Orbit Alignment and Stability

- J. Corbett

Since the introduction of a global orbit feedback system, photon beam stability on the SPEAR beam lines has steadily improved over the past few years. The present feedback system employs 30 electron beam position monitors and 30 correctors per plane. In addition, each beam line is equipped with a local fast servo system to stabilize the vertical photon beam position. The combined global and local systems help to maintain photon beam position and angle at experimental stations tens of meters from photon beam source points.

In the past year, the global feedback system has benefited from other programs instituted to improve orbit stability and ring performance. In particular, a new lattice was implemented to eliminate strong quadrupoles from the colliding beam interaction regions. By eliminating these magnets and reducing the strength of nearby quadrupoles by about 80%, a major source of diurnal orbit drift was eliminated.

Our nominal beam delivery sequence proceeds as follows on a 24-hour schedule. First, we inject beam at 2.3 GeV, ramp to 3.0 GeV, steer the photon beam lines and activate the beam line servos. Next, the orbit feedback system holds the electron beam orbit constant while the beam line servos correct higher frequency fluctuations.

During a 24-hour delivered beam period horizontal and vertical stability at the BPMs is presently 80 µm rms and 60 µm rms, respectively. Fill-to-fill orbit reproducibility, after the 2.3 to 3 GeV energy ramp, is less than 100 µm rms. These values represent a factor of 5 or more reduction in orbit motion, and a factor of 10 improvement in the fill-to-fill orbit reproducibility. For the 130 nm-rad emittance SPEAR beam, the stability at the photon beam source points is now approxi-mately 10% of the horizontal and 20% of the vertical beam size.

For the 1996 run, more high resolution BPMs were installed to increase orbit sampling capacity, and mechanical supports for these and other BPMs were upgraded to reduce transverse thermal motion. A new style of flange mounted "drop-in" style BPMs were installed in chamber sections when they were removed to retrofit new photon beam exit ports. To date, 11 new BPMs have been either replaced or added to SPEAR. In addition, more orbit correctors will be added to SPEAR during the 1996 shutdown.

In conjunction with the new BPMs, a BPM signal multiplexer and rf receiver are being developed. The processor will sample 30 or more BPMs sequentially around the ring.

With an orbit update rate from the BPM processor of 1 to 2 ms, a closed- loop feedback bandwidth of 30 to 50 Hz will be achievable. The final feedback system probably will require four parallel BPM processors to reach its full bandwidth.

To locate the electrical BPM centers, current shunts were installed on each quadrupole. The shunts allow us to center the beam in each quadrupole ( indicated when a shunt does not induce an orbit shift) and thereby to measure BPM offsets (some were a few millimeters). Due to the realignment of SPEAR and the improved BPM system, the absolute orbit through the beam lines is now <1 mm peak in both planes, and the orbit feedback behaves more linearly.

The orbit feedback system is now entering the second phase of the developmental program. The Phase I version of the orbit feedback system resides on the main SPEAR VAX. In this system, the electron and photon BPM readings are accessed directly from the SPEAR control database, and the correctors are set through the database system.

The Phase II feedback system carries out the orbit data, and feedback processing functions in a remote VME crate. In this configuration, a VME-based µ ;VAX (CPU-1) handles exchange of BPM data, photon beam line data, and corrector setpoints with the SPEAR VAX. A second µVAX (CPU-2) performs feedback calculations and orbit adjustments.

We are now working to augment CPU-2 with a 32-bit floating point digital signal processor (DSP) to achieve maximum computation rates.

Another large project directed toward improving beam stability at SPEAR was a complete realignment of the ring in the 1995 shutdown. In the past, many components in the ring had drifted by up to several mm in absolute coordinates. With new beam lines scheduled for installation, we decided a complete realignment of the ring in terms of absolute coordinates was appropriate. Previous alignment efforts were based on local "smoothing" of the ring. The absolute alignment process brought all quadrupole magnets to within +/- 200 µm horizontal and +/-100 µm vertical. These numbers will improve with subsequent surveys and realignment. The photon beam lines were then aligned relative to the ring magnets. The complete project required ~200 two-man shifts of the SLAC alignment team and extensive effort on the part of SSRL staff.

We view the alignment procedure and quadrupole shunt installation as important steps toward improving the beam stability in SPEAR. The injection and ramp processes are now more efficient and reliable, and control of the electron beam orbit and lattice optics has improved. Better knowledge of the electron beam orbit allows us to accurately steer the photon beams down the beam lines, and to avoid restricting apertures. With improved BPM readback reliability, a fully aligned ring, steered orbit, and improved BPM signal processor, the digital feedback should operate more efficiently.

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December 2, 1996