  We report observation of kHz-pulsed-laser-accelerated electron energies up to
3 MeV in the -$k_\text{laser}$ (backward) direction from a 3 mJ laser
interacting at normal incidence with a solid density, flowing-liquid target.
The electrons/MeV/s.r. >1 MeV recorded here using a mJ-class laser exceeds or
equals that of prior super-ponderomotive electron studies employing lasers at
lower repetition-rates and oblique incidence. Focal intensity of the
40-fs-duration laser is 1.5 $\cdot$ 10$^{18}$ W cm$^{-2}$, corresponding to
only ~80 keV electron ponderomotive energy. Varying laser intensity confirms
electron energies in the laser-reflection direction well above what might be
expected from ponderomotive scaling in normal-incidence laser-target geometry.
This direct, normal-incidence energy spectrum measurement is made possible by
modifying the final focusing off-axis-paraboloid (OAP) mirror with a central
hole that allows electrons to pass, and restoring laser intensity through
adaptive optics. A Lanex-based, optics-free high-acquisition rate (>100 Hz)
magnetic electron-spectrometer was developed for this study to enable
shot-to-shot statistical analysis and real-time feedback, which was leveraged
in finding optimal pre-plasma conditions. 3D Particle-in-cell simulations of
the interaction show qualitative super-ponderomotive spectral agreement with
experiment. The demonstration of a high-repetition-rate, high-flux source
containing >MeV electrons from a few-mJ, 40 fs laser and a simple liquid target
encourages development of future $\geq$kHz-repetition, fs-duration
electron-beam applications.
