1. Slice Parameter Measurements at the SwissFEL Injector Test Facility
Eduard Prat Costa
PSI/DESY/KIT Mini-Workshop on Longitudinal Diagnostics for FELs
11 March 2013

2. Contents Introduction Longitudinal phase-space measurements
Slice emittance measurements
High-level applications
Summary and next steps 2

3. SwissFEL Injector Test Facility (SITF)
Phase 1: Electron source and diagnostics (03/2010 – 07/2010)
Phase 2: Phase 1 + (some) S-band acceleration (08/2010 – summer 2011)
Phase 3: The full machine
Summer 2011: installation of bunch compressor.
All S-band rf available from April 2012
X-band: power expected in April 2013
quadrupole
BPM+screen
screen
Gun
S-band
X-band
Bunch compressor
FODO section
Transverse
deflector
High-energy
spectrometer
E= 7 MeV
E= 250 MeV
Phase 4: Installation of new PSI gun + undulator experiment (08-09/2013)

4. Design optics for phase 3
CSR effects
βx < 10m between 3rd and 4th dipole of BC
Longitudinal resolution
At the TD: βy = 40m
Between TD and profile monitor: sin(Δy) = 1
resolution ~ 4μm for VMAX=5MV
(assuming εy=0.5μm, E=250MeV)
Energy spread resolution
At the spectrometer:
βx = 0.214m
Dx = 0.267m
resolution ~ 14keV
(assuming εx=0.5μm, E=250MeV)
Optics for symmetric single quad scan
βx = βy = 15m
αx = αy = 1.385 TD
FINXB
entrance BC 4

5. Transverse Deflector (TD)
A Transverse Deflector (TD) is used to measure the longitudinal bunch profile and the slice parameters of the beam (energy, energy spread, transverse position, transverse size and emittance)
TD gives a transverse (vertical) kick which depends on the longitudinal position of the beam.
Calibration: the phase of the deflector is changed to obtain the calibration between vertical position and bunch arrival time. The centroid shift gives an idea of the equivalent time jitter of the beam.
y at observation point
y’ at TD position
Calibration: S = 1.43mm/ps
equivalent jitter = 0.11ps
At the TD: βy = 40m
Between TDC and monitor: sin(Δy) = 1
resolution ~ 4μm for VMAX=5MV
(assuming εy=0.5μm, E=250MeV) 5

6. Longitudinal phase-space measurements
Combining the TDC with an energy spectrometer the longitudinal phase-space can be measured
At the SIFT these measurements are done at the spectrometer at the end of the lattice
The bunch length is the beam size in the vertical plane normalized by the TD calibration
Beam centroid position and beam size give information about energy and energy spread:
σ2 = ε•β + D2•δ2 (δ = Δp/p)
Optics resolution ~14keV
(βx = 0.21m, Dx = 0.27m, εx=0.5μm, E=250MeV)
Present resolution limited at around 80keV due to screen resolution
More studies will be shown in Gian Luca’s talk, and more to be done soon
Measurement example (230 MeV, on-crest)
Slice energy spread and current profile
Beam image
Bunch length
2.73 ± 0.21 ps
Energy spread at the core
76.4 ± 0.32 KeV 6

7. Slice emittance measurements
Streaked beam
Transverse deflection (streaking)
Optics. The optics are scanned using 5 quads between transverse deflector and observation point (end of FODO section).
Generate regular phase-advance in x
Keep beam size under control
Keep longitudinal resolution constant
Transverse deflector calibration. At each optics the phase of the deflector is changed to obtain individual mm-ps calibration for each optics.
Slice analysis. The beam is split into slices, using the centroid from Gauss fit as a reference. Per each slice the beam size from Gauss fit is obtained.
Emittance/mismatch determination. From the beam sizes per each optics the emittance and optics are obtained per each slice.
Projected parameters are also obtained
Reconstructed emittance and optics 7

8. Optics for slice emittance
5 quads (F10D1.MQUA.60/70/80/90/100) are used to:
Scan phase-advance in x
Keep βx under control (35≤βx≤40)
Keep βy small (βy<10)
sin(μy) ~ 1
Beta functions at the screen
k-values
Phase-advances 8

9. Resolution, errors and matching
Beam image close to screen resolution limit
SwissFEL profile monitor (YAG + OTR) is used for emittance measurements.
Beam size resolution is ~15μm, equivalent to an emittance resolution of ~ 3nm (E=250MeV, β=35m)
Signal to noise ratio is good enough to measure slice emittance down to ~1pC level
Errors
Statistical errors from beam size variations (this is what is shown in the error bars of the measurements)
Systematic errors: optics mismatch, energy error, quad field errors, screen calibration and resolution, errors associated to Gauss fit, etc.
Matching
Beam core is always matched to exclude errors due to
optics mismatch
Matching of the core works normally in 1-2 iterations
Successful matching gives us confidence in the
obtained emittance values 9

10. Optimum Emittances 200pC 10pC
By doing a full optimization we have achieved the following emittances (uncompressed beam)
These emittance values fulfill the SwissFEL requirements for uncompressed beams
Emittance values are stable in short-term and optimum settings are reproducible
200 pC
10 pC
Projected emittance
~0.30μm
~0.15μm
Slice emittance
~0.20μm
~0.10μm
200pC
10pC 10

11. Example application: “thermal emittance” measurements
Goal: compare thermal emittance for different wavelengths (267.5nm is our standard value).
Method: scan the laser beam size and measure the slice emittance at low charge, keeping the charge density constant (10pC with 1.8mm aperture, bunch length of 10 ps).
Measured values (~600nm/mm) are much better than simulation assumptions (910nm/mm).
Degradation when going from 267.5nm to 260nm is only 5-10%.
High order contributions possibly due to space charge or rf fields (not seen in simulations with ASTRA)
We need further investigations (investigate space-charge effects, gun gradient contribution, etc.)
λ=260.0nm
λ=267.5nm
Thermal emittance
~630nm/mm
~585nm/mm

12. High-level applications
High-level applications written in Matlab (they use the generic toolbox ADAQA)
One emittance tool for all type of measurements (projected and slice)
Automatic analysis, data saving, plotting of results and logbook entry
Routinely used in operation
User needs to do some preparation and checks:
Measure the beam energy
Switch on TD and check the zero-crossing phase
Be sure that the used screen is in and that the ROI is properly chosen
For emittance measurements, check that the quads do not steer out of the ROI
Plot for slice emittance measurement 12

13. GUI for longitudinal phase-space measurements 13

14. GUI for emittance measurements 14

15. GUI for optics matching 15

16. Summary and next steps Summary
Methods to measure slice beam parameters well developed and tested at the SwissFEL Injector Test Facility
Longitudinal phase-space
Longitudinal resolution = 4μm (for VMAX = 5 MV)
Energy spread resolution = 14keV (now limited to ~80keV due to screen resolution)
Slice emittance
Emittance resolution around 3nm
Successful optics matching
Great results achieved for uncompressed bunches
High-level applications developed for these measurements, routinely used in operation
Next steps
Emittance optimization for compressed bunches with X-band cavity
Long. phase-space measurements for compressed bunches with X-band cavity
Improve profile monitor at energy spectrometer 16