Supersonic jets of hydrogen and helium for laser wakefield acceleration

The properties of laser wakefield accelerated electrons in supersonic gas flows of hydrogen and helium are investigated. At identical backing pressure, we find that electron beams emerging from helium show large variations in their spectral and spatial distributions, whereas electron beams accelerated in hydrogen plasmas show a higher degree of reproducibility. In an experimental investigation of the relation between neutral gas density and backing pressure, it is found that the resulting number density for helium is $\sim 30\%$ higher than for hydrogen at the same backing pressure. The observed differences in electron beam properties between the two gases can thus be explained by differences in plasma electron density. This interpretation is verified by repeating the laser wakefield acceleration experiment using similar plasma electron densities for the two gases, which then yielded electron beams with similar properties.

Figure 1

False-color images of five electron beams emerging from (a) ${\mathrm{H}}_2$ and (b) He dispersed by a permanent dipole magnet. In both cases, a 2 mm nozzle was used at a fixed backing pressure of 9.5 bar. The reproducibility of the data is shown by the average of ten individual images of electron spectra for beams emerging from (c) ${\mathrm{H}}_2$ and (d) He. All color scales are normalized to the maximum signal in (a).

Figure 2

Measured charge $Q$ in the electron beams accelerated in a 2 mm gas jet over the scanned pressure range 3–15 bar in ${\mathrm{H}}_2$ (blue circles) and He (red crosses) plotted as functions of the backing pressure. Each point represents the average of ten individual measurements with error bars indicating one standard deviation in each direction. Note that only electrons with an energy exceeding the cutoff (40 MeV) contributes to $Q$ in this figure.

Figure 3

The plasma electron number density $n_e$, 1 mm from the nozzle orifice (2 mm diameter) as a function of the applied backing pressure ($p_0$) for ${\mathrm{H}}_2$ (blue circles) and He (red crosses). Assuming full ionization, the plateau electron number density $n_e$ along the center axis in the laser propagation direction is determined from measurements of the neutral gas number density ($n$) using a setup consisting of an expanded HeNe beam and a wave-front sensor. Each point represents the average of 10–20 individual measurements, and the error bars indicate one standard deviation in each direction. The dashed lines are the theoretical results fitted with regards to $r^{*}$.

Figure 4

(a) Measured charge $Q$ from Fig. 2 plotted against plasma electron density $n_e$ (assuming full ionization) instead of backing pressure $p_0$, by using the results of Fig. 3. Again, only electrons with energies exceeding the cutoff energy of the spectrometer setup contribute to $Q$. Each point represents the average of ten individual measurements with error bars indicating one standard deviation. (b) False-color images of five electron beams emerging from He at $n_e=9.8\times {10}^{18}{\mathrm{cm}}^{-3}$ dispersed by a permanent dipole magnet, showing a high resemblance to electron beams accelerated in ${\mathrm{H}}_2$ at similar $n_e$. (c) Average of the full ten-image sequence which is partly shown in (b). The color scales are normalized to the maximum signal in Fig. 1.