Automation and control of laser wakefield accelerators using Bayesian optimisation

Laser wakefield accelerators promise to revolutionize many areas of accelerator science. However, one of the greatest challenges to their widespread adoption is the difficulty in control and optimization of the accelerator outputs due to coupling between input parameters and the dynamic evolution of the accelerating structure. Here, we use machine learning techniques to automate a 100 MeV-scale accelerator, which optimized its outputs by simultaneously varying up to six parameters including the spectral and spatial phase of the laser and the plasma density and length. Most notably, the model built by the algorithm enabled optimization of the laser evolution that might otherwise have been missed in single-variable scans. Subtle tuning of the laser pulse shape caused an 80% increase in electron beam charge, despite the pulse length changing by just 1%

Transverse interferometry of a hydrogen-filled capillary discharge waveguide.

Transverse interferometric measurements are presented of the plasma channel formed in a hydrogen-filled capillary discharge waveguide recently used to generate 1 GeV electrons in a laser-driven plasma accelerator for the first time. The measurements were found to be in good agreement with nonlocal thermal equilibrium simulations, but showed significant differences with the results of a quasistatic model developed by Bobrova et al. [Phys. Rev. E. 65, 016407 (2001)]. The measurements are used to determine scaling laws for the axial electron density and matched spot size of the plasma channel, enabling optimization of the channel to specific applications

Electron acceleration in a gas-discharge capillary

[Abstract unavailable

First milestone on the path toward a table-top free-electron laser (FEL)

Latest developments in the field of laser-wakefield accelerators (LWFAs) have led to relatively stable electron beams in terms of peak energy, charge, pointing and divergence from mmsized accelerators. Simulations and LWFA theory indicate that these beams have low transverse emittances and ultrashort bunch durations on the order of ∼ 10 fs. These features make LWFAs perfectly suitable for driving high-brightness X-ray undulator sources and free-electron lasers (FELs) on a university-laboratory scale.With the detection of soft-X-ray radiation from an undulator source driven by laser-wakefield accelerated electrons, we succeeded in achieving a first milestone on this path. The source delivers remarkably stable photon beams which is mainly due to the stable electron beam and our miniature magnetic quadrupole lenses, which significantly reduce its divergence and angular shot-to-shot variation. An increase in electron energy allows for compact, tunable, hard-Xray undulator sources. Improvements of the electron beams in terms of charge and energy spread will put table-top FELs within reach. © 2010 American Institute of Physids

Gev plasma accelerators driven in waveguides

During the last few years laser-driven plasma accelerators have been shown to generate quasi-monoenergetic electron beams with energies up to several hundred MeV. Extending the output energy of laser-driven plasma accelerators to the GeV range requires operation at plasma densities an order of magnitude lower, i.e. 1018 cm−3, and increasing the distance over which acceleration is maintained from a few millimetres to a few tens of millimetres. One approach for achieving this is to guide the driving laser pulse in the plasma channel formed in a gas-filled capillary discharge waveguide. We present transverse interferometric measurements of the evolution of the plasma channel formed and compare these measurements with models of the capillary discharge. We describe in detail experiments performed at Lawrence Berkeley National Laboratory and at Rutherford Appleton Laboratory in which plasma accelerators were driven within this type of waveguide to generate quasi-monoenergetic electron beams with energies up to 1 GeV

Laser-wakefield acceleration of electron beams in a low density plasma channel

The generation of quasimonoenergetic electron beams, with energies greater than 500 MeV, in a laser-plasma accelerator driven by 2.5 J, 80 fs laser pulses guided in a low density plasma channel, is investigated. The laser energy required to achieve electron injection is found to depend strongly on the quality of the input laser focal spot. Simulations show that, although the matched spot size of the plasma channel is greater than the self-focused spot size, the channel assists relativistic self-focusing and enables electron injection to occur at lower plasma densities and laser powers than would be possible without a waveguide

- som om digtet var sindets neutrum : subjektivitet i Brev i april av Inger Christensen

Denne oppgaven gjennomfører en analyse av diktsamlingen "Brev i april" av Inger Christensen, med sikte på å undersøke hvordan dette diktverkets ulike betydningsnivåer kan samles i et begrep om utvidet subjektivitet. Det ligger til grunn for problemstillingen at Christensens fire siste diktsamlinger - "det" (1969), "Brev i april" (1979), "alfabet" (1981) og Sommerfugledalen (1991) - alle kan karakteriseres som systemdikt, noe som innebærer at den formelle sruktureringen av verkene er formaliserbar, og på en eller annen måte er bestemmende for verkenes utfoldelse. Oppgaven fokuserer på hvordan systemet i "Brev i april" ikke må forstås isolert, men som en integrert del av diktverkets uttrykk og betydning. Den sentrale tesen er at systemet kan forstås som en del av verkets selvrefleksjon, som en kommentar til og påpekning av dets egen verkkarakter for leseren, som likevel inngår i, og ikke nedriver, det betydningsrommet som diktverket bygger opp. Ved å etterspore diktverkets sentrale subjektstematikk, avdekker oppgaven hvordan diktverkets selvrefleksjon, dets evne til å bevege seg inn og ut av sin egen verksgrense, faller sammen med dets undersøkelse av et grunntrekk ved språket: dets samtidige atskillelse fra og tilhørighet til omverdenen. I kapittel I klargjøres oppgavens problemstilling, og den knyttes til "Brev i april" ved hjelp av en gjennomgang av den ingress til dette verket som er å finne i Christensens "Samlede digte". Kapittel II gjennomgår Christensens egne refleksjoner rundt språket, kunsten og sammfunnet i essaysamlingen "Hemmelighedstilstanden", og setter dessuten "Brev i april" i sammenheng med hennes øvrige lyriske forfatterskap. Gjennom dette vises også hvilke utfordringer verkets sammensatthet og selvrefleksjon stiller leseren overfor. Denne problematikken er relevant for diskusjonen av kommentarlitteraturen til Christensens forfatterskap, som er fordelt over kapittel II og III. Ut fra dette tydeliggjør og begrunner kapittel III oppgavens egen teoretiske posisjon, og i dette kapittelet settes også "Brev i april"s selvrefleksjon inn i en lyrikkteoretisk ramme. De påfølgende fire kapitlene er deretter viet selve verksanalysen, som viser hvordan "Brev i april"s ulike nivåer gradvis utfoldes og flettes sammen. Nivåenes samtidige atskilthet og sammenvirkning danner "Brev i april"s utvidede subjektivitet, og denne sammenfattes i kapittel VII, ut fra den leserposisjon som verket krever

Tunable Laser Plasma Accelerator based on Longitudinal Density Tailoring

Laser plasma accelerators have produced high-quality electron beams with GeV energies from cm-scale devices and are being investigated as hyperspectral fs light sources producing THz to {gamma}-ray radiation and as drivers for future high-energy colliders. These applications require a high degree of stability, beam quality and tunability. Here we report on a technique to inject electrons into the accelerating field of a laser-driven plasma wave and coupling of this injector to a lower-density, separately tunable plasma for further acceleration. The technique relies on a single laser pulse powering a plasma structure with a tailored longitudinal density profile, to produce beams that can be tuned in the range of 100-400 MeV with percent-level stability, using laser pulses of less than 40 TW. The resulting device is a simple stand-alone accelerator or the front end for a multistage higher-energy accelerator