Switched resistor current shunts have been installed on all SPEAR quadrupoles to help to characterize the electron beam orbit and magnet optics. The shunts were implemented following a method suggested by and successfully used at MAXLAB. Each shunt consists of a water-cooled resistor and MOSFET switch to bypass ~1-3% of the quadrupole coil current on command, depending on the quadrupole family. The shunts are used to determine when the stored beam passes through the magnetic center of each quadrupole, which in turn permits measurement of the combined electrical and mechanical offsets of nearby beam position monitors (BPMs). In addition, the the shunts are used to measure betatron amplitudes at each quadrupole by recording the induced betatron tune shift.
At present, electron beam position monitors are used to measure the orbit perturbation induced by each shunt (proportional to off-center displacement); to first order, there is no kick when the beam is centered. With the 10 micron resolution of the BPMs, horizontal centering in horizontally focusing (QF) quadrupoles and vertical centering in horizontally defocusing (QD) quadrupoles can be measured with better than 100 micron resolution by taking the difference between orbits measured before and after shunt energization. Vertical centering in QF magnets and horizontal centering in QD magnets can also be measured with this resolution by switching the shunts at a fixed frequency (~2 Hz to assure field penetration through the ring vacuum chamber) and using a spectrum analyzer or lock-in amplifier to determine when the detected frequency component is minimized. Typically, we measure horizontal and vertical BPM offsets by centering the beam in two quadrupoles flanking the BPM.
Most of the horizontal and ~2/3 of the vertical BPM offsets have been measured so far; many of these offsets are more than a millimeter, and some are many millimeters. Combined with information from the recent ring alignment and survey, we now know actual SPEAR beam position with sub-millimeter accuracy for the first time in two decades of operation. This information has also permitted us to correct the orbit to less than one millimeter peak-peak through the ring sectors containing beamline sourcepoints. Work is continuing to make high resolution BPM offset measurements under computer control and to measure and correct the lattice beta functions.
Selection of Experimental Shift Summaries Relevant the Subject
Shift Summary: "Shunt Studies" on January 8, 1996
Shift Summary: "Quadrupole Shunt Studies" on February 12, 1996