Accelerating an object with light has been the dream of mankind for more than a century. In experiments with a so-called light mill, today sold as souvenirs in sunny places, Lebedev  could measure the forces from the radiation pressure of light for the first time.
With our contemporary knowledge the explanation is simple. Photons carry momentum and energy. An object in the light path thus experiences a force and gains kinetic energy, facilitated by the redshift of the reflected photons. Utilizing this force say for space travel is complicated by the weight of the cargo. For example, to accelerate a satellite with a mass of m=30 kg to 70% of the speed of light c, would require the light energy approximately equivalent to its rest energy, i.e. mc2=2.7⋅1018J which could be delivered by a 10 GW laser over 10 years . Such a continuously operated laser remains science fiction, but the most powerful Chirped Pulse Amplification (CPA. Fig. 1) laser systems, where the laser pulse is temporally stretched for amplification , today reach peak powers of up to 1 PW for a very short duration of around 20 fs. The pulse energy of 20 J is equivalent to the rest energy of a carbon disc with a diameter of 3 µm and a thickness of 10 nm. This may be close to the lightest object from which could be reflected a focused laser with 1 µm wavelength. The tiny disc constituting 1010 carbon ions would then be radiation pressure accelerated (RPA) to about 70% c over a distance of 3 µm within 20 fs, i.e. within the pulse duration of the laser.
This simple consideration has at least one severe limitation. When tightly focusing the laser light on a target (for example a thin foil) down to a 3 µm diameter, the intensity approaches ~1022 W/cm2
. Most target materials are vaporized at light intensities near 1010-1012 W/cm2
. In standard CPA-lasers, the intensity gradually rises and eventually exceeds ~1013…1015 W/cm2
(Fig. 2), equivalent to an electric field strength of 1…10 V/Å that binds electrons to their nuclei in atoms. Somewhere between these intensity values, the target is transformed into a plasma.
Being the lightest particles in the plasma, the now free electrons oscillate in the transverse laser electric field absorbing and reflecting part of the laser energy. If the intensity did not rise any further, the absorbed laser energy would drive a typical plasma expansion such as has been studied since the 1960s. The next important stage at 1018 W/cm2 is of particular interest as the electric field amplitude exceeds MV/µm, the plasma electrons gain a kinetic energy comparable to their rest energy (0.5 MeV ~ 0.1 picojoules) and approach the speed of light during phases of the rapid oscillatory motion. Because of this relativistic motion, the Lorentz-force (v×B) which pushes electrons in the direction of laser propagation becomes larger than the transverse oscillatory motion. In other words, the radiation pressure of the laser directs the electrons predominantly forward.
Ideally, forward directed electrons accelerate lagging ions as they pull them along. However, if the intensity rises too slowly (blue curve in Fig. 2), the electrons are heated in an intensive and uncontrolled manner, pulling the ions in all directions normal to the original target boundaries. This so-called target-normal sheath acceleration (TNSA) mechanism, to some extent the modern version of plasma expansion driven by relativistic electrons, has yielded proton beams with energies of up to 70 MeV and dominated most laser-plasma experiments over the past two decades [4,5]. Though already interesting for applications, TNSA inhibits the more desirable radiation pressure acceleration (RPA) mechanism which requires laser pulses with even sharper temporal profiles, i.e high temporal contrast, in order to avoid premature target expansion (orange curve in Fig. 2).
Steepening the leading edge of the laser pulse temporal profile is a tremendous technological challenge which was initially overcome for small scale 10 TW laser systems by implementing the plasma mirror that acts as a fast temporal shutter. Irradiating 5 nm thin diamond-like carbon (DLC) foil targets  resulted in ~108 carbon ions traveling at 6-8% of c. Recently, we extended ultrafast temporal steepening to 100 TW laser pulses by utilizing the nonlinearities arising from the relativistic mass increase of the high energy plasma electrons . In a low density carbon-nanotube plasma directly attached to a 10 nm thin DLC-foil, the laser pulse is focused even more strongly (to higher intensity and field) while at the same time the leading edge becomes considerably steeper, delaying pre-expansion of the DLC-foil. This plasma photonic ‘trick’ enabled acceleration of ~107 carbon ions to 15-20% of c.
The advent of the PW lasers, for example in the context of the European Extreme Light Infrastructure (ELI) or the Center for Advance Laser Applications (CALA) in Garching, for which we cautiously assume initial operation at the single shot level (or very low repetition-rate), mandates finessed engineering and design of sophisticated optical controls. This control requires single shot diagnostics of all laser pulse attributes (energy, spectrum, temporal and transverse profiles, time-dependent intensity contrast, etc) with a repetition-rated, rapid readout capability, ultimately feeding forward into (plasma-based) optical elements that manipulate the laser pulse on an ultrashort timescale. The transition from relativistic laser-plasma science to relativistic laser-plasma engineering refers to the design of such plasma-photonic elements that can play a crucial role in PW-laser particle acceleration.
We have already indicated that ion bunches, abruptly accelerated from micron-sized sources within less than a picosecond, are emitted with a characteristic angular divergence, large energy spread and intrinsically short bunch duration at the source. It would seem that laser-acceleration of particles is fundamentally far removed from conventional acceleration technology. The major distinctions of the laser-driven case are new accelerator challenges, some of which can be viewed as timely opportunities for realizing new diagnostic approaches and suitable applications that exploit laser capabilities and unique laser-driven acceleration features. The currently accessible energies extending up to 10s of MeV/u offer a large potential for applications on the path to one of the most ambitious goals, laser-induced ion beam radiotherapy (LIBRT).
As we learn to walk towards LIBRT, initial radiobiological applications of in-vitro cell experiments have already demonstrated capability to realize all prerequisites for biomedical sample irradiation at more easily accessible low energies of a few MeV/u, meeting all important requirements of absolute dosimetry in a well controlled delivery of multiple-  or single-  laser shots to perform dedicated cell experiments. Further steps to be undertaken as intermediate milestones with presently feasible laser systems include small-animal in-vivo studies and biological experiments using the availability of simultaneous multiple ion species (e.g., protons and carbon ions) produced in the same laser-target interaction. Additional applications on the near horizon deemed to exploit the specific laser characteristics such as large beam divergence and broad energy spread include ion-based imaging, for which proof-of-principle experiments have been already started in Garching. Final performances will strongly depend on ongoing developments in terms of available beam energies and stability, repetition rates and progress in online detector systems.
The current state of laser-particle acceleration and application is encouraging for the PW era, in particular presenting an opportunistic picture of relativistic plasma optical engineering for control of PW systems, for laser-plasma ‘tailoring’ and for advancing laser-driven accelerator machines as integrated systems. In fact, the PW laser era has already begun. In addition to several key high power laser development programs, PW systems can now be purchased from commercial suppliers and the timeframe for many required developments outlined above is immediate. Applying the momentary high pressure of intense laser light to drive and control abrupt acceleration schemes in extreme field plasma environments will demonstrate the clear need for relativistic plasma optics (photonics, electronics and ionics) as a burgeoning new engineering field.
J. Schreiber, P. R. Bolton, K. Parodi
Lehrstuhl für medizinische Physik, Fakultät für Physik, Ludwig-Maximilians-Universität München, Garching/München, Germany
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Lehrstuhl für medizinische Physik
Fakultät für Physik