1. LCLS Bunch Length Monitor Marc Ross - SLAC
LCLS Bunch Length Monitor Conceptual Design Review Instrument Design Considerations February 23, 2006
2/23/2006
LCLS Bunch Length Monitor Marc Ross - SLAC

2. LCLS Bunch length monitor system:
Two subsystems:
Deflecting structure ‘LOLA’
Accurate, calibrated, complex
Destructive – ‘pulse stealing mode ok’
Expensive
Tested
Radiation monitors
Dipole radiation
Gap radiation
Simple sensors
Cheap
Complementary devices
2/23/2006
LCLS Bunch Length Monitor Marc Ross - SLAC

3. Bunch length monitor system
Dipole
radiation
Gap
radiation
LOLA
Video signal
mm wave
optics WG
Pyro-electric detector
High frequency diode
Single signal device
2/23/2006
LCLS Bunch Length Monitor Marc Ross - SLAC

4. Two coherent radiation monitors for BC1
Simple ceramic gap surrounded by mm-wave diode detectors (paired)
Total radiated energy is 2 uJ
For 1nC, 200 micron bunch.
(energy scales as Q^2/(bunch length)
Tested at SLC and ESA (actually many years…)
Annular reflector directs dipole radiation onto mm-wave ‘optics’ – pyroelectric detector
similar total radiated energy
Tested at FFTB/SPPS
2/23/2006
LCLS Bunch Length Monitor Marc Ross - SLAC

5. Bunch charge distribution
Simple indicator: central frequency of radiated energy

6. Coherent radiation detection strategy:
Each individual detector has ~ factor 2 range
Long bunches: use diode sequence
(100, 200, 400, 1000 GHz)
Down to 100 um rms
Short bunches: use reflector and pyro-electric detectors
Below 150 um rms
RD required to match – see 2007 testing plan
2/23/2006
LCLS Bunch Length Monitor Marc Ross - SLAC

7. gap
CERN Bunch length RF

8. LCLS Bunch Length Monitor Marc Ross - SLAC
ESA 100 GHz Gap and Detector
gap
2/23/2006
LCLS Bunch Length Monitor Marc Ross - SLAC

9. ESA gap monitor and detector
Gap / horn / WG-10 closeup

10. LCLS Bunch Length Monitor Marc Ross - SLAC
400 GHz diode
2/23/2006
LCLS Bunch Length Monitor Marc Ross - SLAC

11. ESA 100 GHz System – Jan 8, 2006
Sig_z_min ~300 um
gap

12. Multi-frequency diode ‘xylophone’
gap
Multi-frequency diode ‘xylophone’
1nC / 200 um example
(Total radiated energy 2uJ)
“Energy in” assumes catalog item waveguide horn
Detector sensitivity is 2.3e-15J into 50 Ohm out.
Good S/N for 100, 200, 400… 1000GHz ?
2/23/2006
LCLS Bunch Length Monitor Marc Ross - SLAC

13. ‘Beam view’ of multi-diode / waveguide assembly
gap
‘Beam view’ of multi-diode / waveguide assembly
2/23/2006
LCLS Bunch Length Monitor Marc Ross - SLAC

14. LCLS Bunch Length Monitor Marc Ross - SLAC
Reflector
(in use at FFTB for plasma wake exp. – Hogan)
Thin Ti foil ‘in the beam’ – polished.
Si window
Simple, direct, detector optics
Typically shorter than LCLS BC1 (not BC2)
LCLS annular reflector will be 30mm diameter with 14 mm aperture
Capture 50% ~ 1 uJ – at best
Low frequency performance reported to be poor (<300 GHz)
2/23/2006
LCLS Bunch Length Monitor Marc Ross - SLAC

15. CTR MICHELSON INTERFEROMETER
Variable
Position
Mirror ∆z
Interferometer Pyro Detector
12.5 µm Mylar
Beam Splitters
RT≈0.17
1mm HDPE
Vacuum Window
(3/4” dia)
Reference Pyro Detector
Alignment Laser
1 µm Titanium Foil at 45º
sx=60 µm, sy=170 µm
N≈1.91010 e-
Experimentalist - Need to measure these short bunches
Use interferometer for average bunch length
Not single shot
No info on head/tail asymmetries
10µm to 1mm tricky
• Interference signal normalized to the reference signal
• Motion resolution ∆zmin=1 µm or ≈14 fs (round trip)
• Mylar: R≈22%, T≈78%, RT≈0.17
reflector

16. reflector CTR Energy Correlates with Bunch Length
Relative end of linac
- For shot-to-shot, use scalar value of the integrated CTR energy
Shorter bunches give more CTR
Karl Bane PAC2003 paper showed good agreement with measured E-loss

17. reflector X-ray Pyro Is Not The Whole Story
Need to Look at Details of the Spectra
Example: Jitter from North Damping Ring:
X-ray
- CTR Energy can be ambiguous, but simple so can quickly reduce the data (to a point)
- Complimentary to E-Spread measurements (more later)
Relative Energy [GeV]
Pyro amplitude is ambiguous
Energy spectra are not
They are complimentary diagnostics
Clear correlation between energy spectrum and E-164X outcome

18. pyro jitter distribution – SLC NRTL stability
reflector
pyro jitter distribution – SLC NRTL stability
Our biggest source of jitter is Energy error in NRTL which multiplied by the large R56 gives big phase error entering the linac. Energy in RTL (mm of high dispersion BPM (544 Paul)). Pyro amplitude is y-axis. Not directly applicable to LCLS but these are the types of things you can hunt down with good simple diagnostics.

19. Spectra vs pyro-electric signal
reflector
Energy spectrum (y-axis, 1 column = 1 shot) correlates strongly with pyro amplitude (x-axis)

20. reflector One pyro vs another meets 5 to 10% resolution goal
Two pyros will track each other with little noise due to electronics. These two had diff amplifier circuits etc. Spread with two of current generation is much smaller (tighter line width). Haven’t had time to fish through the logbook and make some updated plots.

21. Pyro for one band vs another
reflector
Pyro for one band vs another
Pyro response as a function of linac ‘chirp’ (phase - offset)
Pyro amplitude depends which side of phase you are on with respect to best compression. Here y axis has different wavelength band of CTR than primary (x-axis). Peak of x axis is best compression. The redundant y-values are different distributions with differenyt fourier components on either side of best compression phase. Hand scanning phase ramp here.

22. pyro response has position correlations:
reflector
pyro response has position correlations:
Beam position on the foil correlates strongly with CTR amplitude. Symmetric diode array summing all sides is better from this standpoint. If use CDR etc need to think how to align. We put HeNe on beam vector…
Slope is larger on right but started saturating the GADC. Lots of data from Dec Do things better now but don’t make these measurements routinely. Might get a chance to before E-167 is over.

23. MM Wave detector Sensitivity
Pyroelectric: Basically a charge source, approximately 1.5uC/Joule.
Capacitance is 120pf for 3mm detector.
Charge amplifier (AmpTek A250, external FET), has noise 300 electrons RMS.
Corresponds to 30pJ of mm wave energy
Typically pyro detectors are supplied with included amplifier, performance tends to be worse.
Detector is a thermal integrator. Dynamic range is limited by the dynamic range of the amplifier.
For the AmpTek A250, this is approximately 60,000:1.
Commercial pyroelectric detectors (Scientech PHF02) have noise level of 3nJ, approx 100uJ maximum signal.
Note, sensitivity is 100X worse than theoretical, dynamic range is 30,000:1
2/23/2006
LCLS Bunch Length Monitor Marc Ross - SLAC

24. MM wave diode detectors
Sensitivity of ~2V/W (into 50 Ohms) with ~100GHz bandwidth (at 300GHz).
For a 100ps pulse, Bandwidth ~5GHz.
Noise is 2.3e-15J.
Diodes typically linear to ~30mV output.
1.5pJ. Dynamic range 700:1
Expect realistic amplifier (10dB noise figure) to limit dynamic range to 250:1
Waveguide for 300GHz is WR-2.8, 0.7X0.35mm.
expected attenuation 0.2dB/cm.
Waveguide for 900MHz is WR-1.0, 0.25X0.13mm.
Expected attenuation 1.1dB/cm
Need something like 20cm of waveguide for dispersion
The initial pulse is very short, with extremely high peak power. Waveguide dispersion spreads the pulse in time, while keeping the original frequency content.
2/23/2006
LCLS Bunch Length Monitor Marc Ross - SLAC

25. Comparison of pyro and diode detectors
Pyro detectors have much larger dynamic range (>30,000:1, vs ~200:1).
Noise energy diodes is 10^4 lower than for pyro detectors
Pyro area (for sample detector) is ~10 mm^2.
Diode (waveguide)
At 300GHz 0.2mm^2
At 900GHz 0.03mm^2
Diode Sensitivity / Area is 250x at 300GHz, 30X at 900GHz
Not clear how much gain available from horn antenna. (~10dB?)
Diodes more sensitive than pyros at 300GHz.
At 900GHz, diodes probably slightly less sensitive.
Dynamic range of pyro detectors is better
Diode alignment of waveguide is much easier.
2/23/2006
LCLS Bunch Length Monitor Marc Ross - SLAC

26. Controls and data acquisition
These systems are ‘single signal’ systems, i.e. only a simple gated digitizer is needed*
(Some concerns over gating precision and noise – to be tested)
BC1 Feedback will require beam intensity normalization and (possibly) steering correction / feedback
integration with LOLA improves the systematics greatly  we strongly recommend an aggressive approach to LOLA data acq./integration.
Pyro/diode systematics will be different and may require different procedures.
2007 testing plan
2/23/2006
LCLS Bunch Length Monitor Marc Ross - SLAC

27. LCLS Bunch Length Monitor Marc Ross - SLAC
Testing plan
ESA
Minimum bunch length ~200 um ?
Multi-channel resolution test
No independent high accuracy reference
April and July 2006
LCLS injector
Minimum bunch length ~ 50 um
High accuracy reference (29-4 LOLA Transverse cavity
2/23/2006
LCLS Bunch Length Monitor Marc Ross - SLAC