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Two variations of the standard configuration are available: the X-ray diffraction variation (XRD) and Thomson scattering variation (XRTS). User supplied detector cannot be fielded in either standard configuration. User may request all standard MEC beamline devices and diagnostics, and the request needs to be explicitly mentioned in the proposal.
This variation consists of the ns laser drive beams, VISAR, the backward XRTS and a number of CSPADs. Fig.1 shows the layout.
Fig.1. Geometry for XRD standard configurations using two nanosecond, 527nm drive beams. The two laser beams hit the target at an angle of 15° from each side of the x-ray axis. The angle between the target normal and the LCLS x-ray axis is 35° (or 55° between target surface and LCLS x-ray axis). The Backward XRTS looks at a scattering angle of 130°. CSPAD scattering detectors are positioned behind the target, and VISAR is collected normal to the back of the target.
Two CSPAD-560k and four CSPAD-140k will be available. Their positions will be fixed. They will cover the scattering angle θ=2θ_b from 25 degrees to 90 degrees. The exact area that is covered is shown in Fig.2.
Fig.2. Scattering angles collected by the various CSPAD in the X-ray diffraction variation. Y-axis is the scattering angle θ=2θ_b, the x-axis is the azimuthal angle, φ, with θ=90, φ=0 is scattering along the horizontal direction. Light blue is the area which is captured by a detector.
This variation consist of the ns drive beams, VISAR, backward XRTS and Forward XRTS, and a more limited number of CSPADs. See Fig.3 for the layout.
Fig.3. Geometry for XRD/ XRTS standard configurations using two nanosecond, 527nm drive beams. The difference with Fig. 1 is that half of the CSPADs used for diffraction is replaced by the Forward XRTS. The vertical scattering angle of the spectrometer can be changed from 10° to 55°.
The CSPAD in this variation collects X-ray diffraction for scattering angles between 35 and 90 degrees. See Fig.4 for the coverage of theta-phi. Both XRTS spectrometer use HAPG crystals, and can operate between 4 and 8 keV. They use CSPAD as a detector.
Fig.4. Scattering angles collected by the various CSPAD in the XRTS variation. Y-axis is the scattering angle θ=2θ_b, the x-axis is the azimuthal angle, φ, with θ=90,φ=0 is scattering along the horizontal direction. Light blue is the area which is captured by a detector.
The MEC ns glass laser will be used and delivered to the target as shown in Fig.1 and 3. Two arms will impinge on the target at the symmetrically about the LCLS beam line axis. Both arms can be shot simultaneously (7 minutes between shots), or interleaved (one shot every 3.5 minutes). MEC provided phase plates can be used, for spot sizes of 50, 100, 200, 300, 500 and 1000 micron, or focused beam without phase plates can be used. Pulse length can vary between 3ns and 50ns. Flat top temporal profiles will be available; other profiles can be provided if requested at least 2 months in advance. The laser energy is dependent on pulse length and profile, but generally a power of 2J/ns can be provided per arm. For 10ns pulses the maximum beam energy on target will be 40J. The arrival time of the optical laser with respect to the x-ray can be changed during the experiment, and is accurate within 25ps RMS.
X-ray beam operation with be in the standard SASE mode (no seeding) for the X-ray diffraction variation. For the XRTS variation, seeding can be requested. Photon energy and spot size are chosen by users.
The MEC line VISAR will be available as a diagnostic (See Fig.1 and 3). Table 1 lists the etalons available at MEC; alternatively users can bring their own. Users can take up to 15 test shots with the laser drive beam and VISAR before the start of their X-ray beam time.
Fig.5. Optical layout of the MEC VISAR system. Two VISAR beds with streak cameras are provided. Etalons for each bed can be chosen from table below, or user can bring their own.
Table 1. VISAR etalons available at MEC. All etalons are fused silica. User can bring their own etalons, provided the diameter is either 25 or 50mm. Maximum etalon length that can be supported is 110mm.
The user provided targets will need to be mounted on target frames that are compatible with the MEC target holder.
Fig.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, and the visar beam from the right. Dimensions are in inches. The spacing and size of the target holes may be changed, but all other dimension need to stay unchanged. (Detailed drawing available)
Bob Nagler, Hae Ja Lee, Eric Galtier, Michael Greenberg and Alan Fry
SLAC National Accelerator Laboratory, Menlo Park, CA
Operated by Stanford University for the U.S. Dept. of Energy