1. Breakout Session SC3 – Undulator
Undulator BPM Diagnostics
R&D Plans
P. Krejcik

2. Requirements
Single-pulse readback at up to 120 Hz
Resolution of 1 mm or better
Over a bunch charge range of 0.1 to 1 nC
Cavity BPM chosen over striplines
Inherently better resolution
Less prone to systematic offsets

3. Cavity Beam Position Monitors
Frequency choice
Cavity Iris should be masked from SR
Vacuum chamber dimensions for the undulator are now chosen
10 mm aperture
is larger than 8 mm diameter inside quadrupole
X-band chosen to maximize resolution and minimize length.
Issues
BPM location with respect to quadrupoles
Resolution in combination with beam-based alignment with EM quads
Signal processing
5 mm
10 mm

4. X-Band Cavity BPM Resolution exceeds LCLS requirements
Zenghai Li
feedthrough
Ri = 5 mm
18 mm
Resolution exceeds LCLS requirements
Sensitivity of 1.6 mV/nC/mm
20 nm demonstrated at KEK/SLAC experiments – S. Smith et al
Systematic offsets are of greater concern

5. Sources of offset in the cavity BPM
lid
body
Concentric surfaces
Calculate effects from errors in
Concentricity of 3 surfaces
Coupler offsets and tilts
Parallel surfaces
5 um machining tolerances achievable
- C. Pearson

6. Cavity Electric Center Sensitivity

7. Tolerance on Coupling Slots

8. Undulator Cavity BPM locations with respect to quadrupoles
Quadrupole and BPM mounted adjacent on the undulator support cradle to ensure 1 um beam based alignment resolution
Also need to keep the distance between the electron beam and the undulator segment axis to less than 70 microns rms
Considering beam position measurement options at downstream end as well
Quad BPM
assemblies
Optional wire monitors,
Train-linked undulator sections – see H.-D. Nuhn presentation

9. Cavity BPM Signal Processing
X - Y cavity at each undulator short gap plus 1 phase reference cavity per girder at each long gap
High-frequency x-band signal is attenuated in a short distance
Incorporate a local mixer to IF at the cavity
Only a simple passive device in the tunnel
Temperature stable
Relatively low radiation loss environment
Distribution of reference x-band oscillator signal in the tunnel
Choose intermediate frequency to match into the RF front end used for stripline

10. Cavity BPM Signal Processing
Upstairs RF receiver
RF in IF
~300 MHz
BP filter
Dw~5 MHz
RF Amp
Common
Local
Oscillator IF
~50 MHz
BP filter
Dw~10 MHz
IF Amp
In-tunnel X-band heterodyne receiver
ADC
14 bit
e.g Echotek
Digital
processing
Control
system IF
300 MHz
Calibration
Pulse
11.42 GHz
Local
Oscillator
11.1 GHz
119 MHz
Clock
24th harmonic

11. Digital BPM Signal Processing
Use same RF front end for stripline BPMs and output from first mixer for cavity BPMs
Initial desire to use a commercially produced BPM processing module (Libera)
We obtained a try out Libera module
Integration into the control system not proceeding fast enough, e.g. could not access raw data in the module.
Present design solution
Commercial VME 8 channel digitizer
RF front end from discrete, commercial components
See T. Straumann presentation

12. Cavity BPM Prototyping and Test Plan
Components for prototyping and testing can be divided into:
Cavities
Feed-throughs and waveguide
RF receiver electronics and reference oscillator
Digitizer electronics
EPICS integration and application programs
Bench tests
Beam tests

13. Cavity BPM Prototyping and Test Plan
Cavities
Fabrication of 4 X-band BPM cavities and 2 reference cavities in SLAC’s Klystron Engineering Department
Use their experience in machining and brazing X-band structures for linear collider research
Brazing and cold RF testing of waveguide to coaxial feed-through assembly

14. Cavity BPM Prototyping and Test Plan
RF receiver electronics and reference oscillator
Prototype heterodyne receiver assembled from connectorized parts using standard filters, mixers and amplifiers
Similar to assembly for SLAC/KEK cavity BPM tests
Reference oscillator signal derived from SLAC LLRF distribution
Opportunity to test other receivers: AM/PM

15. Cavity BPM Prototyping and Test Plan
Digitizer electronics
We have acquired an Echotek 14-bit 125 MHz 8-channel VME card
Clocked at the 119 MHz SLAC reference harmonic
Over-clocked version of current version of this module
Presently bench testing the card with a low-noise oscillator

16. Cavity BPM Prototyping and Test Plan
Bench tests
RF cold testing will evaluate feed-through and cavity combination
Verify frequency, Q, coupling
Antennae tests on a precision x-y stage to measure the electrical center with respect to the mechanical center

17. Cavity BPM Prototyping and Test Plan
Beam tests
On the SLAC FFTB line
Mount 3 cavities plus a reference cavity on a ~1 m long rigid girder
Check cavity+electronics calibration procedure by moving beam ±1 mm
Correlate the position readings from all 3 cavities for small position changes (beam jitter) and determine system resolution and offsets

18. Draft R&D Schedule

19. Draft R&D Schedule