SEPPL Backlighter Laser
Technical contact: Johannes Hornung
The „Seeded Experimental Plasma Probe Laser“ (SEPPL) is an off-harmonic and synchronized laser system with highly variable pulse duration usable for probing plasmas using shadowgraphy, interferometry or streaked shadowgraphy with a very high signal-to-noise ratio.
SEPPL has a fixed entry port at the target chamber of the PTA, located centered between TB2 and TB3 of the western chamber wall, at a height of 360 mm above the breadboard. The motorized mirror before entering the chamber allows for adjustments to the beam path while operating under vacuum.
The pulse duration can be chosen between 3.5 ps (compressed pulse), 275 ps (chirped pulse) or continuously between 0.4 ns and 3.4 ns (at 1030 nm), by utilizing a novel seed modulation design, based on a Fabry-Perot cavity (FPC) of variable length. Additionally, it is possible to temporally overload the last amplifier of SEPPL resulting in pulses up to 10 ns. Depending on the chosen pulse duration, the temporal shape of the pulse changes from Gaussian like, for the picosecond pulses and the overloaded pulse, to an exponentially decaying shape between 0.4 ns and 3.4 ns. Additionally, it has to be noted that the nanosecond long pulses contain fast temporal oscillations (≥ 1.7 GHz) which might not be visible when using a “slow” detection method. An example for three different FPC lengths and an overloaded (OL) amplifier at 1030 nm is visible in the next figure.
Pulse duration (FWHM)
3.5 ps, 275 ps or 0.4 ns – 10 ns
Signal to PHELIX emission ratio
Temporal pulse shape
Gaussian like < 0.4 ns & > 4 ns, else exponentially decaying
Energy (Strongly setup dependent!)
≤ 7 mJ
≤ 2 mJ
Energy stability over 45 minutes
< 5 % RMS
< 5 % RMS
Timing jitter to PHELIX pulse
< 1.95 ps
Possible delay to PHELIX pulse
± 2.4 ns (shiftable up to ± 200 ns, requiring setup changes)
10 Hz, continuous
Example of side-viewed streaked shadowgraphy of an artificial nanosecond prepulse, with a SEPPL pulse duration of 3.2 ns (@ 515 nm) with a sweep time of 10 ns using a Hamamatsu C10910.
Example of side-viewed interferometry of an artificial nanosecond prepulse, with a SEPPL pulse duration of 2.5 ps (@ 515 nm). The inlet shows the interferometric fringes, which can be used to reconstruct the plasma electron density up to a maximum of 5×1020 1/cm³.