## 10 EUV Beamline Scaling to a 1 MHz Repetition Rate  
The 1 KHz repetition rate of the present laser-driver leads to quite long data acquisition time (tens of hours), which could not only reduce the efficiency of measurements, but also increases the experimental error. In addition, it could limit the investigation of extremely sensitive but novel materials. The repetition rate and the useful photon flux at the sample material (inside TR-Spin-ARPES spectrometer) could be scaled by a factor of up to \(\sim\)1000\(\times\). This could be achieved by upgrading the present laser driver to the MHz repetition rate and \(\sim\)kW average power, femtosecond fiber-lasers. Such laser is now becoming available [51, 52] and has already been used to produce high EUV fluxes by HHG [52]. The 1 MHz repetition rate, for example, could reduce the data acquisition time from tens of hours at 1 kHz to minutes at 1 MHz, for a complete set of TR-ARPES runs. Figure 14 shows a schematic of such an upgraded EUV beamline system. The \(\sim\)1.2 kW average power, MHz fiber laser output would be \(\sim\)1.2 mJ/pulse, \(\sim\)300 fs pulse duration, \(\lambda\) = 1030 nm (1.2 eV). The laser pulse could be compressed to \(\leq\)40 fs, in a hollow fiber with an output of \(\sim\)0.7 mJ/pulse, which is only \(\sim\)2.6\(\times\) smaller than present pulse energy. The upgraded EUV beamline would operate similarly to the present EUV BL. A small fraction of the laser pulse energy would be split to drive the excitation laser pulse, and the main laser pulse energy drives the HHG EUV source. The large laser average power, \(\sim\)kW, will require special heat management to protect the EUV beamline from damage. The laser beam would be transformed into an annular beam, with a small central hole, prior to focusing in the HHG chamber. The HHG source would use a gas jet and cooled gas nozzle. The laser beam would be separated from the EUV beam immediately after HHG. This could be achieved by taking advantage of the laser annular beam and larger divergence compared to the  
Figure 13: TR-ARPES measurements of the ultrafast surface photovoltaic (SPV) effect in p-GaAs (100). (**a**) TR ARPES spectra of the valence band near the \(\Gamma\) point at 0 fs and 650 fs delay after the excitation pulse. Vertical axis: photoelectron (PE) energy (eV); Horizontal axis: PE momenta k (\(\AA^{-1}\)); (**b**) typical energy distribution curves (EDC) of valence band measured before the excitation pulse (\(-\)500 fs) and after the excitation pulse (+5 ps); (**c**) temporal evolution of SPV effect with 650 fs rise-time. Delay steps are 67 fs; (**d**) SPV effect over showing a plateau lasting \(>\)30 ps. EUV beam. The EUV beam passes through the hole of the mirror while the laser power is reflected by the mirror into a cooled beam-dump. Alternatively, the reflected beam could be recycled to run a second HHG beamline. The laser power loss in the HHG process is very small. The repetition rate could be increased further, for example to 20 MHz (or even 100 MHz) by using the same kW-class fibre laser-driver laser with 20\(\times\) (or 100\(\times\)) reduced pulse energy compared to 1 MHz (Active Fibre Systems GmbH, Jena, Germany, Data Sheet). As the Beamline repetition rate is increased above tens of MHz, consideration needs to be given to the effects of heat diffusion and/or material relaxation in the sample. These very high repetition rates may restrict the option of using Mid-Infrared excitation pulse, as proposed in Figure 14 and the paragraph below. An alternative method of achieving these very high EUV beamline repetition rates is using a Yb:fibre frequency comb of 80 W average power and cavity-enhanced high-harmonic source to obtain repetition of 88 MHz [53] and 60 MHz [54]. The beamlines provide EUV pulses of \(\sim\)100 fs [53] and 400 fs [54] with 65 meV and <25 meV spectral resolution, respectively, with sub-100 \(\upmu\)m EUV spot on the sample material.