Progress on laser plasma accelerator development using transversely and longitudinally shaped plasmas

Abstract

A summary of progress at Lawrence Berkeley National Laboratory is given on: (1) experiments on down-ramp injection; (2) experiments on acceleration in capillary discharge plasma channels; and (3) simulations of a staged laser wakefield accelerator (LWFA). Control of trapping in a LWFA using plasma density down-ramps produced electron bunches with absolute longitudinal and transverse momentum spreads more than ten times lower than in previous experiments (0.17 and 0.02 MeV/c FWHM, respectively) and with central momenta of 0.76 ± 0.02 MeV/c, stable over a week of operation. Experiments were also carried out using a 40 TW laser interacting with a hydrogen-filled capillary discharge waveguide. For a 15 mm long, 200 urn diameter capillary, quasi-monoenergetic bunches up to 300 MeV were observed. By detuning discharge delay from optimum guiding performance, self-trapping was found to be stabilized. For a 33 mm long, 300 pm capillary, a parameter regime with high energy bunches, up to 1 GeV, was found. In this regime, peak electron energy was correlated with the amount of trapped charge. Simulations show that bunches produced on a down-ramn and injected into a channel-guided LWFA can produce stable beams with 0.2 MeV/c-class momentum spread at high energies. ResumeProgres dans le developpement d'accelerateurs laser-plasma utilisant des plasmas profiles transversalement et longitudinalement. On presente un resume des progres obtenus au Lawrence Berkeley National Laboratory sur : (1) les experiences sur l'injection dans une rampe de densite decroissante; (2) les experiences sur 1'acceleration dans des canaux de plasmas de decharges capillaires; et (3) les simulations d'accelerateur par champ de sillage laser (LWFA) en configuration multi-etages. Le controle du piegeage dans un LWFA a l'aide de rampes de densite decroissante produit des paquets d'electrons avec des dispersions de moment cinetique longitudinal et transversal plus de dix fois plus faibles que dans des experiences anterieures (respectivement 0,17 and 0,02 MeV/c FWHM) avec une valeur centrale de 0,76 ± 0,02 MeV/c, stable sur la duree de la semaine d'experience. Des experiences ont egalement ete faites a l'aide d'un laser de 40 TW interagissant avec un guide d'onde constitue par une decharge

High quality electron beams from a laser wakefield accelerator

“…Figure 2(b) shows a histogram of 100 consecutive shots as captured using the high energy mode of the electron spectrometer. The rms spread in the central energy is 2.8% which compares favourably with other experiments using either one [23][24][25] or two [8] driving laser beams. It is a common feature of LWFA experiments that laser parameter fluctuations are a significant source of electron beam parameter fluctuations.…”

“…These measurements have two important consequences: firstly, the inferred normalized transverse emittance, ε n , is only ∼0.5π mm mrad or less and, secondly, the reduction in relative energy spread at higher energy indicates a fixed absolute energy spread of ∼0.6 MeV for our acceleration conditions. Preservation of the absolute spread during acceleration [25] is enticing for future scaling of the electron energy commensurate with driving an x-ray FEL (see section 4).…”

Modification of narrow ablating capillaries under the influence of multiple femtosecond laser pulses

“…The capillaries serve for preventing diffraction of the pulse and extending the acceleration length either directly by reflecting the pulse from capillary walls, [8][9][10][11][12][13][14][15][16][17] or indirectly through a specific plasma profile inside. [18][19][20][21][22][23][24][25][26] Capillary-guided laser pulses may find applications for X-ray lasing 27,28 and highharmonic generation. 29 A novel concept of wakefield acceleration in hollow plasma channels, [30][31][32][33][34] which offers a number of advantages over usual plasma acceleration, may also stimulate interest in laser-capillary interaction.…”