| Summary: | 碩士 === 國立中正大學 === 物理所 === 95 === To extend the interaction length in laser-plasma interaction is the key
factor to the success of many important applications, such as the
laser-driven electron accelerator, soft x-ray laser, and high harmonic
generation. In our experiment, we generate an elongated plasma waveguide
by using a combination of an axicon and the ignitor-heater scheme. The
axicon is a lens in a cone shape which can be use to focus the incident light
into a line focus. The ignitor-heater scheme consists of a short and a long
laser pulses to produce and heat electrons efficiently. To produce a plasma
waveguide, an intense short pulse is focused into the gas to generate the
seed electrons via field ionization, and a copropagating high-energy long
pulse arrives later to heat the plasma and drive the hydrodynamic
expansion. After an adequate time, the plasma waveguide is formed
through the hydrodynamic evolution of the laser-produced plasma in gases.
Since the electron density produced from the hydrogen gas jet with a
1-cm slit nozzle is too low to efficiently form plasma waveguide, a method
to increase the atom density of hydrogen is necessary. A cryogenic system
filled with liquid nitrogen is used to cool the valve and gas reservoir of the
gas jet to increase the local atom density. When the temperature of the gas
jet is reduced to 123K, the atom density of hydrogen is found to be 2.5
times higher than the room temperature, and a plasma waveguide is
efficiently generated with a guiding efficiency of 66%. The high-efficiency
plasma waveguide may soon be applied to electron accelerator to produce
high-quality and high-energy electron beams.
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