A laser-driven plasma accelerator can shrink a 100-metre synchrotron down to a few centimetres, but today’s versions fire only 10 pulses per second—too slow for most real-world uses. The problem is the laser. The standard Ti:sapphire laser cannot produce powerful enough pulses at the kilohertz rates that applications demand. New commercial thin-disk lasers can reach those rates, but their pulses are too long to drive the accelerator directly. The Oxford team’s solution—the plasma-modulated plasma accelerator (P-MoPA)—uses a long, high-energy pulse from a thin-disk laser, modulates it with a short seed pulse inside a plasma, then compresses it into a train of short pulses that collectively drive a strong accelerating wave. If P-MoPA works, it could produce electron beams as energetic as those from stadium-sized synchrotrons, but at kilohertz repetition rates and tabletop scale. That would directly benefit compact X-ray sources used for phase-contrast tumour imaging and micro-beam radiation therapy, where current conventional accelerators limit X-ray energy and resolution. A successful P-MoPA would boost X-ray energy, shrink the source to micron scale, and improve imaging resolution at lower radiation dose.
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Laser-driven plasma accelerators (LPAs) can accelerate particles in one-thousandth of the distance needed by conventional machines. For example, synchrotrons use beams of high energy electrons to generate radiation tunable from the infra-red to X-rays, with applications throughout science and industry. The accelerators at the heart of a synchrotron are about 100 m long, but a LPA can generate beams of the same energy in only a few centimetres. Laser-driven plasma accelerators could therefore drive a new generation of very compact sources of energetic particles and radiation, with myriad scientific and technical applications. A major roadblock to realizing these applications is the low pulse repetition rate of the lasers used to drive LPAs. Most LPAs today use a Ti:sapphire laser, which can only generate laser pulses of the required energy at a rep-rate of around 10 Hz (10 pulses per second), whereas many applications of LPAs would require operation in the kilohertz range (thousands of pulses per second). However, commercially-available laser systems --- such as thin-disk lasers (TDLs) --- have recently been developed that can generate laser pulses of the required energy at kilohertz repetition rates. The pulses generated by TDLs are too long to drive an LPA directly, but the Oxford group has proposed a concept to solve that problem: the plasma-modulated plasma accelerator (P-MoPA). This has three stages: The “modulator”. A long (1 ps), high-energy (1 J) “drive” pulse from a TDL co-propagates with the low-amplitude plasma wave driven by a short (50 fs), low-energy (100 mJ) “seed” laser pulse. This adds sidebands to the frequency spectrum of the long pulse. The “compressor”. The spectral phase of each sideband is removed, which converts the spectrally-modulated drive pulse to a train of short laser pulses spaced by the wavelength of the plasma wave driven in the modulator. The “accelerator”. The pulse train is focused into a second plasma of the same density as the modulator. Since the pulses are spaced by the plasma wavelength, the low-amplitude plasma waves driven by each of them add up to generate a large-amplitude plasma wave behind the pulse train. This is analogous to pushing a child on a swing by a series of properly-timed small pushes. In this project we will investigate each of the key steps in the P-MoPA scheme, and we will seek to demonstrate electron acceleration in a P-MoPA for the first time. If successful this could enable a new generation of very compact, kilohertz-repetition-rate accelerators generating electrons with energies comparable to those used in stadium-sized synchrotrons. To give an example, Compton X-ray sources have been used for phase-contrast imaging of early-stage tumours and tests of micro-beam radiation therapy. These work by colliding a beam of electrons with a visible laser pulse, but are currently limited to low-energy X-rays by the conventional accelerators that are used. Using a P-MoPA could boost the X-ray energy substantially whilst also reducing the X-ray source size to the micron-scale, which would improve resolution in imaging applications at reduced radiation dose.
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