Laser-plasma-accelerator's potential to radically transform space radiation testing

Hidding, B. and Karger, O. and Königstein, T. and Pretzler, G. and Rosenzweig, J.B. (2014) Laser-plasma-accelerator's potential to radically transform space radiation testing. [Report]

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    Laser-plasma-accelerators are relatively new accelerator devices which are characterized by being very compact, which is the result of the giant electric accelerating fields present in strongly focused, high-power ultrashort laser pulses. Peak intensities of modern laser systems can reach $10^{22}\,\mathrm{W/cm^2}$ or more, which is many orders of magnitude larger than the complete sunlight incident on Earth, if it were collected and focused at the same time onto an area of a tip of a pencil. Such intensities make such laser systems attractive for many applications, as exotic as inertial confinement fusion and producing ultrashort electron beams with GeV-scale energies or advanced light sources such as free-electron lasers, or those based on inverse Compton scattering and betatron radiation. The woldwide booming community in this fields works towards these applications which have highly stringent demands on beam quality, as an alternative to well-established accelerators based on radiofrequency cavity based accelerators such as linacs (for electrons) and cyclotrons (for protons and ions). Breakthroughs were achieved in 2004, when for the first time instead of spectrally very broadband and rather divergent particle beams, pencil-like electron beams with quasi-monoenergetic electron bunch distribution were generated. Beam quality in terms of narrow energy spread and larger energies (beyond the GeV barrier) improves continuously and rapidly, fueled by progress in terms of understanding and by ever increasing laser power and technology readiness. In contrast to such highest-quality beams which are needed for example for free-electron-lasers, space radiation which harms electronics and living systems outside Earth's protective magnetic fields, is always very broadband. In fact, conventional accelerators always automatically produce very narrowband particle beams, which are unnatural. It has been proposed (and patented) for the first time in 2009 to use compact laser-plasma-accelerators to produce broadband radiation such as present in space and to use this for radiation hardness tests. Such broadband radiation is the inherent regime of laser-plasma-accelerators. The difficulty of laser-plasma-accelerators to produce monoenergetic beams is turned into a noted advantage here. Since producing broadband radiation is possible since many years with laser-plasma-accelerators, this application is one which has been ''left behind'' for many years now due to the community seeking to produce more monoenergetic beams such as with conventional accelerators. Recent proof-of-concept experiments in a project which merged state-of-the-art space radiation testing with state-of-the-art laser-plasma acceleration has shown that by using laser-plasma-accelerators it is possible to reproduce the spectral characteristics of radiation belt ''killer electrons'' for example, which populate the radiation belts on GEO orbits, for instance. This especially prominent type of space radiation was for the first time produced in the laboratory here on Earth in a well-controlled manner and seems to be a a natural candidate as a benchmark for other radiation sources, which produce monoenergetic beams based on which also the use of degraders cannot reproduce space radiation which is characterized by a decreasing (often exponentially decreasing) spectral flux towards higher particle energies. Spectral flux shaping by tuning the laser-plasma-interaction parameters has been demonstrated, for example to reproduce the electron flux incident on satellites on GPS orbits according to the AE8 model. Sophisticated diagnostics, readily available from the laser-plasma-community as well as the traditional accelerator community, which are increasingly merging (again), have been used to characterize and monitor the flux. State-of-the-art radiation hardness testing techniques have been adapted to the laser-plasma radiation source environment, test devices have been exposed to laser-plasma-generated space radiation and it was shown that the performance of these electronic devices was degraded. With the exception of doing radiation tests directly in space, these irradiation campaigns may have been the most realistic space radiation tests to be carried out in the laboratory here on Earth to date. The approach of reproducing space radiation flux directly in the lab has hitherto not been accessible, which is why approximative techniques employing monoenergetic beams had to be used. This clearly demonstrated the applicability of laser-plasma-accelerators for space radiation reproduction, and is currently triggering large interest in the laser-plasma-community. Other advantages of laser-plasma-accelerators are that they can produce electrons, protons and ions alike -- even at the same time -- as well as enormous peak flux, which may allow for exploration of nonlinear response of electronics and biological systems. Both fields, laser-plasma-acceleration on the one hand and space radiation testing on the other are highly vibrant fields, which have been disjunct so far. Connecting both fields, and to introduce laser-plasma-accelerators as complementary radiation sources for improved space radiation testing is highly advisable. It shall be emphasized that both the traditional radiation sources as well as laser-plasma-accelerators have inherent advantages, and that it is expected that the combination of both types of radiation sources will be highly fruitful for the further development of the space radiation field. Obvious strenghts of laser-plasma-accelerators are the production of broadband particle flux, and the enormous flexibility, compactness and tunability. For example, the devlopment of a test standard for radiation belt electron radiation effects with laser-plasma-accelerators seems advisable, which could then serve as a benchmark for other radiation sources. On the other hand, it is much harder to produce higher energy protons and ions with laser-plasma-accelerators than electrons. This said, the progress in the laser-plasma-accelerator tecnology is rapid, and protons and ions with several hundreds of MeV have already been produced. The highest proton and ion energies are always reached with large, cutting edge laser facilities, but it has been learned from the last years that steady and ongoing advances in laser technology quickly converts prototype, cutting edge laser technology to commercially available off-the-shelf products. Highest power (hundreds of TW or even PW) laser systems are also characterized by relatively low repetition rate (typically, 10 Hz or less), but there is much movement on this front, too, and kHz systems are already available. Generally speaking, the higher the obtainable particle energies, the lower the repetition rate. This further supports the advised strategy to start the establishment of laser-plasma-accelerators in the space radiation field with reproduction of broadband, lower energy electrons and protons. In this regime, the laser shot repetition rate can be very high, currently up to hundreds of kHz, which increases the average flux. It is estimated that with such systems, for example satellite-relevant fluence can be produced within irradiation times which are orders of magnitude shorter than at large facilities. The development of high power thin-disk and fiber lasers and optical parametric amplification (OPA) technology deserves special attention. Such lasers do not only allow for highest repetition rates, but also for an especially compact setup, best cost-effectiveness and a very high wall-plug power efficiency. Such compact devices with ever increasing powers, repetition rates and therefore obtainable radiation flux levels may end up in the future as compact radiation sources without proliferation issues available on site at chip and electronic manufacturers, and in the air- and spacecraft community. Further increased communication between the laser-plasma-accelerator community and the space radiation community is highly desirable. This should contain further collaborative R\&D acitivities, as well as networking, ideally on a European level. Such a network could bundle the needs and requirements for the most efficient use of laser-plasma-accelerators, for example to ameliorate the shortness of available beamtime for radiation tests which the space radiation community faces today. Based on such a network, a coordinated strategy should be developed which ideally would integrate the European space entities, as well as the traditional accelerator and the laser-plasma-accelerator community. For example, the establishment of laser-plasma-accelerator systems at space radiation testing clusters, for example at ESTEC, and in turn the formation of a dedicated space radiation testing beamline at application-oriented laser-plasma-facilities such as the Scottish Centre for the Application of Plasma-based Accelerators (SCAPA) or at facilities of the European Extreme Light Infrastructure (ELI) seems promising. Even mobile laser-plasma-accelerator devices, mounted on mobile trucks, may be feasible. At the same time, the use of plasma afterburner stages which may convert monoenergetic in broadband flux should be considered.