Density measurement in a laser-plasma-accelerator capillary using Raman scattering

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.

Figure 1

(a) Top view of the experimental setup. A 532 nm laser (green) is focused by an $f=250\mathrm{mm}$ lens (L1) into the sapphire capillary (CAP). The gas density and profile in the capillary is set by two separate pressure valves that lead to each gas inlet, see Fig. 2a. Raman scattered light (red) is filtered and imaged into a CCD. (b) The image shows the Raman signal, the lines are additionally drawn and indicate the capillary. The increased signal in nozzle region A is due to the high-density gas jet. The stray light to the right of B inside the capillary is caused by the laser hitting the wall. This light was too intense for the filters. The shutter time was 7 s.

Figure 2

Part (a) shows a photograph of the laser-machined capillary with nozzle and the diamond ground gas inlets and outlets. Part (b) illustrates the geometry of the capillary used in FLUENT simulations. The colors denote the number density in ${10}^{19}\times {\mathrm{cm}}^{-3}$. The narrow channel in the middle denotes the capillary; the boxes on the left and right side as well as on top are used to simulate vacuum. The two pipes coming from the bottom are the gas inlets for the nozzle (left) and the inlet (right).

Figure 3

Six density graphs with varying inlet and nozzle pressures (in mbar). The black graphs denote the measurements and the red graphs denote the simulation results. The $y$ axis shows the nitrogen density. The $x$ axis refers to the position along the capillary. The capillary entrance is located at 0 mm.

Figure 4

(a) Applied nozzle pressure versus peak density (experimental results). The red line shows the linear fit. The error bars are RMS. (b) Comparison of an OpenFOAM simulation with a FLUENT simulation under the same conditions.