Direct measurement of coherent ultrahigh wakefields excited by intense ultrashort laser pulses in a gas-jet plasma

The coherent wakefield excited by 2 TW, 50 fs laser pulses in a gas-jet plasma around 10(18) cm(-3) is measured with a time-resolved frequency domain interferometer. The density distribution of the helium gas is measured with a time-resolved Mach-Zehnder interferometer to search for the optimum laser focus position and timing in the gas jet. The results show an accelerating wakefield excitation of 20 GeV/m with good coherency, which is useful for ultrahigh gradient particle acceleration in a compact system. This is the first time-resolved measurement of laser wakefield excitation in a gas-jet plasma. (C) 2002 American Institute of Physics.