. Pulse-to-pulse fluctuations in the emission of ions emitted from a Pb plasma, which was generated by a KrF laser.
J. Krása et al. / Vacuum 85 (2010) 617e621
0.8
0
H
IIC [A]
0.6
+
C F
4+
-40
7+
0.4 -80
UCEA [mV]
620
0.2
C 0.0 0.0
heavy ions as Au50þ [21]. Generally, the fast ions expand with a velocity u 1 106 m/s. A new phenomenon in the emission of fast ions concerning the outbursts of fast ions was observed [8]. This non-linear phenomenon results in pulse-to-pulse fluctuations, for example, of Cqþ (1 q 6) and Hþ ion emitted from the polyethylene (PE) plasma, as Fig. 5 shows. The plasma was generated by the PALS laser facility delivering energy of about 200 J at the wavelength l0/3 ¼ 438 nm. The system was checked to be in the focus at the target surface, FP ¼ 0. The applied intensity related to the laser wavelength was Il2 ¼ 2 1014 W cm2 mm2. The fast ions generated exhibit not only a separation of charges in space and time during their free expansion but also significant pulse-to-pulse fluctuations in the amplitude e i.e. outbursts similar to a selfpulsing instability are present leading to a chaos, see the TOF spectra from 0.1 ms to 0.6 ms. Although the irregular outbursts of fast ions have occurred, the peak of thermal ions shows small pulse-topulse fluctuations. In contrast to the fast ions peaks, which are largely composed of C6þ and C5þ ions, the main peak contains all the partial currents of all Cqþ (1 q 6) and Hþ ions. Their temperature obtained by the deconvolution is T z 1.6 keV [19]. The centre-of-mass energy, ECM, of Hþ ions as well as of Cþ ions is Hþ;Cþ < TÞ; as opposite to mostly lower than the temperature ðECM these single charged ions, Cqþ ions with q 2 reach higher centreC2þ C6þ y5 T to ECM y50 T. of-mass energies ranging from ECM Last peaks in Fig. 5 at TOF z 7 ms were ascribed to the slow ions generated by X-rays emitted from the expanding thermal plasma, which are absorbed by the target, and, thus, both the target ablation and the emission of ions are prolonged [22,23]. The slow peaks are composed of Cqþ (1 q 4) and Hþ ions. The radiation emitted from the thermal plasma can be regarded to be a secondary source of radiation. In contrast to the reproducible generation of thermal ions the generation of slow ions can hardly be expected to be well reproducible due to the chain of involved non-linear processes creating the fast and thermal ions. If the laser intensity is increased, the non-linear interaction between the impacting laser pulse and the expanding plasma results in clear emergence of plasma outbursts, thus, of outbursts of ions, as Fig. 6 shows. The polytetrafluoroethylene (PTFE) target was exposed to the first harmonics with Il2 ¼ 1.4 1016 W cm2 mm2. To provide an effective heating of the plasma, the laser beam was focused 100 mm behind the target surface, FP ¼ 100 mm. In this case the heating is very effective because the plasma region with the maximum absorption coefficient coincides with the region of the highest intensity of the focused laser beam. The thermal ions were produced only as a minority group while the fast ions totally dominated. The fast ions do not constitute a single group as the thermal ones do, but they are separated into a number of outbursts (see the bottom trace in Fig. 6). Every outburst contains fully stripped carbon and fluorine ions, as the coincidence between the
9+
5+
8+
F C ;F
0.4 0.6 Time-of-flight L = 1 m [µs]
-120 0.8
Fig. 6. Outbursts of Cqþ (1 q 6), Fqþ (1 q 9) and Hþ ions emitted by a polytetrafluoroethylene plasma generated by the laser system PALS operated at l ¼ 1315 nm, Il2 ¼ 1.4 1016 W cm2 mm2, FP ¼ 100 mm. The upper trace shows a spectrum of ions which have an energy of q 60 keV after passing through the mass analyzer.
group of C6þ, F9þ, C5þ and F8þ peaks in the upper trace and the outburst at TOF z 450 ns in the bottom trace indicates. The irregular generation of the ion outbursts results from a nonlinear interaction of the impacting laser beam with the expanding plasma. Evidently, this non-linearity is significantly affected by the laser pulse precursor (pedestal/contrast). If the laser pulse with an insufficient contrast is focused onto a target, the pulse precursor ionizes the ablated material which is later heated by the main part of the pulse [16]. Fig. 7 shows the temporal shape of an applied laser pulse. Owing to the laser pulse precursor, about 1.2 J from the total 150 J delivered is deposited onto the target during the first 100 ps before the main part of the pulse impacts. In comparison with the experiment presented in Fig. 1, the fluence delivered by the precursor is about 150 times higher because it reaches a value of w20 kJ/cm2. Varying the laser pulse energy, the space-time characteristics of the preformed plasma are varied and, thus, the focus position dependence shows a complex interaction between the preformed plasma and the main pulse of the laser beam. The precursor-created underdense plasma can form a plasma region with the maximum absorption coefficient coinciding with the region of the highest intensity of the focused laser beam. This nonlinear interaction inevitably affects the evolution and stability of the emitted ion group properties. Therefore, the pulse-to-pulse fluctuations of emitted currents of fast ions can be significant, as irregularities in the ion emission from the plasma of a heavy element demonstrated in Fig. 8. The Ta target of 2 mm in thickness was exposed to the first harmonics with intensity of 7.5 1015 W cm2 mm2, related to the wavelength. The fastest Ta ions gained kinetic energy of 50 MeV while the maximum energy gained in the second laser shot was only 25 MeV. The highest charge-state of 48 of Ta ions was recognized in a spectrum of the cylindrical electrostatic ion analyzer. One should note that the effective voltage accelerating the Ta ions should reach a value of
1.0 I(t)
Fig. 5. Pulse-to-pulse fluctuations in the emission of Cqþ (1 q 6) and Hþ ions emitted by a polyethylene plasma generated by the laser system PALS operated at l ¼ 438 nm. Il2 ¼ 2 1014 W cm2 mm2; FP ¼ 0.
0.2
6+
0.5 0.0 0.0
Precursor 1J
0.5
1.0
1.5
Tim e [ns] Fig. 7. Temporal shape of the first harmonics 1u0 pulse integrated over the beam cross section of the iodine photodissociation laser PALS. The precursor is seen near the leading edge. The pulse energy was 150 J.
J. Krása et al. / Vacuum 85 (2010) 617e621
621
IIC [mA]
Acknowledgements
200
Ta target
This work was partly supported by the Academy of Sciences of the Czech Republic (project AV0Z10100523) and by the Grant Agency of the ASCR (Grant IAA100100715).
100
References
0 0.1
0.2
Time-of-flightL=1m [µs]
1
2
Fig. 8. Pulse-to-pulse fluctuations in the emission of Ta ions generated by the laser system PALS (l ¼ 1315 nm, Il2 ¼ 7.5 1015 W cm2 mm2, FP ¼ 0).
about 1 MV. The highest current density at a distance of 1 m from the target was measured to be 170 mA/cm2. The outbursts of Ta ions as well as of C and F ions, which are shown in Figs. 6 and 8, well represent significant fluctuations in the emission of ions from a two-temperature plasma due to the created fast electrons.
4. Conclusions The classification of laser ion sources concerning their pulse-topulse reproducibility in the ion emission was presented. In particular, the analysis of time-of-flight spectra of expanding ion flux, which were generated under various experimental conditions, made it possible to estimate the threshold laser intensity limiting the range of the high degree of reproducibility of the ion emission. The ion emission induced by pulsed lasers can be characterized as follows. (1) If the plasma is produced by a short laser pulse having the lowcontrast of w106, the laser intensity of w3 1014 W/cm2 focused onto a target is a threshold value below which pulseto-pulse fluctuations in the ion emission are not significant. Since the kinetic energy per nucleon of thermal ions reaches only w30 keV/u, the multiply charged ions can be accelerated by a high voltage to extract them to form very stable monoenergetic ion beam. (2) Above the threshold, the yields of the different charge states of the fast ions vary strongly from pulse-to-pulse due to the nature of the ion outburst instability. This deficiency can be partially balanced by a possibility to produce highly charged ions of heavy elements, e.g. Ta48þ, and by a high overall current density which reaches a value of 170 mA/cm2 at a distance of 1 m from the target. Although the pulse-to-pulse fluctuations are specific for the currents of fast ions, a high number of shots can eliminate such a trouble in some practical applications. (3) The precursor effect being responsible for the occurrence of the pulse-to-pulse fluctuations can be eliminated if a higher harmonics of the high-power laser radiation is used to heat a target because in the case of higher harmonics, the precursor intensity is strongly reduced. Although the produced plasma remains dense and collision dominated, the intensity threshold for several non-linearities can be exceeded due to, e.g. a transient and time periodic ponderomotive self-focusing.
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Emission characteristics and stability of laser ion sources Josef Krása a, *, Andriy Velyhan a, Eduard Krouský a, Leos Láska a, Karel Rohlena a, Karel Jungwirth a, Jirí Ullschmied b, Antonella Lorusso c, Luciano Velardi c, Vincenzo Nassisi c, Agata Czarnecka d, Leszek Ry c d, Piotr Parys d, Jerzy Wo1owski d a
Institute of Physics, ASCR, v.v.i., Na Slovance 2, 182 21 Prague, Czech Republic Institute of Plasma Physics, ASCR, v.v.i., Za Slovankou 3, 182 20 Prague, Czech Republic c Department of Physics, University of Salento, Laboratorio di Elettronica Applicata e Strumentazione, L.E.A.S. I.N.F.N. sez. di Lecce, Via Provinciale Lecce-Arnesano, CP 193, 73100 Lecce, Italy d Institute of Plasma Physics and Laser Microfusion, 00-908 Warsaw, Poland b
a b s t r a c t Keywords: Laser ion sources Ion emission reproducibility Thermal and fast ions Ion temperature Centre-of-mass velocity
A new classification of laser ion sources concerning their pulse-to-pulse reproducibility in the ion emission is proposed. In particular, we distinguish between plasmas according to the electron distribution and changing its characteristics at a laser intensity threshold of 1014 W/cm2. Well reproducible continuous pulsed ion currents are typical for the intensity below the threshold. In contrast to this plasma the “twotemperature” plasma arising for the intensity above this threshold shows not only a separation of charges in space and time during the expansion but it also shows outbursts of ions similar to a self-pulsing instability leading to a chaos. The sequence of fast ion outbursts visible on the time-resolved ion currents is sensitive to the details of non-linear interaction of the laser beam with the generated plasma. Analysis of ion sources based on a time-of-flight signal function is applied to quantify their emission characteristics as the hidden partial currents of the ionized species constituting, the total measured current, the centre-of-mass velocity of individual charge states, their abundance and the ion temperature. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The laser-produced plasma expanding into the vacuum can be used as a source of multiply and highly charged ions. The emitted ions expand with energies in the eV [1] up to multiply-MeV [2] range depending on characteristics of a focused laser beam. The influence of external electromagnetic field, e.g. strong laser field or intrinsic weak electrostatic and magnetic fields, on the generated plasma and its expansion has become a problem of topical interest. For specific application purposes, such as sources of MeV ions, a sufficient reproducibility in time-of-flight spectra of pulsed ion currents is needed. The pulse-to-pulse fluctuations can be significantly suppressed by employing a laser exhibiting well reproducible pulse properties and by stable target surface properties obtained by careful target preparation. Nevertheless, if the laser beam is realigned, then the maximum ion energy and ion current amplitude can show other values for the same target and laser
* Corresponding author. Fax: þ420 286 890 265. E-mail address: [email protected] (J. Krása). 0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.08.021
parameters [3]. Since this effect corresponds to an imperfect realignment, the focus of the laser beam (i.e. the minimum focus spot) must be positioned with regard to the target surface with accuracy better than 50 mm [4]. The accuracy in the focus setting is demanded mainly due to the process of optimum absorption of the laser radiation by the created plasma [5]. The laser radiation penetrates into the plasma plume only as far as the electron density is lower than the critical density ncr. Since the laser-produced plasma is inhomogeneous due to the decreasing electron density from ncr at the core to the outer expanding boundary of the plasma plume, the absorption of laser radiation is the strongest near the critical density. If the laser beam is exactly focused onto the surface of a thick metal target (the focus position FP ¼ 0), it was observed that the absorption is less due to its higher reflectivity [5]. The electron temperature increases with the energy deposited per unit volume and, thus, the heating is most effective if the plasma region with the maximum absorption coefficient coincides with the spot position of the region of the highest intensity of the focused laser beam. It was also experimentally demonstrated that the maximum ion energy, the highest charge states of ions, and the highest intensity of emitted hard X-rays corresponding to the maximum electron
J. Krása et al. / Vacuum 85 (2010) 617e621
2. Experimental arrangement Three different lasers were employed in our experiments. The first one was the high-power iodine laser system at the PALS Research Centre ASCR in Prague (sFWHM z 300 ps; l0 ¼ 1315 nm;
1.5 Fe target Nd:YAG laser
1.0 Inpurities
temperature are reached when the laser beam focus is positioned in front of the surface of a thick metallic foil target. This phenomenon appears if the focused laser intensity exceeds a threshold value of about 3 1014 W/cm2 [6]. For the laser intensity above the threshold, the focus-position dependence of plasma parameters shows a quasi-periodic modulation, e.g. in the ion current amplitude, the maximum charge state as well as in the hard x-ray emission accompanying the generation of fast electrons [7]. Moreover, outbursts of fast ions were identified under these conditions [8]. In the search for a phenomenon which could lead to the creation of outbursts in the ion emission from laser-produced plasma, a variety of phenomena in the underdense expanding plasma responsible for seeding instabilities can be considered. For example, the formation of hot spots with the radiation intensity many times higher than the average laser beam intensity can be induced by the filamentation which may be due to both the thermal and the ponderomotive mechanisms above a threshold ranging from 7.9 1013 W/cm2 to 4.5 1014 W/cm2 [9]. Also, the repeated bursts of back-reflected laser emission at 1u0 and its harmonics from the expanding laser-produced plasma, which disappear below 3 1014 W/cm2, should be mentioned [10,11]. Evidently, the seeding of instabilities in the transient laserproduced plasma at intensity above the threshold can result into significant pulse-to-pulse fluctuations in the ion emission. The reproducibility of ion beams generation requires stabilized laser pulse parameters including a very high contrast ratio of w107. Also in this case there is a pre-pulse at 1e2 ns ahead of the main high-power pulse which can deposit on the target an intensity above 1012 W/cm2, which is high enough to create a plasma. The main high-power pulse will therefore always interact with a plasma and never with a solid target as a result of the finite contrast ratio [12]. Then it is not surprising that the observed pulseto-pulse reproducibility of ion emission can be, for example, w50% for MeV C5þ ion bunches produced by an ultrahigh-intensity [13]. For practical laser generation and acceleration of ions, the plasma should be a highly stable medium. It cannot be generated if the laser pulse propagation in the plasma is unstable due to the instabilities. Apparently, there are two ways how to adjust the laser ion source stability. The first one is to employ a laser delivering very low intensities just above the threshold of ablation which avoids the onset of instabilities. Then an ion beam can be extracted and modified applying external electric and magnetic fields [1,14]. On the other hand, some of the state-of-the-art lasers can deliver ultraintense, ultrashort laser pulses with a very high contrast ratio which could significantly minimize the plasma formation by the pre-pulse [15,16]. Nevertheless, fluctuations in the ion production remain large [13]. Since various responses of ion generation to the laser pre-pulses should be generally anticipated in each experiment directed on the production of high-energy ions as it can be deduced from Ref. [17], the pulse-to-pulse reproducibility remains a topical problem for these laser ion sources. The goal of our contribution is to give an account of reproducibility of the ion emission from a laser-produced plasma regarding the focused laser intensity below and above a threshold value of about 3 1014 W/cm2 for nanosecond and sub-nanosecond laser pulses, respectively. The laser ion sources will be studied with respect to the stability of the laser pulse propagation in the generated plasma. Ion collector signals will be analyzed to characterize the pulse-to-pulse fluctuations in the ion emission.
IIC [mA]
618
0.5
0.0
0
10
20 30 40 Time-of-flightL=1m [µs]
50
60
Fig. 1. Pulse-to-pulse fluctuations in the emission of ions emitted from a Fe plasma generated by a Nd:YAG laser.
minimum focal spot diameter of 80 mm) [18]. The laser energy EL ranged up to 500 J. The laser beam was focused with an aspherical lens onto a target at 30 with respect to its surface normal. The second laser employed was the KrF laser at INFN, Lecce, Italy operated at l ¼ 248 nm with sFWHM ¼ 23 ns and EL ¼ 600 mJ. Laser pulses were focused onto a target at 70 with respect to the normal to the target surface. The third laser was a Nd:YAG laser at IPPLM, Warsaw delivering laser pulses of energy EL ¼ 0.8 J at l ¼ 1064 nm and sFWHM ¼ 3.5 ns. The laser beam illuminated the target surface at an angle of 45 with respect to the target normal. Ion currents, i.e. time-of-flight (TOF) spectra, were detected with the use of ion collectors (IC) positioned along the normal to the target surface. The mass spectra of ionized species were measured employing a cylindrical electrostatic ion analyzer. Both the time-of-flight and current scales were rescaled to 1-m distance applying the similarity relations: tL1 ¼ (L1/L2) tL2 and JL1 ¼ (L2/ L1)3 JL2, where JL is related to the 1-cm2 surface of IC active area. 3. Results and discussion 3.1. Laser intensity below threshold of 1014 W/cm2 Fig. 1 shows an example of pulse-to-pulse fluctuations of 8 TOF spectra of ions produced by the Nd:YAG glass laser with a fluence of 122 J/cm2 (intensity of 3.5 1010 W/cm2) focused onto a Fe target. The ion current emitted from the Fe plasma is a sum of partial currents of all the ionized species, as primary Feqþ (1 q 4) ions and the ionized surface impurities of Cqþ (1 q 3), protons e Hþ and Oqþ (1 q 3):
jIC ðL; tÞ ¼
X
jFe q ðL; tÞ þ
X
jCq ðL; tÞ þ jH ðL; tÞ þ
X
jO q ðL; tÞ:
(1)
The ionized impurities compose fast small peaks detected at the leading edge of the ion current (1e6 ms). The central peak of the TOF spectra is largely composed of the current of Fe2þ and Fe3þ ions, and, thus, the pulse-to-pulse fluctuations in the ion emission is mainly due to fluctuations in generation of the Fe2þ and Fe3þ ions and recombination processes during the plasma free expansion. The recovery of the hidden partial currents of ionized species constituting the expanding plasma is shown in Fig. 2. The partial currents of the ionized species constituting the expanded plasma were retrieved by the deconvolution of a measured detector’s signal by fitting a detector’s signal function, which was derived from a three-dimensional velocity distribution [8,19]. The deconvolution revealed that the highest partial current of z1 mA/cm2 at a distance of 1 m from the target was exhibited by Fe3þ ions. The deconvolution also gave the abundance of ionized species shown in Table 1. The signal function used
J. Krása et al. / Vacuum 85 (2010) 617e621
619
3
uCM [10 m/s]
100
50
0
1
2
3
4
q Fig. 3. Charge-state dependence of centre-of-mass velocity of Feqþ (1 q 4) ions generated by a Nd:YAG laser. The applied fluence was 30.6 J/cm2, see Fig. 2.
Fig. 2. Comparison of partial ion currents uncovered by deconvolution of a collector signal UIC (bottom curve e observed), with cylindrical electrostatic energy analyzer signal, UCEA (uppercurve), induced by ions of Fe plasma having kinetic energy per charge state of 840 eV. The time scale of the CEA signal was rescaled for the IC distance of 1 m from the target. The applied fluence was 30.6 J/cm2.
fIC ðL; tÞfL2 t 5 exp½bðL=t uCM Þ;
(2)
where b ¼ m=2kT, makes the determination of the temperature T of ions and their centre-of-mass kinetic energy ECM ¼ m u2CM =2 possible. The average value of the temperature of Feqþ (1 q 4) ions was fitted to be z150 eV. The charge-state dependence of the centre-of-mass velocity uCM(q) of Feqþ ions is shown in Fig. 3. The linear increase in the velocity uCM fq can be explained by the balance between the ambipolar electric field E and the collisions between the ion species expressed by the rate f for all Feqþ ions generated and, consequently, by the term eE/f in Eq. (3) for the relative velocity, ur, between the ion species
q2 q eE 1 : m2 m1 f
ur ¼ u2 u1 ¼
(3)
3.2. Laser intensity above threshold of 1014 W/cm2 Increasing the intensity above a threshold of 1014 W/cm2, a multigroup distribution of hot electrons is created [6,20]. The laser-produced plasma is characterized by both the temperature of hot electrons and by the temperature of the background cold plasma electrons; a two-temperature model elucidates this observable fact [20]. The hot electrons generate, e.g. highly charged
25 20 IIC [µA]
The measured dependence indicates in the first approximation that Feqþ ions are accelerated with the same reduced electric field (eE/f ¼ const) for all the ion charge states (1 q 4) of the main ion group. Similar observations were reported in Ref. [8,19]. As for the impurities, C3þ and O2þ ions exhibited the highest peaks in the mass spectra obtained with the use of the cylindrical electrostatic ion mass analyzer. The total abundance of impurities was estimated to be 2%. The deconvolution of the IC signal shown in imp y100 T imp . The temperature Timp of the Fig. 2 gave the value of ECM ionized impurities ranged from z20 eV to z30 eV. The effective electric field Eimp accelerating the impurities was about 2 times higher then E accelerating the group of Fe4þ ions. The centre-ofmass energy of Feqþ ions (1 q 4) increases non-linearly with q due to the collisions between the ion species, as Eq. (3) shows. The charge-state dependence of the ratio of the centre-of-mass energy to the temperature ECM/T obtained for the Feqþ ions confirms this
effect, as presented in Table 1. The value of the temperature fitted as a free parameter was 135 23 eV. The abundances of impurities and of Feqþ (1 q 4) ions are also presented in Table 1. Fluctuations of time-resolved ion currents can be quantified as the fluctuations of the total charge carried by expanding ion flux if the shape of the TOF spectrum does not show any significant pulseto-pulse variation. The determined standard error of the total charge generated by a single laser shot is 0.62%. This value indicates a good reproducibility of the ion emission. As one might expect, the impurities exhibit more substantial plus-to-pulse fluctuations in contrast to the Feqþ (1 q 4) ions due to their coincident absorption on the target surface. Similar low pulse-to-pulse fluctuations were observed employing a KrF laser for a generation of Al, Cu, Mo and Au ions. Higher fluctuations were exhibited by Ni, Nb, W, Ta and finally by Sn and Pb, as Fig. 4 illustrates for the ion emission from a Pb plasma. The applied laser fluence was 2.4 J/cm2 (I z 1 108 W/cm2, l ¼ 248 nm). The corresponding standard error of the total charge carried by the Pb ions is 2%. The corresponding error of the peak current is 3.2%. The shape of the TOF spectra shown in Figs.1 and 4 is characteristic of so-called thermal ions. Although the total ion current is composed of partial ion currents, the TOF spectrum of thermal ions generally shows only a single peak with no visible evidence of separated maxima corresponding to the partial ion currents, as Eq. (1) indicates.
Table 1 Abundance and ratio of centre-of-mass kinetic energy and ion temperature, ECM/T, of Feqþ (1 q 4) ions generated by the Nd:YAG laser, which was operated at l ¼ 1064 nm and delivered a fluence of 30.6 J/cm2. The abundance of ionized impurities was deduced from the first peak of the ion collector signal at TOF ¼ 4.5 ms. þ
2þ
Ionized species
Fe
Fe
Abundance [%] ECM/T
54 5e4
24 1.9
3þ
Fe
17 4.3
3 21
Fe
4þ
Impurities 2 z100
Pb target KrF laser
15 10 5 0
0
50
100 150 200 Time-of-flightL=1m [µs]
250
300
Fig. 4. Pulse-to-pulse fluctuations in the emission of ions emitted from a Pb plasma, which was generated by a KrF laser.
J. Krása et al. / Vacuum 85 (2010) 617e621
0.8
0
H
IIC [A]
0.6
+
C F
4+
-40
7+
0.4 -80
UCEA [mV]
620
0.2
C 0.0 0.0
heavy ions as Au50þ [21]. Generally, the fast ions expand with a velocity u 1 106 m/s. A new phenomenon in the emission of fast ions concerning the outbursts of fast ions was observed [8]. This non-linear phenomenon results in pulse-to-pulse fluctuations, for example, of Cqþ (1 q 6) and Hþ ion emitted from the polyethylene (PE) plasma, as Fig. 5 shows. The plasma was generated by the PALS laser facility delivering energy of about 200 J at the wavelength l0/3 ¼ 438 nm. The system was checked to be in the focus at the target surface, FP ¼ 0. The applied intensity related to the laser wavelength was Il2 ¼ 2 1014 W cm2 mm2. The fast ions generated exhibit not only a separation of charges in space and time during their free expansion but also significant pulse-to-pulse fluctuations in the amplitude e i.e. outbursts similar to a selfpulsing instability are present leading to a chaos, see the TOF spectra from 0.1 ms to 0.6 ms. Although the irregular outbursts of fast ions have occurred, the peak of thermal ions shows small pulse-topulse fluctuations. In contrast to the fast ions peaks, which are largely composed of C6þ and C5þ ions, the main peak contains all the partial currents of all Cqþ (1 q 6) and Hþ ions. Their temperature obtained by the deconvolution is T z 1.6 keV [19]. The centre-of-mass energy, ECM, of Hþ ions as well as of Cþ ions is Hþ;Cþ < TÞ; as opposite to mostly lower than the temperature ðECM these single charged ions, Cqþ ions with q 2 reach higher centreC2þ C6þ y5 T to ECM y50 T. of-mass energies ranging from ECM Last peaks in Fig. 5 at TOF z 7 ms were ascribed to the slow ions generated by X-rays emitted from the expanding thermal plasma, which are absorbed by the target, and, thus, both the target ablation and the emission of ions are prolonged [22,23]. The slow peaks are composed of Cqþ (1 q 4) and Hþ ions. The radiation emitted from the thermal plasma can be regarded to be a secondary source of radiation. In contrast to the reproducible generation of thermal ions the generation of slow ions can hardly be expected to be well reproducible due to the chain of involved non-linear processes creating the fast and thermal ions. If the laser intensity is increased, the non-linear interaction between the impacting laser pulse and the expanding plasma results in clear emergence of plasma outbursts, thus, of outbursts of ions, as Fig. 6 shows. The polytetrafluoroethylene (PTFE) target was exposed to the first harmonics with Il2 ¼ 1.4 1016 W cm2 mm2. To provide an effective heating of the plasma, the laser beam was focused 100 mm behind the target surface, FP ¼ 100 mm. In this case the heating is very effective because the plasma region with the maximum absorption coefficient coincides with the region of the highest intensity of the focused laser beam. The thermal ions were produced only as a minority group while the fast ions totally dominated. The fast ions do not constitute a single group as the thermal ones do, but they are separated into a number of outbursts (see the bottom trace in Fig. 6). Every outburst contains fully stripped carbon and fluorine ions, as the coincidence between the
9+
5+
8+
F C ;F
0.4 0.6 Time-of-flight L = 1 m [µs]
-120 0.8
Fig. 6. Outbursts of Cqþ (1 q 6), Fqþ (1 q 9) and Hþ ions emitted by a polytetrafluoroethylene plasma generated by the laser system PALS operated at l ¼ 1315 nm, Il2 ¼ 1.4 1016 W cm2 mm2, FP ¼ 100 mm. The upper trace shows a spectrum of ions which have an energy of q 60 keV after passing through the mass analyzer.
group of C6þ, F9þ, C5þ and F8þ peaks in the upper trace and the outburst at TOF z 450 ns in the bottom trace indicates. The irregular generation of the ion outbursts results from a nonlinear interaction of the impacting laser beam with the expanding plasma. Evidently, this non-linearity is significantly affected by the laser pulse precursor (pedestal/contrast). If the laser pulse with an insufficient contrast is focused onto a target, the pulse precursor ionizes the ablated material which is later heated by the main part of the pulse [16]. Fig. 7 shows the temporal shape of an applied laser pulse. Owing to the laser pulse precursor, about 1.2 J from the total 150 J delivered is deposited onto the target during the first 100 ps before the main part of the pulse impacts. In comparison with the experiment presented in Fig. 1, the fluence delivered by the precursor is about 150 times higher because it reaches a value of w20 kJ/cm2. Varying the laser pulse energy, the space-time characteristics of the preformed plasma are varied and, thus, the focus position dependence shows a complex interaction between the preformed plasma and the main pulse of the laser beam. The precursor-created underdense plasma can form a plasma region with the maximum absorption coefficient coinciding with the region of the highest intensity of the focused laser beam. This nonlinear interaction inevitably affects the evolution and stability of the emitted ion group properties. Therefore, the pulse-to-pulse fluctuations of emitted currents of fast ions can be significant, as irregularities in the ion emission from the plasma of a heavy element demonstrated in Fig. 8. The Ta target of 2 mm in thickness was exposed to the first harmonics with intensity of 7.5 1015 W cm2 mm2, related to the wavelength. The fastest Ta ions gained kinetic energy of 50 MeV while the maximum energy gained in the second laser shot was only 25 MeV. The highest charge-state of 48 of Ta ions was recognized in a spectrum of the cylindrical electrostatic ion analyzer. One should note that the effective voltage accelerating the Ta ions should reach a value of
1.0 I(t)
Fig. 5. Pulse-to-pulse fluctuations in the emission of Cqþ (1 q 6) and Hþ ions emitted by a polyethylene plasma generated by the laser system PALS operated at l ¼ 438 nm. Il2 ¼ 2 1014 W cm2 mm2; FP ¼ 0.
0.2
6+
0.5 0.0 0.0
Precursor 1J
0.5
1.0
1.5
Tim e [ns] Fig. 7. Temporal shape of the first harmonics 1u0 pulse integrated over the beam cross section of the iodine photodissociation laser PALS. The precursor is seen near the leading edge. The pulse energy was 150 J.
J. Krása et al. / Vacuum 85 (2010) 617e621
621
IIC [mA]
Acknowledgements
200
Ta target
This work was partly supported by the Academy of Sciences of the Czech Republic (project AV0Z10100523) and by the Grant Agency of the ASCR (Grant IAA100100715).
100
References
0 0.1
0.2
Time-of-flightL=1m [µs]
1
2
Fig. 8. Pulse-to-pulse fluctuations in the emission of Ta ions generated by the laser system PALS (l ¼ 1315 nm, Il2 ¼ 7.5 1015 W cm2 mm2, FP ¼ 0).
about 1 MV. The highest current density at a distance of 1 m from the target was measured to be 170 mA/cm2. The outbursts of Ta ions as well as of C and F ions, which are shown in Figs. 6 and 8, well represent significant fluctuations in the emission of ions from a two-temperature plasma due to the created fast electrons.
4. Conclusions The classification of laser ion sources concerning their pulse-topulse reproducibility in the ion emission was presented. In particular, the analysis of time-of-flight spectra of expanding ion flux, which were generated under various experimental conditions, made it possible to estimate the threshold laser intensity limiting the range of the high degree of reproducibility of the ion emission. The ion emission induced by pulsed lasers can be characterized as follows. (1) If the plasma is produced by a short laser pulse having the lowcontrast of w106, the laser intensity of w3 1014 W/cm2 focused onto a target is a threshold value below which pulseto-pulse fluctuations in the ion emission are not significant. Since the kinetic energy per nucleon of thermal ions reaches only w30 keV/u, the multiply charged ions can be accelerated by a high voltage to extract them to form very stable monoenergetic ion beam. (2) Above the threshold, the yields of the different charge states of the fast ions vary strongly from pulse-to-pulse due to the nature of the ion outburst instability. This deficiency can be partially balanced by a possibility to produce highly charged ions of heavy elements, e.g. Ta48þ, and by a high overall current density which reaches a value of 170 mA/cm2 at a distance of 1 m from the target. Although the pulse-to-pulse fluctuations are specific for the currents of fast ions, a high number of shots can eliminate such a trouble in some practical applications. (3) The precursor effect being responsible for the occurrence of the pulse-to-pulse fluctuations can be eliminated if a higher harmonics of the high-power laser radiation is used to heat a target because in the case of higher harmonics, the precursor intensity is strongly reduced. Although the produced plasma remains dense and collision dominated, the intensity threshold for several non-linearities can be exceeded due to, e.g. a transient and time periodic ponderomotive self-focusing.
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