Demonstration of kilohertz operation of hydrodynamic optical-field-ionized plasma channels

We demonstrate experimentally that hydrodynamic optical-field-ionized (HOFI) plasma channels can be generated at kHz-scale pulse repetition rates, in a static gas cell and for an extended period. Using a pump-probe arrangement, we show via transverse interferometry that the properties of two HOFI channels generated 1 ms apart are essentially the same. We demonstrate that HOFI channels can be generated at a mean repetition rate of 0.4 kHz for a period of 6.5 h without degradation of the channel properties, and we determine the fluctuations in the key optical parameters of the channels in this period. Our results suggest that HOFI and conditioned HOFI channels are well suited for future high-repetition rate, multi-GeV plasma accelerator stages.

Figure 1

(a) Schematic diagram of the experimental setup used to generate HOFI plasma channels at kHz repetition rates. See the main text for a detailed description. (b) Schematic diagram of the generation of probe and pump pulses, showing: (b1) the train of 800 nm pulses from the laser front-end, and the selection of pairs of 800 nm pulses, the pairs being separated by $t_{\mathrm{seq}}=1/f_{\mathrm{seq}}$; (b2) the conversion of each pair of pulses into a pump-probe pair; and (b3) gating of the CCD to record the interferogram generated by the second pump-probe pair. (c) Measured focal spot of the channel-forming beam. (d) Raw interferogram measured 50 ps after the passing of the channel-forming beam, using a 350 mbar backfill pressure of ${\mathrm{H}}_2$.

Figure 2

(a,b,c) Phase maps retrieved from the interferometric measurements. (a,c) show the plasma generated by (a) a single channel-forming pulse, and (c) the second of a pair of channel-forming pulses separated by 1 ms, measured at $\tau =0.5\mathrm{ns}$ after the passing of the channel-forming beam. (b) Measured phase shift 50 ps prior to the arrival of the second channel-forming pulse. (d) Temporal evolution of the transverse electron density profile $n_{\mathrm{e}}(r)$ produced by: (left) a single channel-forming pulse; and (right) the second of a pair of channel-forming pulses separated by 1 ms. For each plot the line and shading show, respectively, the average and standard deviation of the density measured over 50 shots. For these data $P_{\mathrm{b}}=(350\pm 3)\mathrm{mbar}$. The inset in (d) depicts a zoom-in of the electron density profiles for $\tau \ge 1\mathrm{ns}$, clearly showing the plasma channel formed at those times.

Figure 3

Demonstration of the long-term stability of HOFI channels. (a) The measured phase map; the apparent slow drift in the transverse position of the plasma is an artifact caused by a slow drift in the position of the probe beam. (b) The transverse electron density profiles of the HOFI channels formed at a delay $\tau =1.5\mathrm{ns}$ after the second of the pair of channel-forming pulses separated by 1 ms are shown for data recorded at $f_{\mathrm{seq}}=200\mathrm{Hz}$ over a period of 6.5 h. For these data, the initial gas pressure was $P_{\mathrm{b}}=350\mathrm{mbar}$.

Figure 4

Long-term evolution of the total measured phase shift and the key optical properties of the $⟨f_{\mathrm{rep}}⟩=0.4\mathrm{kHz}$ HOFI plasma channels shown in Fig. 3. In all cases, the solid lines and shading show, respectively, the moving average and standard deviation over 50 shots. (a) Maximum phase shift measured ($\mathrm{\Delta }{\varphi }_{\max }$). The red error bar indicates the standard deviation of the averaged measured noise for the shots in the run. (b) The measured shock position $r_{\text{shock}}$, defined by the position of the peak electron density, (c) the electron density at the shock front $n_{\mathrm{e}}(r_{\text{shock}})$, and (d) the axial density $n_{\mathrm{e}0}$. For (b–d), the black error bars indicate the standard deviation of the plotted parameter deduced from a Monte Carlo simulation of the effect of noise on the Abel inversion. The $y$-position of the centre of the error bars is equal to the mean value during the run. The error bars are shown at $t=6\mathrm{h}$; this is an arbitrary choice, although it does allow for direct comparison with the data at the time when the noise of the phase maps is highest.