Meter-scale plasma waveguides for laser wakefield acceleration*

Laser wakefield acceleration (LWFA) can deliver electron beams of multi-GeV energies. Achieving such high energy gains, while avoiding dephasing and depletion, typically requires the laser driver to propagate over many Rayleigh lengths at high intensity (normalized vector potential a>1) through low density plasma (~10¹⁷ cm^-3). We have recently demonstrated two methods, based on optical field ionization (OFI), to generate low-loss, meter-scale plasma waveguides (in hydrogen plasma) where high intensity guided modes can propagate hundreds of Rayleigh lengths [1,2]. The first method uses two time-separated Bessel beam pulses, where the first pulse (a J₀ beam) generates the core of the waveguide and the second pulse (a high order Bessel beam) generates the waveguide cladding. The resulting plasma density profile does not rely solely on hydrodynamic expansion and it allows independent control of core and cladding height, which is crucial for reducing guided mode leakage. In the second method, a J₀ Bessel beam heats the hydrogen to yield a neutral hydrogen channel with a central density depression. The leading edge of a high intensity laser pulse, injected into the end of this structure, self-generates its own plasma waveguide as it propagates. Both methods employ our recently developed long supersonic hydrogen gas jets. I will describe our recent LWFA experiments employing 20-30 cm long gas jets and injection of multi 100 TW laser pulses and achieving multi-GeV electron acceleration.