HEBT, Ring, and RTBT Pickup Electrode Mechanical and Electrical Design

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HEBT, Ring, and RTBT Pickup ElectrodeMechanical
and Electrical Design

• Peter Cameron
• July 23, 2002

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Outline

• Table of BPM Types
• Mechanical Construction
• Tolerance Stackup
• Stripline Impedance
• Impedance Implications
• Wire Measurements
• Conclusions/Action Items

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Table of BPM PUE Types
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Mechanical Construction

• Direct descendant of RHIC PUEs economical,
  accurate, robust
• Stainless steel annealed before machining, copper
  brazed with integral fixturing to maintain
  precise mechanical alignment
• Contact from stripline to feedthru
• Shorted stripline stripline cantilever is
  spring
• Open striplines CuNi bellows

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21cm HEBT PUE
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21cm Ring PUE
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Ring 21cm PUE Ceramic Supports and Contact
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Ring 21cm PUE Tolerance Stackup

• Estimated accuracy to the center of quad
• Shoulder screw location .008
• Brazing surface on the outer shell .005
• Rad. Clearance for the brazing .001
• Weld post on the outer shell .005
• Length of ceramic standoff .005
• Groove depth on the stripline .005
• Quad. Surface flatness .020
• Total(rms) .024 lt .040 design req.

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PUE on Quad w/o Corrector
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Feedthrus, Cables, and Standoffs

• Feedthrus
• Lithium silicate glass/ceramic
• Thermal expansion matched to stainless steel
• Ultra-reliable mechanically
• Very good electrically (much better than
  Kovar-ceramic)
• Cables
• PUE to tray - flexible coaxial
• Tray to service building - 1/4 low loss heliax
• Ceramic Standoffs
• All ring components to be titanium nitride coated
• Results in shielded standoff design to avoid
  shorting

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Stripline Impedance

• Direct scaling from RHIC PUEs
• Dual plane, four electrodes
• 70 degrees subtended angle, maximum linearity
• Sum mode matched to 50 ohms
• Simplistically, four relevant impedances
• Sum mode Zs 50 ohms
• Difference mode Zd 28 ohms
• Matched Z0 sqrt(Zs Zd ) 38 ohms
• Termination Zc 50 ohms

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Stripline Impedance 2

• Argument is that if you adjust sum and difference
  mode impedances such that
• Z0 sqrt(Zs Zd ) 50 ohms (for instance)
• Then both sum and difference mode will be matched
• Not everyone believes this yet, measurements are
  in progress
• S21 and TDR with a wire
• BPM into electron beam at ATF

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Stripline Impedance Implications

• No implications for narrowband measurements
• No implications for HEBT, Ring and RTBT position
  measurements
• No accelerator physics implications (impedance,
• Significant implications for broadband
  measurements - eg TOF
• It may be necessary to modify the six 12cm HEBT
  TOF PUEs - not yet brazed
• Alternatively, it may be necessary to modify the
  LINAC PUEs?

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21cm Ring PUE on wire scanner
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WireScanner Measurement

• Permits measurement of
• Transfer function (and mapping)
• Relative mechanical and electrical centers
• Presently no plan to do antenna measurement (ala
  RHIC) to find electrical center of BPM relative
  to quad fiducials due to vacuum access problems.
• Beam-based alignment will accomplish the same
  thing, only better.

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Conclusions/Action Items

• Check current capacity of contacts (especially
  bellows)
• Decision on PUE-to-tray cable, 1/4 vs 3/8 heliax
• Pursue Stripline impedance calculations and
  measurements (including ATF) - motivation is
  certainty that we have TOF right
• Will 30cm Injection Dump PUE (open) work with
  402.5MHz electronics?
• Wire scanner measurements