| Summary: | 碩士 === 國立中央大學 === 物理學系 === 101 === There are two kinds of targets in laser-plasma interaction experiments,
gas target and solid target. Gas target is used in table-top electron accelerator,
high-harmonic generation, EUV laser, and mid infrared generation.
Solid target is used in braking radiation and plasma mirror. In these experiments
a very important condition is that the prepulses must not ionize the
target before the main pulse arrives. Prepulses are the small pulses that appear
before the main pulse. For this reason laser contrast is a very important
parameter in laser-plasma interaction experiment.
In general the ionization threshold of gas target is about 1014 W/cm2 and
that of the solid target is about 1010 W/cm2. The intensity of National Central
University 100 TW laser system can reach 1019 W/cm2. Under this condition,
the required contrast is 105 and 109 for gas and solid targets respectively.
To ensure the laser has a high enough contrast, a 1010 high dynamic
range autocorrelator is needed as a diagnosis tool in a high performance lase
system.
The dynamic range of our existing third-order autocorrelator is only 108,
much lower than the theoretical estimation of 1010. In this experiment, we
change the signal processing for PMT (photo multiplier tube) by inserting
a current amplifier to raise the dark current signal of the PMT above the
quantization noise of the analog-to-digital converter. After this modification,
the dark current can be observed easily and the signal is amplified by
two orders of magnitude without increasing the noise. The dynamic range is
increased by two orders of magnitude by this method.
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