X-Ray Diffraction

This configuration of MEC supports diffraction measurements on targets shocked to pressures up to several Mbar, with shocks propagating along the X-ray direction. Four or three ePix10k measure diffraction, with angular range optimized for liquid diffraction at high photon energies (>15 keV). A dual line VISAR diagnostic measures shock break-out to determine pressure. Forward X-ray Thomson scattering (FXTS) is available with the removal of one quad and a reduction in angular coverage; backward X-ray Thomson scattering (BXRTS) is always available at 125° scattering angle. In summary, the available modes for this standard configuration are:

• Full
• FXRTS

They are both detailed below.

Standard Configuration #1: Full mode

The base configuration consists of the ns laser drive beams, VISAR, the backward XRTS (optionally) and four ePix10k quad detectors. Fig.1 shows the layout with all of the quads.

Figure 1. Geometry for XRD standard configuration using two nanosecond, 527 nm drive beams.

The lasers hit the sample at an angle of 20° from each side of the x-ray axis. The angle between the target normal and the LCLS x-ray axis is 0° (so that each drive beam is incident at 20°). VISAR is collected normal to the back of the target. 4 Quad detectors are arranged to spatially resolve wide angle scattering from 8 to 72º. BXRTS will be located around 125 degree.

Figure 2. 3D view with all four quads.

The XRD detection is performed using 4 “quad” cameras from the SLAC ePix10k family. The sensors on each quad are ¼ of a full ePix10k-2M. The MEC variety use twice the standard thickness of Si, 1 mm, to achieve higher quantum efficiency at high photon energies, and are packaged to be robust against EMP from laser-target interaction. The pixel size is 100 µm x 100 µm.  Added during run 18, these detectors have better noise and higher dynamic range than the previous CSPADs, for a similar detector area. For diffraction experiments, the detectors are oriented around the primary interaction point in a Debye-Scherrer transmission geometry, with their positions in the standard configuration illustrated in Figure 3. Here four Quads, labeled as Q0, Q1, Q2 and Q3, are used to record the diffraction signal.

Figure 3. 4 Quads on the standard configuration geometry, XRD variation are shown. This arrangement is designed to cover a wide 2-theta range of 8-72º while accommodating the VISAR diagnostic for normal geometry, with the wider angles achieved in the horizontal plane, because of the vertical polarization of the X-rays.

The top two quads, Q0 and Q1 access the lowest angles from above the horizontal plane, while Q2 and Q3 are oriented for wider angles in the horizontal plane as shown in Figure 4.

Figure 4. Phi and 2-theta coverage of the four quads in the default standard configurations. The FXRTS variant and configuration #2 (PCI) both remove quad Q2.

For the reconstruction of the diffraction pattern, dioptas is available as an analysis package at LCLS. Dioptas may be run on our psana analysis machines, accessing experiment data via NFS.

Dioptas webpage

Dioptas code repository

Standard Configuration #1: FXRTS mode

Quads Q2 can be removed to allow a forward X-ray Thomson scattering diagnostic to be placed. The FXRTS spectrometer will be in the horizontal plane with a fixed scattering angle of 30 degree.

Figure 5 shows the experimental layout with FXRTS. The 3 ePix10k Quads as XRD detectors cover from 8 to 60º.

Figure 5. Geometry for XRD standard configuration with quad Q2 replaced with a FXRTS spectrometer for high photon energy.

Optical Laser Parameters and Geometry

The full frequency-doubled energy of the long pulse laser is delivered to the target with angles of ± 20 deg. in the horizontal plane relative to the X-rays, with the target oriented normal to the X-rays. Both beamlines can be used simultaneously (7 minutes between shots), or staggered (one shot every 3.5 minutes). The 72 mm beams are focused using 250 mm focal length aspheric lenses (F/3.5), and the focal plane of each beam relative to the target can be adjusted from best focus to a ~100 µm spot. MEC uses phase plates generating circular focal spots of 150, 300 and 600 micron by CPP. Phase plates can be manually exchanged during a standard configuration run, requiring a chamber vent. Note that desired pulse shapes and phase plates must be submitted at least 2 months before beamtime.

Targets

The user provided targets will need to be mounted on target frames that are compatible with the MEC target holder. Please contact an instrument scientist for more information about target mounting. An example target mount design is illustrated in Figure 6.

Figure 6. Drawing of the target frames that are compatible with the MEC alignment stage. The targets are mounted on the right side of the side view (on reference plane A). In the side view, the laser comes from the left. Dimensions are in inches. The spacing and size of the target holes may be changed, but all other dimension need to stay unchanged. The chamfer in the front side of the target hole is 60°. (Download detailed drawing)

Parameter table

To be considered for scheduling in this standard configuration, users will be required to include a table in the proposal that lists the specific experimental parameters to ensure compatibility with these configurations. If the experimental parameters are not compatible with the standard configuration or if the table of parameters is incomplete, the proposal will be reviewed and considered for scheduling as a general user proposal. Please see the table of required parameters of the MEC Standard Configuration #1. No fundamental changes to the standard configurations will occur, but some details of the configuration may be updated in response to inquiries, so users should recheck the website before submitting your proposal to confirm that you have the latest information. Address any questions to the instrument staff.

X-ray Imaging

This configuration supports diffraction, phase contrast imaging (PCI) and direct X-ray Imaging measurements on targets shocked to pressures up to several Mbar, with shocks propagating perpendicular to the X-ray direction. The new MEC X-ray Imager (MXI), located upstream of the target, focuses the beam to a few 100s of nanometer size before the target, with the ability to shift between three lens sets for different magnification. The expanding X-rays pass through the target and are registered on an X-ray microscope ~4.5 m from TCC. The layout within the chamber is shown in Figure 1. X-ray diffraction is achieved with three out of four ePix10k quads of the XRD platform covering a range of 8 to 60 degrees. VISAR is also provided to measure shock velocity.

Figure 1. Geometry for MXI standard configurations using two nanosecond, 527nm drive beams in orthogonal beam direction to X-ray beam.  A detector for MXI will be located after the fly-tube, which is ~4.5m away from TCC.

Figure 2. Isometric of the Standard configuration #2 showing the MXI, the long pulse laser perpendicular delivery , the VISAR system and the XRD detectors.

Optical Laser Parameters and Geometry

The full frequency-doubled energy of the long pulse laser is delivered to the target with angles of ± 20 deg. in the horizontal plane relative to the target normal, which has an angle of 90 deg. relative to the X-rays. Both beamlines can be used simultaneously (7 minutes between shots), or staggered (one shot every 3.5 minutes). The 72 mm beams are focused using 250 mm focal length aspheric lenses (F/3.5), and the focal plane of each beam relative to the target can be adjusted from best focus to a ~100 µm spot. MEC uses phase plates generating circular focal spots of 150, 300 and 600 micron by CPP. Phase plates can be manually exchanged during a standard configuration run, requiring a chamber vent. Note that desired pulse shapes and phase plates must be submitted at least 2 months before beamtime.

Target Pillars

A stair-step target mount design allows for laser and VISAR access to a target for side-on X-ray imaging of the target. A pillar design is used to mount columns of targets in this way, as shown below:

Figure 2. Mechanical drawing of a target pillar designed for the PCI standard configuration (download PDF drawing).

Parameter table