Laser–plasma-accelerators—A novel, versatile tool for space radiation studies

Nuclear Instruments and Methods in Physics Research A 636 (2011) 31–40

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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

Laser–plasma-accelerators—A novel, versatile tool for space radiation studies a ¨ Bernhard Hidding a,b,, Thomas Konigstein , Oswald Willi a, James B. Rosenzweig b, c,d a Kazuhisa Nakajima , Georg Pretzler a

Institute of Laser and Plasma Physics, Heinrich-Heine-University, D¨ usseldorf, Germany Particle Beam Physics Laboratory, Department of Physics and Astronomy, University of California, Los Angeles, CA, USA c KEK, Tsukuba, Ibaraki, Japan d Shanghai Jiao Tong University, PR China b

a r t i c l e i n f o

abstract

Article history: Received 22 October 2010 Received in revised form 19 January 2011 Accepted 22 January 2011 Available online 4 February 2011

The potential of laser–plasma-based accelerator technology for future advanced space radiation studies is investigated. Laser–plasma accelerators have been shown to be capable of robust generation of particle beams such as electrons, protons, neutrons and ions, as well as photons, having a wide range of accessible parameters. Further, instead of maximum accelerating fields of the order of MV/m as in state-of-the-art accelerators, laser–plasma acceleration operates with fields up to TV/m and can thus be used to reach as yet inaccessible parameter regimes, but which are very relevant to space radiation studies. Due to their versatility and compactness, the same laser–plasma-accelerator can be used in university-scale labs to generate different kinds of particle and photon beams, each yielding up to kGy doses per shot, and allowing combinations of different kinds of radiation production simultaneously. Laser–plasma-accelerators provide the advantage of cost-effective radiation generation, thus ameliorating the current shortage of beam time for testing of radiation resistivity of electronic components. Beyond this, laser–plasma-accelerators can be used to reproduce certain aspects of space radiation, e.g. broad, decreasing multi-MeV-scale spectra, with substantially improved level of fidelity, as compared to state-of-the-art technology. This can increase the significance of electronic components testing, and in turn yield increased reliability and safety of future manned or unmanned space missions, highaltitude flights, as well as the electronic components used in harsh radiation environments in general. Laser–plasma-accelerators may therefore become indispensable tools for next-generation space radiation studies. & 2011 Elsevier B.V. All rights reserved.

Keywords: Space radiation Laser–plasma-acceleration

1. Introduction More national and international programs worldwide are making substantial efforts to take part in space exploration, and quantity as well as quality of space missions are increasing. This broad push includes further missions to the moon, to Mars, and beyond. The radiation environment on such missions is highly complex and may also undergo substantial changes during the mission course. This violent radiation environment can be highly dangerous for biological systems (astronauts) as well as for electronical systems, which can malfunction or crash as the result of impinging space radiation. Electronic components and circuits are essential to space missions and satellites, but also for military applications and the civil aerospace sector, as well as for facilities dealing with

 Corresponding author at: Institute of Laser and Plasma Physics, Heinrich ¨ -Heine-University, Dusseldorf, Germany. E-mail address: [email protected] (B. Hidding).

0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.01.090

nuclear reactions such as atomic power plants and research centers. There is a growing need for beam time for testing the effects of various kinds of radiation on such electronics, and for novel radiation sources and test methods in order to better understand and mitigate these effects. For example, radiation occurring in space often has exponential or power-law energy spectra, since the acceleration mechanisms produce distributions that follow statistical laws. They are therefore not easy to reproduce in the environment of standard accelerators, which due to reliance on resonance produce nearly monoenergetic distributions. In this article, we investigate the potential of laser–plasmaaccelerators (LPAs) for studies of space radiation effects. We will conclude and show that LPAs are highly suited for such studies due to their enormous versatility, the relatively smaller space (and cost) required for an LPA, and the ability to access highly relevant space radiation parameter regimes otherwise inaccessible so far with traditional accelerator technology. In this sense, our current proposal is diametrically opposed to the current trends in LPA research, in which the goal is to create mono-energetic beams

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similar to those produced in current accelerators. Here we seek to exploit advantages inherent in the laser–plasma accelerator per se.

2. Fundamentals of laser–plasma accelerators Today’s laser systems are capable of generating ultra-high powers up to the petawatt-regime based on chirped pulse amplification (CPA) [1]. This scheme has been conceived in order to avoid that the laser intensity exceeds the damage threshold of the amplifier media, which would destroy it. Instead, the pulse is stretched significantly in time using chromatic effects (‘‘chirping’’) before amplification, for example by a grating system. Such long pulses can be amplified by several orders of magnitude without destroying the amplifier medium. After amplification, the pulse is recompressed to (ideally bandwidth-limited) the same duration as before stretching, yielding powers many orders of magnitude higher than those possible without CPA. When such laser pulses are then focussed down to spot sizes of the order of few mm2 , intensities up to 1021 W/cm2 can now routinely be reached with modern laser systems. The effects of such high intensity on media are wavelength-dependent. To see this, one introduces the dimensionless light amplitude a0 ¼ eE=ðm0 ocÞ, where e is the elementary charge, E is the electric field amplitude, m0 is the electron rest mass, o is the laser frequency and c is the speed of light. The a0 factor describes the transition from the nonrelativistic ða0 5 1Þ to the relativistic regime ða0 b 1Þ, where the oscillation of electrons in the focussed laser field is strong enough to cause relativistic mass increase and substantially alter the laser–matter interaction. Using a0, the laser intensity I can 2 be expressed as I ¼ 2a20 e0 cðpme c2 =ðelÞ2 ¼ a20 =ðl ½mm2 Þ  1:37  18 2 10 W=cm . Typically, solid state lasers based on Ti:Sapphire media are used in CPA systems. Such laser systems are very compact, are often dubbed ‘‘table-top’’ and can easily fit into university-scale laboratories. Ti:Sapphire lasers amplify broadband radiation around a central wavelength in the near infrared at lL  0:8 mm. With this wavelength, plasma electron relativistic mass increase effects begin asserting themselves at a focussed intensity of I  1018 W=cm2 , corresponding to a0  1. In this scenario, the motion of electrons in the laser focus is becoming anharmonic, the magnetic field is no longer negligible, and there is a net longitudinal force on the electrons. At the same time, the maximum electric field amplitude E0 ¼ 2pa0 me c2 =ðelÞ ¼ 3:2  2 1012 a0 =ðel ½mm2 Þ V=m is high enough to rapidly ionize media. Therefore media in the laser focus can be turned into a plasma, which is then the source of electrons susceptible to the intense laser field during the interaction process. While electrons can

easily be moved by high-power laser pulses, the much heavier ions remain quasi-stationary in the first phase of the interaction. The generation of plasma and the separation of electrons and ions due to the subsequent plasma electron motion driven by the laser, are the bases for laser-driven acceleration processes. In contrast to state-of-the-art radiofrequency cavity-based accelerator techniques, where the accelerating fields are limited by ionization breakdown which sets in at few tens to few hundreds of megavolts per meter, plasma, already being ionized medium, can support electric fields which are many orders of magnitude higher. This fundamental feature is the reason why laser– plasma-based acceleration is extremely versatile and can be carried out with enormously reduced equipment footprint and space requirement. Fig. 1 shows the principal setup of a laser–plasma acceleration scenario. The focussed high-power laser pulse is incident from the left on a target which can initially be a solid such as a foil, liquid such as a droplet or gaseous such as a gas jet. The rising slope of the laser pulse will ionize the medium in its focus due to the ultrahigh intensities, and the main part of the laser pulse will interact with plasma. The nature and outcome of that interaction strongly depends on the electron plasma density profile, and on the laser beam parameters. In the following, we will show exemplified how various types of radiation can be generated via laser–plasma interaction and how to use these radiation sources for studies of the effects of space radiation. Useful current reviews on particle and photon generation with laser–plasma-accelerators can be found in Refs. [2,3]. It is further not the purpose of this article to describe in detail the various mechanisms by which space radiation generates damages. Instead, we will show how LPAs can generate certain relevant types of radiation which can in turn be used to study these mechanisms and contribute to radiation-hardening approaches.

3. Exponential energy beams—‘‘killer electrons’’ and protons It is well-known that in the van Allen belts, intense beams of so called ‘‘killer electrons’’ [4] can occur. These sudden bursts of intense, high-energy electrons can indeed be extremely deleterious to both man and machine. While in the inner van Allen belt, mostly protons and ions are collected and accelerated (which can also be highly dangerous to spacecrafts, see Section 3.2), encounters with the high-energetic electrons occur mainly in the outer van Allen belt, where Medium Earth Orbit (MEO) satellites such as

Fig. 1. Basic LPA scheme. A focussed high-power laser pulse is incident on matter (solid, liquid, gaseous or plasma). Electrons are set into motion by the focussed electromagnetic fields, which can result either in relativistic pencil-like electron and photon beams propagating in the forward direction (green) with low divergences, and/or in the generation of electrostatic charge separation which in addition to electrons leads to subsequent acceleration of protons and ions with broad-cone emission angles (gray). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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GPS satellites are positioned, for example. The distance as well as the extension of the van Allen belts can vary substantially, so that lower and higher orbits can be affected, too, including geosynchronous orbits [5,6]. Further, other astrophysical objects with magnetic fields such as Jupiter, Saturn, etc. might also create radiation belt electrons that pose a danger to spacecraft [7,8]. The mechanisms for the acceleration of these relativistic electrons have been subject to intense discussion in the last years [7,5,6]. According to NASA standard models AE8/AP8 [9], accelerated and trapped electrons and protons in space have exponential or power-law energy distributions. This means that most particles have relatively low energies, and fewer electrons have higher energies and can penetrate deeper. The detailed process of how the electrons penetrate shielding and accumulate in insulations around satellite electronics, eventually causing catastrophic internal dielectric discharges, for example, is strongly dependent on the energy distribution of the incoming energy flux. Fig. 2(a) shows two examples of exponential electron spectra encountered in space calculated based on the AE8 model. One is calculated for an McIlwain parameter of L¼2 during solar activity minima (AE8min), the other for L¼3 during solar activity maxima (AE8max). However, on Earth, accelerators based on classical technology do not generate such particle energy spectra, but instead give nearly monoenergetic distributions. Furthermore, scattering of such monoenergetic beams in solid matter (e.g., satellite shielding) reshapes the beam energy distributions to forms that are even more unnatural and thus poorly suited to reproduce electron space radiation and to explore their effects. Consider a monoenergetic electron beam of energy E¼5 MeV as produced by a conventional electron accelerator. Based on calculations with MULASSIS [10], we have evaluated the spectral change when the beam straggles through a mm-scale shielding based on a combination of aluminum and plastic. Fig. 2(b) shows the results of these calculations. It can be seen that during passage through the shielding, more and more lower-energy electrons are produced, while the 5 MeV peak gets less pronounced. By comparing Fig. 2(a) and (b), it is obvious that the electron spectra to be encountered in space and those produced on Earth by conventional accelerators are very different. In fact, the spectra generated on Earth after passing a monoenergetic beam through shielding look diametrally opposite to those encountered in space in reality. Considering the energy-dependent stopping powers, it can therefore be concluded that such electron accelerators are far from being optimally suited to simulate electron space radiation. Consequently, the need for radiation sources which can produce exponential spectra suggests itself.

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3.1. Laser–plasma-generation of space ‘‘killer’’ electrons Laser–plasma-accelerators are ideally suited in order to generate electron beams with characteristics such as those occurring in space in the van-Allen belts. This is demonstrated in Fig. 3, where in (a) electron and proton spectra as encountered during a satellite orbit according to the NASA standard models AE8 and AP8 are plotted (here, the KuaFu-B satellite orbit according to Ref. [11] was taken as a reference). In contrast, Fig. 3(b) depicts an example of a typical experimentally measured LPA-generated electron spectrum with exponential energy distribution. The main purpose of this graph is to demonstrate that LPAs can easily generate electron beams with exponential spectra, which is a fundamental characteristic that they share with spaceborne radiation belt electrons. We will later see how the temperature of the electron beam (i.e., its slope) can be adjusted by varying the laser intensity, in order to achieve a better agreement with the actual space electron spectrum. Also, although in both parts of the figure the y-axis gives an equivalence of flux, the units are different (e.g., Fig. 3(a) gives flux per second, while Fig. 3(b) gives flux per laser shot). At the bottom line, the space electron spectrum can in principle be reproduced and thus, studied, here on Earth, while in contrast conventional accelerators cannot be used to produce such electron spectra at all, since the accelerator cavities act as energy filters. In order to generate the depicted spectrum, we used a Ti:Sa laser pulse with a pulse duration of t  80 fs and an energy of E  700 mJ, yielding a power of P  7 TW. This pulse was focussed to a focal spot of 5 mm2 (FWHM), yielding a peak intensity of about I  5  1019 W=cm2 . A 2 mm thick titanium foil was positioned in the laser focus as a target under an angle of 101. There is always a more or less powerful prepulse linked to high-power laser pulses, so one has to take into account that this prepulse already might be intense enough to turn part of the target into a plasma. Hydrodynamical tools such as MULTI-FS [12] can be used for this. Fig. 4 shows the estimated status of plasma expansion due to the prepulse (which has a peak intensity which is in the present case about four orders of magnitude lower than the peak intensity of the main pulse) which arrives at the target about 4 ps before the main pulse as calculated with MULTI-FS. In Fig. 4(a) the plasma electron density and temperature are plotted, while in Fig. 4(b), the ion density is given. The laser pulse is incident from left to right. Plasma electrons are expanding outwards into the vacuum, and form a preplasma with a scale length of ½dðlogNe Þ=dx1  200 nm (Fig. 4(a), left axis), the electrons having a temperature of about 20 keV (Fig. 4(a), right axis). The expansion in terms of the

Fig. 2. Electron fluences in space compared to fluences producible by conventional accelerators. (a) Electron fluence in space, showcases: L¼ 2 in the AE8min model and L¼ 3 in the AE8max model. (b) Electron fluence change when an incident monoenergetic 5 MeV electron beam from a conventional accelerator is straggling through aluminum/plastic shielding.

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Fig. 3. (a) Electron as well as proton fluxes in the van-Allen belt on KuaFu-B satellite orbit according to NASA AP8 and AE8 models [11] (raw data provided by Hong-Fei Chen). The energies are in the MeV range and the spectra are exponential. LPAs are ideally suited to generate such spectra, whereas it would be extremely hard to produce such spectra with conventional accelerator technology. (b) A typical experimentally generated electron spectrum, which is easy to produce with laser–plasma-accelerators.

Fig. 4. Plasma expansion calculated by MULTI-FS for a typical laser prepulse incident on a solid target. (a) The scale length of the plasma density amounts to about 200 nm, while the electron temperature is approximately 20 keV. (b) Corresponding plot of the ion density.

Fig. 5. Setup necessary to generate an electron beam with an exponential energy spectrum as given in Fig. 3. (a) Computer reproduction, (b) photo highlighting the most important elements parabola, target, and electron spectrometer.

much heavier ions is depicted in Fig. 4(b). With the dashed line, the unperturbed initial solid density profile of the titanium foil (density r  4:5 g=cm3 ) is given here, while the solid line gives the density profile after the prepulse, when the main pulse arrives. Next to outward ion expansion into the vacuum, ions are also driven into the solid, where they generate an inward travelling shock front. Fig. 5 shows the setup in the vacuum chamber used to generate the electron beam with the spectrum as shown in Fig. 3(b).

Electrons are emitted perpendicular to the target foil/in the laser direction with considerable divergence. The whole setup fits into a chamber of size  0:5 m3 only [13,14]. In contrast to particle beams generated by classical cavitybased accelerators, where the cavities act as energy selectors and filters and inherently produce monoenergetic beams, complementary the generation of electron beams with exponential energy spectra are inherent characteristics of laser–plasma-interaction. In fact, even in the pre-CPA era scalings have already been

B. Hidding et al. / Nuclear Instruments and Methods in Physics Research A 636 (2011) 31–40

developed which can be used to predict the temperatures of the exponential electron beams [15–17] and have been further developed as relativistic laser intensities had become routinely accessible [18]. The effective electron temperature follows a 2

Teff pðIl Þz scaling, the exponent z typically amounting to values pffiffi between 1/2 and 1/3 [19]. In Ref. [20], a I dependency of the 2

temperature was found for the intensity range from Il  1:3  2

18

19

2

beam charge can be of the order of tens of nC [14]. The exponential spectrum means that most of the electrons are of low energy and therefore have comparably low penetration depths. As we have seen above (compare Figs. 2 and 3), the same situation exists in space, where the outer shielding receives most of the charge and dose, but the higher energy electrons can penetrate.

2

10 to Il  1:4  10 W mm =cm and which is Teff,Wilks ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð 1þ I ½W=cm2 l ½mm2 =ð1:37  1018 Þ1Þm0 c2 . Other experimental works with sub-ps laser pulses and intensities up to 1019 W/ cm2 [21] lead to the slightly different scaling Teff,Beg ¼ 2

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0:1ðI17 l Þ1=3 MeV where I17 gives the intensity in multiples of 1017 W/cm2. This scaling yields Teff ¼ 2.0 MeV for the experimental parameters as described above. Thus, by changing the intensity, and by changing the preplasma parameters, one can easily steer the temperatures of the generated electron beams to emulate electron beam radiation in space with unprecedented level of realism. For example, the temperatures in Fig. 2(a) can exactly be reproduced on Earth by changing the incident laser intensity of a laser–plasma-accelerator. In practice, this can be done via various parameters, for example by changing the laser pulse energy, duration or simply by moving the target on the sub-mm scale outside of the laser focus. Not only the electron beam temperature, but also their divergence follows intensity-dependent scalings. In Ref. [22], experimental data in the intensity range between 1018 and 1021 W/cm2 have been assembled and analyzed. Most of the data stems from PW-class lasers with pulse durations in the ps-scale, but data (previously unpublished) taken with a 80-fs laser fits nicely into those data. Fig. 6(a) shows, how the divergence increases as intensity increases in these measurements. It ranges from about 251 up to about 551. Fig. 6(b) summarizes the pulse durations of the lasers used to establish the data which range from 80 to 5000 fs. The divergence is important for studies of radiation effects, because it strongly influences the doses received by electronic samples. Broad divergence of electron beams can have several advantages. On the one hand, one can change the dose incident on the sample by orders of magnitude simply by adjusting the distance of the sample to the laser target. In addition, very broad areas can be irradiated. This is in diametral contrast to conventional accelerators based on rf-cavity technology, since these cavities act not only as energy filters, but also as divergence filter, rendering the output beam needle-like. The fluences and the absorbed dose per laser shot attributable to electrons depend on the distance to target. The total electron

3.2. Proton and ion acceleration During laser–plasma-interaction with solids, not only electrons are accelerated, but as a secondary effect, also protons and ions. The light electrons move first, but while the most energetic electrons forming the tail of the exponential spectra can have energies of many tens and even hundreds of MeV, most of them gain energies which are limited to the (multi-)keV range. These moderately hot electrons form quasi-stationary electrostatic fields which are strong enough and live long enough to set the comparatively heavy positively charged protons and ions at the target outer faces in motion. This process is commonly called Target Normal Sheath Acceleration (TNSA) [28] and is the dominant scheme for laser-based proton and ion acceleration. Typically, the acceleration process leads to exponential proton and/or ion energy spectra, too. Proton energies up to tens of MeV can routinely be generated. Similar to electrons, proton beams with exponential energy distributions constitute the major part of protons occurring in space (compare Fig. 3). Therefore, reliability and safety considerations suggest to use these laser-generated exponential beams for space radiation effect studies. Proton beams generated by laser–plasma-interaction typically are emitted in a cone with opening angles similar to electron beams. Typical values are approximately 10–301. Protons, due to their high stopping powers, have much less penetration depths in matter than electrons of comparable energy. In addition, the energies of protons typically are significantly lower than those of the simultaneously produced electrons. This means that the deposited doses close to the target due to protons can easily amount to kGy or higher. Therefore, one can place the test samples substantial distances away from the target, still being able to receive high doses, and at the same time having the option to harvest large-area irradiation. While the majority of electrons in the van-Allen belts have energies up to 15 MeV, protons can have energies up to hundreds of MeV. In contrast, while LPAs are easily able to produce electron spectra up to even hundreds of MeV, the high-energetic protons encountered in space are currently well beyond the limit of LPAbased generation. However, it shall be pointed out that the most important aspect is the exponential energies generated by LPAs,

Fig. 6. Divergence of electron beams generated by various laser–plasma-interactions, obviously depending on the laser intensity but not on the laser pulse duration. Data from: Stephens [23], Santos [24], Lancaster [25], Kodama A [26], Kodama B [27], Green A [22], Green B [22], Hidding et al.

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electronic test component in the detection plane, smooth energy tuning can be performed. This is at the same time convenient and remarkable, because changing the output energy in conventional accelerators can be time-consuming, and by far not every accelerator is designed to deliver any energy desired by the experimentalist. Since protons and electrons are deflected in opposite directions, various ports for different measurements exist. For example, the response of one component towards lower-energy electrons could be examined, while another one could be positioned where high-energy electrons leave the magnetic deflector device. Simultaneously, on the other side of the spectrometer, the incidence of protons on electronics can be tested. In addition, on axis behind the target, particle-free irradiation of electronic components with electromagnetic noise generated during laser– plasma-interaction can be used. Fig. 8 summarizes the ultraversatile possibilities of multi-testing of electronic components based on laser–plasma-interaction with solid targets. Various testing ports are indicated in the figure, enabling multiple electron-only (e.g., ports #1 and #2) as well as proton/ion-only (e.g., ports #4 and #5) testing at discrete energies, needle-like testing with photons (port #3, on axis), as well as combined, large-area electron/proton/ion testing with exponential energy distributions (e.g., ports #6 to #9).

and even the tens of MeV cutoff limit which is currently the LPA limit for protons, are very interesting for electronics testing. As regards ion energies, multi-MeV/u acceleration of ions such as carbon [29] and various other types of ions [30], including heavy ions [31], have been demonstrated, and schemes for acceleration up to multi-GeV energies of ions are currently being developed [32]. In this context, it shall be noted that MeV electrons might also be useful to examine certain effects of space proton irradiation [33]. As a basic principle of laser–plasma acceleration the electrons are moved first by the laser, while protons and ions, due to their higher masses, have much slower velocities even at MeV energies. This means that emitted protons lag behind the faster electrons, which can be measured with Faraday cups, for example (see Fig. 7(b)). This shows vividly, how electron and proton beam emission are connected. It furthermore means that normally one will have both electrons and protons (and ions) impinging on the test sample as the result of a single laser shot. However, it is easily possible to separate electrons from protons and ions in magnetic fields due to their opposite charges. This simple procedure also opens up the option to investigate the effects of discrete parts of the energy spectra of electrons or protons and ions, for example. Although the particle beams with exponential energy distribution are a far more realistic particle source when it comes to modelling space radiation when compared to state-of-the-art accelerators, monoenergetic beams can be useful to examine the effect of particle irradiation with varying intrusion depths and energy deposition. This can be easily done with LPAs, too. Since beams with exponential energy distributions simultaneously provide all energies up to their cutoff energy, energy resolution via magnetic fields or via quadrupoles can be used to ‘‘cut out’’ a specific energy from the exponential spectrum. By translating the

4. Using monoenergetic electron bunches for simulation of cosmic ray ionization tracks Besides the inherent and outstanding ability of LPAs to generate particle beams with exponential energies, there is another mode of laser-particle-acceleration by which it is possible to generate electron beams with quasimonoenergetic energy distribution and

target

protons

amplitude / a.u.

incident laser pulse

protons

3

+ + ++ ++ ++ + ++ + ++ + +

2 1 0 -1

electrons -2 -2

electrons 0

2

4

6

8

time / µs Fig. 7. Secondary acceleration of protons and ions. (a) Measured typical signal at Faraday cup due to plasma expansion.

Fig. 8. Multiple, simultaneous component testing scheme. The exponential-energy electrons and protons/ions generated during a laser shot are deflected according to their energy and mass in a magnetic field based on permanent magnets. Multiple measurement ports (e.g., #1 to #9) are possible to test multiple samples during the same shot.

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ultrasmall dimensions. Here, we put to discussion if these electron bunches might be useful for space radiation studies, too, for example as a unique method of mimicking the ionization effects of cosmic energy particles. Already in 1979 the laser-based acceleration of electrons had been proposed [34]. Later it was shown, that a relativistic (electron beam) driver pulse can very effectively expel electrons away from its propagation axis due to their high radial electric fields, leaving behind the quasistationary positive plasma charges and a nearly electron-free plasma ‘‘blowout’’ [35], which ploughs through the plasma at the same speed of the driver. The extensive charge displacement causes high electric wakefields which can be used to accelerate electrons. Fig. 9 visualizes this acceleration mechanism with the help of snapshots of the laser–plasmainteraction retrieved from a particle-in-cell simulation code [36,37]. In Fig. 9, the time evolution of the electric fields and the electron plasma density are given for three different points of time: (a) at the beginning of the interaction, (b) after 150 fs, and (c) after 190 fs. The simulation assumes a (Gaussian) FWHM laser pulse duration of t ¼ 10 fs and a peak (vacuum) focus intensity of I ¼ 8  1018 W=cm2 , and a plasma electron density of ne  3:2  1025 m3 . At the beginning of the interaction, the longitudinal electric fields (see Fig. 9(a), left hand side) of the plasma wave, which is running in x-direction with approximately the speed of light vph  c, are very homogeneous. The plasma wavelength, which is defined by the plasma electron density ne, amounts to lp ¼ c½4p2 e0 m2 =ðne e2 Þ1=2  6 mm, where m and e are the electron mass and charge, respectively. Two consecutive wave buckets are shown behind the driving laser pulse (not visible here), the longitudinal fields amounting to 1.1 TV/m (decelerating, at the front of the bucket) and  0.9 TV/m (accelerating, and the end of the bucket). The transversal electric fields (depicted in

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the middle of Fig. 9(a)) are of the same order of magnitude and exert electron defocussing forces in the first half of the blowout and focussing forces in the second half of the bubble. It is these ultrahigh fields that lead to trapping and acceleration of electrons in the plasma wave. However, as of yet, as can be seen in the right part of Fig. 9(a), there are not yet any electrons injected and trapped in the plasma blowouts. However, the situations change within a few hundreds of fs, corresponding to less than 100 mm, as can be seen in Fig. 9(b). As is revealed by the electron density plot on the right hand side of the figure, electrons have already been injected in the (first) plasma blowout and can harvest its accelerating and focussing co-moving electric fields. The exact mechanisms of self-injection and trapping into the ‘‘blowout’’ or ‘‘bubble’’ [38] can be complicated [39–43] and shall not be discussed within the scope of this article. Here, it is important that it is now possible to produce electron bunches with durations as short as tb  2 fs, lateral sizes of sr  200 nm, and charges Q of tens of pC [44,45]. The electric self-fields of electron bunches scale favorably with beam dimensions, the radial field being Er ðrÞ ¼ Q =½ð2pÞ3=2 sz e0 rð1er

2 =ð2 2 Þ r

s

Þ

ð1Þ

where sz is the length of the bunch [46]. As can be seen from Fig. 9, the electric fields can approach the TV/m-regime. These are values which are beyond the ionization thresholds even of gasses [47]. In addition with the high energies of the bunch electrons, which can be as high as more than a GeV [48] with energy spreads down to about 1% [49,50], these bunches and the connected electric fields would travel through matter like ‘‘relativistic ionizing arrows’’ with penetration depths into solid matter up to the meter scale. In electronics, they would leave behind an intense ionization track which could be used to study single event

Fig. 9. Laser–plasma wakefield acceleration of monoenergetic electrons. (a) Longitudinal (left) and transversal (middle) electric plasma fields, and according electron number density at the beginning of the laser–plasma-interaction. (b) After 150 fs, electrons are already injected from the vertex of the plasma cavity. (c) After 190 fs, a monoenergetic electron bunch has formed in the middle of the plasma blowout/bubble.

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effects such as those generated by cosmic-ray particles. This unique property of such laser–plasma-generated electron bunches is completed by the tiny dimensions of the bunches, which are of the order of the smallest structures which are used in modern microelectronics. Therefore, ionization events and tracks would be similar to those generated by cosmic-particle impacts. Such high-energy nuclei (HZE particles) have also high penetration depths and are highly charged and therefore, very densely ionizing. This said, it shall be noted that the ionization by the unipolar fields of such ultrasmall fields is a novel, fundamental research topic in its own right [44]. It shall also be noted that the generation of highly relativistic, ultrashort electron bunches with characteristics capable to act like ionizing arrows is much more difficult and complex than the production of exponential-energy beams (such as described in Section 3), and is limited to high-end laser–plasma-accelerators. Nevertheless, it is pointed out that the unique properties of these bunches might be a unique access to disclose properties and mechanisms of ionizing radiation such as high-energy cosmic particles in more detail.

5. Experimental implementation towards proof-of-concept In order to implement laser–plasma-accelerators into the repertoire of experimentally viable technologies for the radiation effects testing communities, it is in a first experimental step advisable to stick closely to standard laser–plasma-acceleration experiments, which are today carried out routinely in various labs all over the world. These experiments and setups are well understood, and reliable in terms of beam diagnostics. A lot of these diagnostics are adapted from traditional accelerator technology, while in some cases the special environment of laser– plasma-acceleration gave birth to special diagnostic techniques. For the determination of flux, and at the same time, also of radiation uniformity, reusable image plates (IPs) have been established as highly reliable diagnostics for electrons and are widely accepted and used in LPA radiation experiments. These IPs had originally been invented for X-ray detection, but later turned out to be excellent tools for detection of electrons, and of protons. IPs are easy-to-use, reusable plates with large detection areas (many square centimeters), a very high dynamic range of five orders of magnitude and high resolution (typically 50 mm). Since 2005, several groups have established and refined the use of IPs for electron detection making use of various radiation sources, including well-characterized linacs in a wide electron energy range from keV to GeV [51–54]. These works have led to IP electron sensitive curves and a good understanding of the physical principles behind the detection and readout process. At the bottom line, they are based on the reversible excitation of long-living, metastable states by electron impact (data on received electron flux is stored), and the triggered relaxation of these states in a specific readout system by laser light of a specific wavelength, which is accompanied by emission of light of different wavelength. By collecting and measuring this light emission in the readout scanner, the electron flux collected earlier during exposition can be quantified. IPs work both inside a vacuum chamber and at air, but the IP has to be shielded from visible light by a thin foil such as commercial aluminum foil in order to protect relaxation of metastable states by visible light. So in an experiment, one would as a first step determine the electron number/flux accumulated over a number of laser shots on an IP. Because IPs are large area and high resolution, they can be used at the same time to examine the spatial radiation uniformity. As a side note, IPs could for the same reason also be helpful (additional) tools to determine flux

and radiation uniformity of standard radiation sources used in radiation effects testing, such as linacs and Co-60 facilities, and might thus be of interest for the radiation effects testing community in their own right. When detecting the electron beam inside a vacuum target chamber, it is necessary to break the vacuum to remove the IP and put it into the scanner system after each exposure. Although no UHV is needed, and in a comparably small LPA vacuum chamber pressures of  104 are already good enough to perform the irradiation experiments, this is nevertheless time-consuming. To reduce the impact of this principal drawback of IPs of being offline diagnostics, mechanical translation stages can be used in order to move previously unexposed, fresh IP areas into the beam in between shots, such as used in Refs. [55,54]. Substantial shielding (often with a low-Z–high-Z sandwich structure (see Fig. 5(b)) is used to protect those IPs which are not to be irradiated from particle bombardment and electromagnetic noise. Reference image plates to integrate the 4p background radiation during exposure [54] provide additional information helpful in determining the absolute flux. This way, for example shot-to-shot fluctuation can be measured without having to break the vacuum too often. After having thus fully characterized the spatially resolved total flux and radiation uniformity, one can then proceed to place the DUT at the position of the previously irradiated IP(s). The flux and radiation uniformity received during an arbitrary number of shots by the DUT is known from the IP measurements. In addition, one can place additional IP monitors into the setup, for example around the shielding aperture at the front (this is done in the setup displayed in Fig. 5(b)), for example, as well as behind the DUT. Analysis of the flux accumulated on these monitor IPs after the exposure of the DUT can be used to cross-check the total dose received by the DUT with the one predicted by the previous IP-only calibration runs. In addition, it is also possible to monitor online the radiation flux and uniformity. Integrating Current Transformers (ICTs) as non-intrusive and Faraday cups as intrusive devices for charge measurements are used in LPA experiments as additional means for beam characterization. Also, various optical beam viewers belong to the standard repertoire in LPA radiation diagnostics. These screens scintillate upon exposure of electrons, and by imaging the scintillation by triggered CCDs one can determine the level and uniformity of the incident beam. These screens have been absolutely calibrated [56] and can thus also be used for absolute charge/flux determination [57]. Since the imaging geometry and setup influences the charge, a constant reference light source based on tritium-gas-powered scintillators [58] can be used for imaging-independent calibration of the beam viewers [56]. These screens can be moved in front of the DUT for single test shots to cross-check the impinging flux on the DUT. The combination of these diagnostics can provide the accurate knowledge of flux level and uniformity the DUT is exposed to required in a proof-of-concept experiment for benchmarking with the effects of irradiation from classical electron sources.

6. Conclusion It is shown that it might be imperative to use laser–plasmaaccelerators as complementary, additional radiation sources for future space radiation studies, since they can reproduce or mimic certain kinds of space radiation with a much higher level of realism than state-of-the-art techniques. This holds for radiation belt electron and proton radiation, for example, but might also be true for cosmic energy particle bombardment. Due to their versatility, laser–plasma-accelerators can generate electron, proton,

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ion and photon beams with durations from few fs to many ns, exponential or quasimonoenergetic energies from eV to the GeV regime, divergences from tens of degrees down to few mrad, energies from mJ up to kJ, and doses up to kGy or more. Depending mainly on the power of the laser pulses, repetition rates range from less than 1 Hz to kHz. Setups capable to generate electron radiation such as those in the radiation belts, for example, are now off-theshelf products which are, at steadily decreasing costs, affordable enough even for small university institute-scale laboratories and companies. Operating with TV/m-scale accelerating fields, laser– plasma-accelerators can be much smaller when compared to stateof-the-art accelerators which are limited to MV/m-scale fields, and can access parameter regimes inaccessible by conventional accelerators. One and the same laser–plasma-accelerator can be used to generate electron, proton, ion and photon beams. Switching from one type of radiation to the other as well as energy and flux variation consumes practically negligible time, and all four fundamental types of radiation can even be generated at the same time. It is possible to irradiate large areas with homogenous flux, and even with a combination of the radiation types simultaneously. Laser– plasma acceleration and component testing can be carried through either in vacuum chambers with pressures up to  104 mbar (which leads to much more relaxed component handling when compared to the UHV vacua often necessary for state-of-the art accelerators), or at air, the vacuum chambers then taking on the role of spacecraft shielding, for example. The high fluxes and the feasible exponential-energy, multi-component radiation make it possible to test a whole variety of electronic components at the same time, and if required under different aspects. So far, testing of microelectronics for space missions are among the most expensive and time-consuming processes in spacecraft design [59]. The use of laser–plasma-accelerators could decrease substantially the costs as well as the time consumption of these tests, up to a level to potentially give new momentum to radiationhardening by design approaches. In consequence, their use could in future decrease the lag of performance of electronic components used in space today when compared to standard microelectronics used on Earth. Thus, the performance of electronics on future space missions and satellites could be improved beyond the current progress. Furthermore, using laser–plasma-accelerators instead of radioactive radiation sources is desirable under proliferation aspects. Ultimately, laser–plasma-accelerators could lead to an improvement of reliability of electronic components onboard of space vessels, and thus to increased reliability and safety of both manned and unmanned space missions. Laser–plasma-accelerators might therefore become an indispensable tool for future, advanced testing and certification of electronic components for use in space, for highaltitude flights and in other harsh-radiation environments.

Acknowledgments We would like to thank E. Daly and V. Ferlet-Cavrois/ESA, and S. ¨ Metzger, J. Kuhnhenn and S. Hoffgen at the Fraunhofer INT, Euskirchen, Germany, and L.Z. Scheick, R.D. Harris and S. McClure/ NASA, for fruitful discussions, and H.-F. Chen, Institute of Space Physics and Applied Technology, Peking University, PRC, for KuaFu-B particle flux data. This work has been supported by the Strategischer ¨ Forschungsfonds, Heinrich-Heine-University Dusseldorf, by the DFG SFB TR18, and by ESA’s Networking/Partnering Initiative (NPI). References [1] D. Strickland, G. Mourou, Opt. Commun. 56 (3) (1985) 219. [2] V. Malka, J. Faure, Y.A. Gauduel, E. Lefebvre, A. Rousse, K.T. Phuoc, Nat. Phys. 4 (6) (2008) 447.

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