Studies of the beam dynamics of Elliptical Polarizing Undulators (EPUs) at the ALS

This page tries to collect data from measurements, simulations and discussions with other people concerning the impact of Elliptical Polarizing Undulators (EPU) at the ALS. The group of people currently studying the effects are:

Jin-Young Jung, Steve Lidia, Steve Marks, Laurent Nadolski, David Robin, Ross Schlueter, Christoph Steier, Andy Wolski

(External communications with P. Elleaume, J. Chavanne, O. Chubar, B. DiViacco)

Main problem areas are:

1. Variation of field integrals with EPU phase (causing orbit distortions). Problem is solved fairly well using a combination of a fast feed forward and a slow orbit feedback.
2. Variations of the (mostly vertical) beamsize (both with gap and with phase):
   1. Due to focusing changes (systematic focusing terms from the bulk of the undulator). Can be compensated with local quadrupole trim coil (or potentially shims ...), plus improvements in orbit feedback.
   2. Due to coupling terms (skew quadrupole like or solenoid like). This effect seems to be the dominating one for the vertical beamsize variations at the moment and is a problem for many other experiments.
3. Higher order effects impacting the dynamic (or momentum) aperture, for example due to the feld roll-off, which is quite significant and systematic in circular polarization mode. This effect is not a problem with the 5cm period devices at the moment, but needs to be studied for long period EPUs (sub-meV beamline).

Collection of articles and transparencies

• Article about the orbit impact (field integrals), the feed-forward compensation and the initial asessment of beam size variations (at that time mostly tune induced) (EPAC 2000)
• The minutes of the inaugural task force meeting (January 2002) of the EPU task force gives a nice overview of the situation as far as it was understood back then (2002-1-23)
• Summary of communication with Pascal Elleaume (David's transparencies from 2002-10-07)
• Horizontal field integral as a function of phase/shift (2002-10-07): simulation compared to estimates from measured feed forward tables. Conclusion: Measured kicks are a factor of more than 5 larger than the ones calculated in RADIA (calculation for ideal device including the ends and the deviation of dB/dH from 1).
• Comments from Pascal Elleaume regarding phase dependent skew terms induced by EPUs.
• Horizontal field integral as a function of phase/shift (2002-10-21): Comparison of estimates from measured feed forward tables and magnetic measurements with a straight wire on a bench. Conclusion: Magnetic bench measurements and measurements with beam agree well.
• Derivative of horizontal field integral (dIx/dx) as a function of phase/shift based on magnetic bench measurements (2002-11-04): Conclusion: The initial agreement of the amplitude of the variation of the derivative of the horizontal field integral with the EPU phase for minimum gap in sector 11 and the amplitude of the beamsize variation of the beam seemed to be coincidental.
• Orbit response matrix analysis to quantify skew quadrupole components in the ALS EPUs (2002-11-04): Conclusion: The fit results for the skew quadrupole strength are very consistent with the measured variation of the vertical beamsize.

Current minutes and lists of Action Items:

• Minutes and Action items of 2002-09-30
• Minutes and Action items of 2002-10-07
• Minutes and Action items of 2002-10-21

Task force on EPU problems (in early 2002)

• Charge to the task force.
• Minutes of first task force meeting.

Background information from other Labs:

• Termination of EPUs at ESRF
• Shimming of EPUs at ESRF to reduce phase dependent tune-shift
• Operational Aspects of Elliptical Undulators at ELETTRA (2001)
• Commissioning of EPUs in ELETTRA
• Development of EPUs at ELETTRA (2000)
• Development of EPUs at ELETTRA (1999)
• Construction of Elliptical Undulators for ELETTRA (1998)
• Coupling correction at ESRF
• Using RADIA to simulate nonlinear problems with permanent magnets (demagnetization!)
• Nonlinear Dynamics in SPEAR wigglers