The role of near time coherent contrast on ultrafast laser driven radiation sources
This work presents experimental results from two campaigns using the Gemini laser system at RAL and the JETI 40 laser in Jena. Simulations using EPOCH are also presented. The overall theme is the effect of near time, femtosecond - tens of picoseconds, contrast on different mechanisms produced by laser - plasma interaction. The Gemini experiment is one which examines the effect of a controlled prepulse introduced into a set-up for ion acceleration via Target normal sheath acceleration (TNSA). Using the optical streaking method, ultrafast absorption dynamics induced by the accelerated protons in H_20 are observed. By changing the arrival of the controlled prepulse the maximum energy of the incident protons, and by extension their stopping depth in the H_20 sample, can be finely tuned. The experimental results from the JETI 40 examine the effect of third order dispersion (spectral phase) on high harmonic generation. The third order dispersion (TOD) changes the shape of the incident pulse creating either leading or trailing satellite pulses which can influence the harmonics generated. A scan of TOD is undertaken, and the harmonics examined. This provides new insight into how the near-time contrast on the femtosecond time scale affects the harmonic generation. A controlled prepulse like that used on the Gemini laser system is then introduced on the JETI 40 and the same scan of TOD is undertaken. It is found that a combination of both a prepulse and TOD are significant for obtaining the highest efficiency harmonics. This implies that contrast on these timeframes must be considered for proposed applications using this source. Finally EPOCH simulations are performed to model the JETI 40 results in a simplified fashion. These simulations examine the role of a shoulder peaking 100 fs before the main pulse.
High harmonic generation, ion acceleration, contrast
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