Photographic Tour of STELLA Experiment

  The STELLA Experiment is located on Beamline #1 in the Experimental Hall of the BNL ATF.  A schematic plan-view of the experiment is given in Fig. 1.  For clarity the laser beam is drawn in the plane of the figure.  In reality, the laser beam enters the beamline perpendicular to the plane of the figure.  Thus, the lens, window, and vacuum tube supporting the window actually extend perpendicular out of the plane of the figure.

Figure 1.  Schematic plan-view of STELLA experiment.

    Starting from the right side of Fig. 1, we shall present a photographic tour of the major components of the experiment.  The ATF CO₂ laser beam is expanded in size using a convex mirror.  This is to avoid damaging the lens and window, which are made from uncoated NaCl for transmission of the infrared laser light.  Two large copper mirrors are suspended from the ceiling as shown in Fig. 2 and direct the laser beam downward through the NaCl lens (see Fig. 2) and into the vacuum chamber indicated in Fig. 1.  The top of this vacuum chamber is shown in Fig. 3.

Figure 2.  Laser mirrors and NaCl lens positioned above vacuum chamber containing parabolic mirror.	Figure 3.  Top of vacuum chamber containing parabolic mirror

Inside the vacuum chamber is a remote-controlled 90° off-axis parabolic copper mirror, which focuses the laser beam into the center of the accelerator undulator (IFEL2) located downstream of the vacuum chamber.  This mirror is shown in Fig. 4.  A 1-mm dia. hole is drilled through the center of the mirror for transmission of the e-beam.  The odd-shape of the base platform supporting the mirror is because of the need to fit between existing equipment inside the vacuum chamber shared by another experiment.

Figure 4.  Parabolic mirror with central hole for e-beam.	Figure 5.  Permanent-magnet (PM) buncher lying on its side.

Downstream of the parabolic mirror is a triplet and then the permanent-magnet (PM) buncher (IFEL1).  The buncher is shown resting on its side in Fig. 5 to facilitate seeing inside it.  During the experiment the buncher gap is oriented vertically.  A wide gap is necessary because of the large laser beam size at this location.  Magnetic field clamps on both ends of the device help control fringe field effects and ensure the buncher introduces minimal steering of the e-beam.

The purpose of the PM buncher is to impart a small (~±0.5%) sinusoidal energy modulation on the e-beam.  A hybrid permanent-magnet/electromagnet chicane converts this energy modulation into spatial modulation by creating microbunches at its output.  A photograph of the chicane is given in Fig. 6.  The gap (i.e., magnetic field) of the chicane is oriented orthogonal to the IFEL1 and IFEL2 undulators in order to minimize interactions of the laser beam co-propagating with the electrons inside the chicane.  The permanent magnets of the chicane are designed to cause bunching of the electrons at the entrance to IFEL2 when the e-beam energy is 45 MeV with ±0.5% energy modulation.  A main electromagnet provides the ability to adjust the phase of the microbunches with respect to the laser field in IFEL2.  Trim coils at the ends of the chicane compensate for steering effects when the main coil field is varied.

Figure 6.  Chicane in normal orientation.	Figure 7.  Tapered undulator lying on its side.

          Downstream of the chicane is the gap-tapered undulator (IFEL2), which is shown lying on its side in Fig. 7.  It consists of 20 poles with a period of 33 mm and a gap that is 11% smaller at this output end.  The magnets are NdFeB.  The untapered wiggler K parameter is 2.9.  Figure 8 shows the tapered undulator installed on the beamline.  Manual translation stages are used to lift the undulator away from the beamline when the undulator is not in use.

The buncher, chicane, and undulator were all designed, fabricated, assembled, and characterized at STI Optronics, Inc.

Figure 8.  Tapered undulator installed on beamline.	Figure 9.  Electron energy spectrometer with wide energy acceptance.

At the end of the beamline is the electron energy spectrometer, which consists of a dipole, focusing optics, and a phosphor target at its output.  A high-sensitivity CCD video camera views the phosphor target.  This part of the spectrometer is shown in Fig. 9.  The angle of deflection of the dipole can be varied, which permits adjusting the energy acceptance of the diagnostic.  Because of the very large energy gain during the STELLA experiment (>20%), a large energy acceptance window is needed in order to view the entire energy spectrum in a single shot.  At this large energy acceptance, the spectrometer has a measured resolution of 0.14% (1s).

Figure 10 completes our photographic tour and shows the PM buncher, chicane, and tapered undulator positioned on the beamline.  When not needed, these devices can be removed from the beamline.  The PM buncher lifts off the beamline using a lifting bar (see Fig. 10).  The chicane slides horizontally away from the beamline with a counterweight used to provide balance.  As mentioned, the tapered undulator lifts vertically off the beamline using manual translation stages.

Figure 10.  Photograph of PM buncher, chicane, and tapered undulator on Beamline #1.  The chicane has been retracted horizontally off the beamline.