4.2. VISAR

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Procedure: VISAR in DAQ preparation

Information

Etalon inventory

VISAR time scaling

VISAR1_time_scaling_v2017.TXTVISAR2_time_scaling_v2017.TXT

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Introduction

Line-imaging VISAR (Velocity Interferometer System for Any Reflector) is a useful diagnostic for measuring shock wave velocity of matter in extreme conditions and is used in many laboratories as a tool to study high pressure, shock, and Z-pinch physics. MEC uses VISAR analysis to study shock waves propagating through materials subject to laser-driven plasma-induced shocks and is capable of measuring shock velocities on a nanosecond timescale. Here we describe MEC's experimental setup dedicated to studying VISAR data and our in-house data analysis code which allows users to analyze their data in-situ.

An overview of the theory and mathematical formalism of VISAR analysis can be viewed in the following document:

A presentation about the principles of VISAR analysis can be viewed in the following document:

Experimental Setup

The MEC experimental endstation can be seen in Fig. 1. An illustratio​​n of the original design of the VISAR setup as a part of the experimental endstation can be seen in Fig. 2. The current setup has been modified from its previous design such that the optics and streak cameras are now contained within a two-level enclosure.

Figure 1. MEC experimental endstation.

Figure 2. Original design of the MEC VISAR setup as part of the experimental endstation. In the current setup, the VISAR optics and streak cameras are contained within a two level enclosure in order to save space on the table.

A schematic diagram of the basic optical arrangement of needed for making a VISAR arrangement can be seen in Fig. 3. Following the arrows provides a flow-chart for how a VISAR measurement is made, starting from the fiber-coupled single-mode laser.

Figure 3. Basic optical arrangement for a VISAR measurement.

As portrayed in the above diagram, as a shock front collides with the object of interest, causing it to propagate forward, light emanating from the fiber-coupled single-mode laser is reflected off of the moving surface. The Doppler-shifted light of wavelength lambda is then directed toward an interferometer, where it is split into two separate beams. One of the beams is temporally delayed by the insertion of an etalon in its beam path, and then the two beams are recombined, resulting in a fringe pattern which is recorded by a streak camera. A shift in the fringe pattern corresponds to a phase shift, which can be used to calculate the breakout velocity of the object of interest.

VISAR Laser

The VISAR laser is an Nd:YAG injection-seeded laser with pulse shaping capabilities. It is driven at a wavelength of 532 nm with a pulse width of 100 ns and a pulse repetition frequency (PRF) of 10 Hz. The total possible output energy of the laser is 30 mJ, however usually no more than ~5 mJ is used so as to not damage the optical fiber that is used for transport. The output of the laser is fiber-optically coupled to the VISAR endstation, since the laser itself does not sit next to the setup (see Fig. 4). The multimode fiberguide used has a core diameter of 1 mm.

Figure 4. VISAR laser location at MEC. The VISAR laser is isolated within the split-mode laser enclosure. It is fiber-optically coupled to the VISAR experimental setup.

The fiber output at the MEC endstation is then directed toward the sample in the ​​​​​​​​target chamber (TC), and the reflected light is then redirected back to the VISAR test setup where it is sent through an interferometer and recorded by the streak cameras. A schematic of this can be seen in the following diagram in Fig. 5.

Figure 5. Schematic diagram of the VISAR endstation. The output of the fiber-coupled laser is directed to the right toward the TC. The light that is reflected off of the sample is then re-directed back to the left and passed through a series of interferometers before reaching the streak cameras.

Relay Imaging System

The sample on which the VISAR laser impinges is imaged back onto the slits of the streak cameras. Cylindrical lenses are used to have a different magnification in the direction parallel and perpendicular to the slits of the streak camera, to increase the contrast of the fringes and maintaining a high resolution for the phase measurement.  The design of this imaging system, with the current magnification and Field of View can be seen in Fig. 6.

Figure 6. VISAR relay imaging system. M_x is the magnification parallel to the streak camera slits, M_y the magnification perpendicular to the slits. The yellow 300mm lens can be changed for different magnifications and Field of View, but this needs to be discussed with POC before the experiment. The final focusing lens to the target is generally now 200mm instead of 150mm, which decreases the magnification by 25% and increases the FOV by 33%, compared to the values in the figure. The final focusing lens to the target is generally now 200mm instead of 150mm, which decreases the magnification by 25% and increases the FOV by 33%, compared to the values in the figure.

While the above setup stays the same for the most part, some changes can be made (i.e. swapping out one lens for one with a different focal length) to conform to the users' specifications. These changes should be discussed with the MEC Point of Contact (POC) and/or instrument scientists for the experiment.

Mach-Zehnder Interferometers

MEC's VISAR analysis uses two Mach-Zehnder interferometers for the separate VISAR beds. Each bed has an etalon of differing thickness, therefore the phase delay between the two beams in each interferometer will be slightly different, thus leading to differing fringe shifts. A 3D diagram of the Mach-Zehnder interferometers can be seen in Figs. 7 and 8.

Figure 7. Top-down view of the two Mach-Zehnder interferometers.

Figure 8. Side view of one of the Mach-Zehnder interferometers.

The Mach-Zehnder interferometers include etalons with lengths <15 cm on motorized linear stages that can travel ~5 cm. The end mirror sits on a motorized stage that can tip, tilt, and translate as needed. Furthermore, they include a white light alignment station with a sensitivity of <2 microns, as well as a flip-up beamblock for remote target imaging and a flip-up reticle for remote CCD alignment check.

By comparing the two different free-surface velocity profiles obtained by the separate VISAR beds, one can find the correct number of 2pi fringe jumps to correct for such that the two free-surface velocities match up.

Optical Streak Cameras

A diagram of the optical streak camera used in MEC's VISAR analysis can be seen below in Fig. 9.

Figure 9. Hamamastu C7700s optical streak camera setup.

The optical streak cameras used to record the VISAR data are Hamamastu C7700s. They have a high dynamic range of 1500:1 and a temporal resolution of <100 ps depending on the sweep range (sweep windows: 0.5 ns, 1 ns, 5 ns,..., 1 ms). They can capture images in a single shot mode or with repetition frequencies up to 5 Hz. They are connected to output optics and a CCD/CCD control unit.

Timing calibration

The images of both the streak cameras can be saved directly to the DAQ. A typical image is shown in the figure10. The horizontal axis is the 1D spatial imaging axis, while the vertical axis is the time axis. The mapping from pixel to time is given by the polynomial in equation  1, where Δt is the time difference between pixel p and pixel p+1, and the coefficients C0, C1 and  C2 given in Table 1 for both streak cameras.

Table 1 . Coefficients for Streak Camera 1

Table 2. Coefficients for Streak Camera 2

Figure.10. Typical image of visar streak camera. The horizontal axis is the spatial imaging direction, and the vertical axis is the time direction.