1. Hall C Readiness Review – Small Angle SHMS Operation
Mitigation and Monitoring of Primary Beam Steering Due to
Field Leakage From SHMS Magnets
Roger Carlini,
Jefferson Laboratory
&
Steve Lassiter, Michael Moore, Jure Bericic, Jay Benesch,
Buddhini Waidyawansaa, Silviu Covrig, Brad Sawatzky
Page 1

2. Summary: Kinematics for Hall C Small q Experiments
Kinematics from proposals  Note: All these settings may not actually be used
(Limited early small qSHMS running planned & no small qSHMS at maximum PSHMS)

3. Many Approaches Investigated to get SHMS Small Angle Operation “Off the Ground”

4. Dump Face Well Beyond Acceptable Left-Right Range
SHMS has “Complex Ion Optics” With Strong Fringe Fields Along Beamline – Which If Not Mitigated Will Steer Primary Beam at
Dump Face Well Beyond Acceptable Left-Right Range
Magnet “as build” Fringe Fields
Along Beam Line
(need to make Bleft & Bright cancel)
Simplest solution is to
suppress these components
Note: There is no significant fringe field from Dipole

5. Definition of SHMS Central Angle (qSHMS ) of Acceptance
Close up of deflection at beam dump face. “r” is horizontal displacement.
Need r < about 4 cm
Mechanical center of SHMS is displaced 3o w.r.t. ion optics coordinate system.
Solid red line (z-axis) is path to center of dump without deflection. Dashed red line is deflection from SHMS field leakage (fringe fields).

6. X The SHMS Magnets Consequential Fringe Fields Negligible Fringe Field
Note: Current densities generated from OPERA for maximum momentum setting of the SHMS
Consequential
Fringe Fields X
Negligible
Fringe Field

7. Modify HB (the biggest Contributor) by Addition of Fe Pieces for
Contouring & Suppression of “Fringe Fields”
Additional Yoke Material
“Wedges”
Front Field Clamp
Note: Pieces for extended yoke and wedges are being purchased

8. Individual Magnet Fringe Field Integrals along Beam Line
as Function of PSHMS (for qSHMS = 5.5°) Q1 HB
(after modification) Q2 Q3
Negligible
Fringe Field

9. BY [G] Field Integrals Along the Beam Line
qSHMS = 5.5° to 14o, with PSHMS = EBeam = 11 GeV
(electron polarity)
Worst Case FC HB Q1 Q2 Q3
BY [G]
14°
Notice the Integral including drifts space effects
is now closer to canceling!
5.5°
Target
Z beam Line [cm]

10. Now Raytrace to the Dump Face
Employ simple raytracer to examine the beam deviation at the beam dump (51.8m from target).
Takes into account local deviations from straight path and drift spaces.
Traces only in x-z plane and accounts only for By
Generate tables of ∫Bdl and beam deviations for different field excitations, SHMS angles and incident beam energies.
Summary of best cases found (details on next few slides)
Modifying HB alone (shown previously) – sufficient for 7 commissioning exps.
For all SHMS kinematics planned – we need also to suppress to ~1/2 its strength the fringe field of Q2. Simplest solution looks to be a thin Fe collar around beam pipe along side of Quad (engineering still underway).
All other magnets are left as designed.

11. } Current Acceptable Range GeV Suppress ½ Q2 fringe field
Displacement at Dump Face [cm]
Effective SHMS Scattering Angle q [deg]
GeV
Current Acceptable Range
Currently taken to be same as Hall A (±4 cm).
May expand range, Keith Welch (Rad Con) modeling.
Engineering to provide heat calculations to accelerator division for consideration of expansion.
Suppress ½ Q2 fringe field }
Best universal kinematics solution
that also minimizes Fe around
Beam pipe adjacent to Q2
No shielding of Q2 fringe field
Suppress all of Q2 fringe field
Displacement at Dump Face [cm]
Displacement at Dump Face [cm]
Sufficient for kinematics of the
7 commissioning exps!
Ideal solution, but requires thick
Fe beam pipe collar adjacent to Q2
Effective SHMS Scattering Angle q [deg]
Effective SHMS Scattering Angle q [deg]

12. Results after Optimizing Designs of Fringe Field
Displacement at Dump Face [cm]
SHMS Central Momentum [GeV/c]
Results after Optimizing Designs of Fringe Field
Contouring / Suppressing “add on Fe” to HB & Q2
Incident Beam Energy
Note: For q > 10o deflection decreases
(fringe fields fading )

13. Auxiliary: Big BPM’s Downstream of Target
Additional real time knowledge (x,y,x’,y’) of full intensity primary beam after target desirable when SHMS configured for small forward angles.
Two prototype large diameter BPM’s to be installed in downstream beam line separated by drift space.
Mounted inside dedicated 1.5m removable sections of 24” diameter beam pipe. Minimal mass structures of extremely radiation hard materials (Al, few small SS screws and ceramics).
Designed to work with standard JLab BPM electronics (newest generation). Outputs appear like other BPM’s to MCC operators and facilitate straightforward tune up and augmented real time monitoring.
Exact fit but hollow 24” beam pipe replacement sections available should removal/repair of Big BPM’s be desired/required.

14. Summary
For qSHMS < 10o the “as designed” magnet fringe fields leak into the post target primary beam transport line and will steer the beam off acceptable region (±4 cm left- right) at the beam dump face.
Situation is mitigated for the 7 commissioning exps with only the addition of shaped Fe pieces to the HB.
Appears situation should be mitigated for all SHMS kinematics by addition of the shaped Fe pieces to the HB plus thin Fe shield (~1/4”) Fe around the transport line adjacent to Q2.
Addition of auxiliary “Big BPM” to existing downstream beam steering tools “viewers & ion-chambers” should simplify and reduce setup time for spectrometer configuration changes.