Z-Beamlet typically generates a single laser pulse that creates a single diagnostic image per shot. A second image of the same event is especially valuable because individual features can be traced throughout their evolution where as only statistically similar features can be studied with single images from multiple experiments.
Several possible approaches for generating multiple images where considered and angular multiplexing was chosen as the most appropriate. The studies and design for the multi frame backlighting (MFB) was performed in a collaboration with the LS&T group at LLNL, several of whom where involved in the original design and implementation of the Beamlet laser at LLNL.
Z-Beamlet uses a 4 pass main amplifier architecture where the laser pulse is reflected from plane cavity end mirrors back and forth through the main amplifiers 4 times before being directed out of the cavity by a full aperture polarization switch. The 4 passes are off set from each other by a small angle and pass through individual pinholes on each pass within the cavity spatial filter.
Adding a second set of 4 pinholes offset slightly from the first set allows a second beam to be propagated through the same laser system at a slightly different angle to the original beam, the new second beam is equivalent in size and energy to the original but due to its angular offset it can be focused to a separate location.
Before being injected into the main laser cavity, with the required angular offset, the second beam is also delayed in time using a longer optical path. The result being a second laser focus spot that is at a different location and at a different time to the original focus. The two laser spots then generate separate x-ray sources that produce two sequential images of the experiment being performed.
The imaging system used in the experiments is a bent crystal imager developed by Dan Sinars specifically for imaging Z experiments with the Z-Beamlet laser. The picture shows the imaging system during alignment. The cylinder in the middle of the view is the Z experiment of interest, the bright white objects in the lower right is the two bent crystals used to image the x-rays. The red and green laser beams define the path taken by the x-rays generated from the two laser spots on the target foil, upper right end of the beams, behind the shielding block. The lasers and the x-rays cross paths at the experiment and bounce of their respective crystals and into the entrance holes of the film holder that records the images, lower right end of the beams.