P. S. Foster

Monoenergetic beams of relativistic electrons from intense laser-plasma interactions

High-power lasers that fit into a university-scale laboratory can now reach focused intensities of more than 1019 W cm-2 at high repetition rates. Such lasers are capable of producing beams of energetic electrons, protons and γ-rays. Relativistic electrons are generated through the breaking of large-amplitude relativistic plasma waves created in the wake of the laser pulse as it propagates through a plasma, or through a direct interaction between the laser field and the electrons in the plasma. However, the electron beams produced from previous laser–plasma experiments have a large energy spread, limiting their use for potential applications. Here we report high-resolution energy measurements of the electron beams produced from intense laser–plasma interactions, showing that—under particular plasma conditions—it is possible to generate beams of relativistic electrons with low divergence and a small energy spread (less than three per cent). The monoenergetic features were observed in the electron energy spectrum for plasma densities just above a threshold required for breaking of the plasma wave. These features were observed consistently in the electron spectrum, although the energy of the beam was observed to vary from shot to shot. If the issue of energy reproducibility can be addressed, it should be possible to generate ultrashort monoenergetic electron bunches of tunable energy, holding great promise for the future development of ‘table-top’ particle accelerators.

Enhanced proton beams from ultrathin targets driven by high contrast laser pulses

The generation of proton beams from ultrathin targets, down to 20 nm in thickness, driven with ultrahigh contrast laser pulses is explored. the conversion efficiency from laser energy into protons increases as the foil thickness is decreased, with good beam quality and high efficiencies of 1% being achieved, for protons with kinetic energy exceeding 0.9 MeV, for 100 nm thick aluminum foils at intensities of 10(19) W/cm(2) with 33 fs, 0.3 J pulses. To minimize amplified spontaneous emission (ASE) induced effects disrupting the acceleration mechanism, exceptional laser to ASE intensity contrasts of up to 1010 are achieved by introducing a plasma mirror to the high contrast 10 Hz multiterawatt laser at the Lund Laser Centre. It is shown that for a given laser energy on target, regimes of higher laser-to-proton energy conversion efficiency. can be accessed with increasing contrast. The increasing efficiency as the target thickness decreases is closely correlated to an increasing proton temperature. (c) 2006 American Institute of Physics.

The plasma mirror - A subpicosecond optical switch for ultrahigh power lasers

Plasma mirrors are devices capable of switching very high laser powers on subpicosecond time scales with a dynamic range of 20–30 dB. A detailed study of their performance in the near-field of the laser beam is presented, a setup relevant to improving the pulse contrast of modern ultrahigh power lasers (TW–PW). The conditions under which high reflectivity can be achieved and focusability of the reflected beam retained are identified. At higher intensities a region of high specular reflectivity with rapidly decreasing focusability was observed, suggesting that specular reflectivity alone is not an adequate guide to the ideal range of plasma mirror operation. It was found that to achieve high reflectivity with negligible phasefront distortion of the reflected beam the inequality csΔt<λLaser must be met (cs: sound speed, Δt: time from plasma formation to the peak of the pulse). The achievable contrast enhancement is given by the ratio of plasma mirror reflectivity to cold reflectivity.

Bright quasi-phase-matched soft-x-ray harmonic radiation from argon ions

Selective enhancement (>10(3)) of harmonics extending to the water window (approximately 4 nm) generated in an argon gas filled straight bore capillary waveguide is demonstrated. This enhancement is in good agreement with modeling which indicates that multimode quasi-phase-matching is achieved by rapid axial intensity modulations caused by beating between the fundamental and higher-order capillary modes. Substantial pulse energies (>10 nJ per pulse per harmonic order) at wavelengths beyond the carbon K edge (approximately 4.37 nm, approximately 284 eV) up to approximately 360 eV are observed from argon ions for the first time.

Evidence of photon acceleration by laser wake fields

Photon acceleration is the phenomenon whereby a light wave changes color when propagating through a medium whose index of refraction changes in time. This concept can be used to describe the spectral changes experienced by electromagnetic waves when they propagate in spatially and temporally varying plasmas. In this paper the detection of a large-amplitude laser-driven wake field is reported for the first time, demonstrating photon acceleration. Several features characteristic of photon acceleration in wake fields, such as splitting of the main spectral peak and asymmetries between the blueshift and redshift for large shifts, have been observed. The experiment is modeled using both a novel photon-kinetic code and a three-dimensional particle-in-cell code. In addition to the wide-ranging applications in the field of compact particle accelerators, the concept of wave kinetics can be applied to understanding phenomena in nonlinear optics, space physics, and fusion energy research.

Relativistic electron mirrors from nanoscale foils for coherent frequency upshift to the extreme ultraviolet

Reflecting light from a mirror moving close to the speed of light has been envisioned as a route towards producing bright X-ray pulses since Einstein’s seminal work on special relativity. For an ideal relativistic mirror, the peak power of the reflected radiation can substantially exceed that of the incident radiation due to the increase in photon energy and accompanying temporal compression. Here we demonstrate for the first time that dense relativistic electron mirrors can be created from the interaction of a high-intensity laser pulse with a freestanding, nanometre-scale thin foil. The mirror structures are shown to shift the frequency of a counter-propagating laser pulse coherently from the infrared to the extreme ultraviolet with an efficiency >104 times higher than in the case of incoherent scattering. Our results elucidate the reflection process of laser-generated electron mirrors and give clear guidance for future developments of a relativistic mirror structure.

Diagnostic of laser contrast using target reflectivity

Using three different laser systems, we demonstrate a convenient and simple plasma based diagnostic of the contrast of high-power short-pulse lasers. The technique is based on measuring the specular reflectivity from a solid target. The reflectivity remains high even at relativistic intensities above 10(19) W/cm(2) in the case of a high-contrast prepulse-free laser. On the contrary, the specular reflectivity drops with increasing intensities in the case of systems with insufficient contrast due to beam breakup and increased absorption caused by preplasma.

A tuneable ultra-compact high-power, ultra-short pulsed, bright gamma-ray source based on bremsstrahlung radiation from laser-plasma accelerated electrons

The laser driven plasma wakefield accelerator is a very compact source of high energy electrons. When the quasi-monoenergetic beam from these accelerators passes through dense material, high energy bremsstrahlung photons are emitted in a collimated beam with high flux. We show how a source based on this emission process can produce more than 109 photons per pulse with a mean energy of 10 MeV. We present experimental results that show the feasibility of this method of producing high energy photons and compare the experimental results with GEANT4 Montecarlo simulations, which also give the scaling required to evaluate its suitability as method to produce radioisotopes via photo-nuclear reactions or for imaging applications.

Dependence of laser accelerated protons on laser energy following the interaction of defocused, intense laser pulses with ultra-thin targets

The scaling of the flux and maximum energy of laser-driven sheath-accelerated protons has been investigated as a function of laser pulse energy in the range of 15-380 mJ at intensities of 10(16)-10(18) W/cm(2). The pulse duration and target thickness were fixed at 40 fs and 25 nm, respectively, while the laser focal spot size and drive energy were varied. Our results indicate that while the maximum proton energy is dependent on the laser energy and laser spot diameter, the proton flux is primarily related to the laser pulse energy under the conditions studied here. Our measurements show that increasing the laser energy by an order of magnitude results in a more than 500-fold increase in the observed proton flux. Whereas, an order of magnitude increase in the laser intensity generated by decreasing the laser focal spot size, at constant laser energy, gives rise to less than a tenfold increase in observed proton flux.

Investigation of the role of plasma channels as waveguides for laser-wakefield accelerators

The role of plasma channels as waveguides for laser-wakefield accelerators is discussed in terms of the results of experiments performed with the Astra-Gemini laser, numerical simulations using the code WAKE, and the theory of self-focusing and self-guiding of intense laser beams. It is found that at a given electron density, electron beams can be accelerated using lower laser powers in a waveguide structure than in a gas-jet or cell. The transition between relativistically self-guided and channel-assisted guiding is seen in the simulations and in the behaviour of the production of electron beams. We also show that by improving the quality of the driving laser beam the threshold laser energy required to produce electron beams can be reduced by a factor of almost 2. The use of an aperture allows the production of a quasi-monoenergetic electron beam of energy 520 MeV with an input laser power of only 30 TW.