Laser-plasma electron acceleration in dielectric capillary tubes

Abstract

Electron beams and betatron X-ray radiation generated by laser wakefield acceleration in long plasma targets are studied. The targets consist of hydrogen filled dielectric capillary tubes of diameter 150 to 200 microns and length 6 to 20 mm. Electron beams are observed for peak laser intensities as low as 5 × 10 17 W/cm 2 . It is found that the capillary collects energy outside the main peak of the focal spot and contributes to keep the beam self-focused over a distance longer than in a gas jet of similar density. This enables the pulse to evolve enough to reach the threshold for wavebreaking, and thus trap and accelerate electrons. No electrons were observed for capillaries of large diameter (250 µm), confirming that the capillary influences the interaction and does not have the same behaviour as a gas cell. Finally, X-rays are used as a diagnostic of the interaction

Enhancement of x-rays generated by a guided laser wakefield accelerator inside capillary tubes

“…11 In the case of Fig. 1(f), it was found to be 5.4 keV, which is higher than in previous observations 1, 12 with similar laser power.…”

“…The use of laser guiding in capillary tubes has been shown to enable electron acceleration and x-ray emission at low plasma density and low laser intensity. 12,13 In this letter, we report on the ability to increase the number of photons produced in the 2-10 keV range by using a lower density, longer plasma inside capillary tubes, compared to the plasma density and length usually achieved with gas jets. Using 16 TW laser pulses, the generated x-ray peak brightness is multiplied by 30 when the laser beam is guided by a 10 mm long capillary tube instead of using a 2 mm long gas jet.…”

“…The capillary provides a long distance for laser evolution to the threshold required for self-trapping 12,13 and helps collecting and refocusing the energy initially in the wings of the laser spot; 16 the excitation of multiple modes and their beating can also give rise locally to higher intensity than in vacuum, thus favoring an increase of a 0 . For the gas jet, electron trapping starts around n e ¼ 11 Â 10 18 cm À3 , which results in lower energy electrons, as electron energy inversely depends on plasma density.…”

Electron beams and X ray radiation generated by laser wakefield in capillary tubes

“…To achieve low density plasmas and guide the intense laser over a long enough distance, using capillary tubes [11] has several advantages [12]: (i) the plasma density inside capillary tubes can be arbitrarily low, as there is no density requirement for guiding, and different plasma density ranges can be explored; (ii) the capillary tube is able to collect the laser energy in the wings around its central focal spot to assist laser guiding over a longer distance than in a gas jet or gas cell [13]; (iii) the capillary tube provides a shock-free gas medium, beneficial for generating stable electrons and X-rays [14].…”

“…In this paper, we report on the study of electrons self-injection in long plasmas produced inside capillary tubes for two values of input laser intensities [13,15,16]. The influence of the plasma density and of the capillary tube parameters on the non-linear evolution of the laser pulse and electron trapping are characterized using the measured electrons and X-ray beams properties.…”