Abstract
Laser wakefield accelerators have shown 1 GeV electron beams with some 10 pC charge from centimeter-length gas capillaries. The electrons are accelerated by the field of a plasma wave trailing an intense laser pulse. For improving the stability, electron injection and acceleration should be separated. One possible scheme is self-injection with a plasma density gradient and subsequent acceleration at constant density. This can be realized by embedding a high-density gas jet into a capillary. A critical parameter for this scheme to work is the realization of a specific density gradient, therefore a robust measurement is desirable. A new method utilizing the density dependence of Raman scattering has been used to characterize the high-density region of a neutral gas within a capillary with a few ten micrometer longitudinal resolution. This allowed us to measure a density drop of a factor of 4 within 200 micrometers.
Highlights
Laser wakefield accelerators (LWFA)—in which electrons are accelerated by the field of a plasma wave trailing an intense laser pulse—have been demonstrated as compact sources of electrons with energies in the GeV range [1,2]
We present a new method towards a pressure measurement of a density profile in a narrow sapphire capillary for LWFA
We succeeded in spectrally distinguishing density photons from stray photons and could detect a well pronounced density peak produced by the gas jet
Summary
Laser wakefield accelerators (LWFA)—in which electrons are accelerated by the field of a plasma wave trailing an intense laser pulse—have been demonstrated as compact sources of electrons with energies in the GeV range [1,2]. First density down ramp experiments [7] using a gas jet have already shown high charges in the order of nC albeit with an energy spread ÁE=E around 25% FWHM for only 1 MeV electrons. Interferometry measurements as used in previous works [8,9] require rectangular capillaries made from polished sapphire surfaces in order to preserve the phase of the transmitted laser beam This method uses the large refractive index of the plasma column, which is large compared to the one of neutral gas, in order to measure the free-electron density in the plasma. The Raman signal is recorded perpendicular to the laser and the capillary This method allows explicitly measuring a geometry that does not require plan parallel walls like the interferometry and is, well suited for the proposed geometry of an embedded gas jet
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