X-Ray Diagnostics for the LCLS

Jan. 19-20, 2004 UCLA

General Assumptions (See CDR)

• Undulators Fixed in Position– Exception: Small motion for alignment

• Fixed Gap Undulators

• Permanent Magnet Quads

• Cavity BPMs following each undulator and prior to the 1st

• Full suite of diagnostics every third undulator

Current Thinking

• Use NdFeB– Radiation Damage Issue

• Real benefit of Sm2Co17 still no known

• Must determine acceptable losses• APS is doing this for operational reasons• 7 Additional undulators planned for use in regular

maintenance schedule

– Temperature Compensation• Sm2Co17 slightly more than a factor of 2 better. Not

enough.

Current Thinking• K Adjustment/Control Strategy

Cant (mrad)

Horizontal alignment accuracy (mm)

Beff/Beff

over 10 mm

Range of hort. motion (mm) for Beff/Beff =

±1.5x10-3 (±5.5x10-4)b

Incremental thickness (mm) of mech. shims

2 0.50 3.0x10-3 ±5.0 (±1.8) 0.010

3 0.33 4.5x10-3 ±3.3 (±1.2) 0.015

4 0.25 6.0x10-3 ±2.5 (±0.9) 0.020

a All calculations were made according to a tolerance of 1.5x10-4 and a total range of 1.5x10-3 (20 Gauss) for Beff/Beff. Spacer thickness steps were chosen to allow

full compensation at half travel of the total range of motion. b The range of horizontal motion listed in the parentheses corresponds to ±1°C temperature compensation. To allow for temperature compensation of ±1°C the additional range of motion listed in parentheses should be provided.

Effect of K Errors

Undulator segment tolerances

Parameter Specified Value

Trajectory excursion (both planes) 2 m

Radiation amplitude deviation 2%

Phase slippage between undulators 10° a

Vertical positioning error 50 m

a Total allowed phase slippage including all errors. The error in the effective magnetic field Beff, totally dominates the contributions.

Phase Error Correction Applied

Calculated gain length and increase in saturation length using a random uniform distribution of K values from one undulator to the next with end-phase corrections applied for the LCLS beam parameters.a

K/K Gain length (m) b Increase in sat. length (m) c

±3.5x10-4 6.46 * -

±7.0x10-4 6.50 ~ 1.7 m

±10.0x10-4 6.60 ~ 3.4 m

a Normalized beam emittance was 1.5 mm-mrad, beam energy spread 2.1x10-4, FODO lattice strength 0.112 m-1, and peak current 3.5 kA. Other parameters same as in Table 1.b The gain length was derived over the next-to-last super period of six undulators.c The increase in saturation length was estimated near 100 m by determining how much additional distance was needed to reach the same level of ln|J|2.* Same value as for K/K = 0.

Current Thinking

• Undulator System Fully Installed on day 1

• Start at 800 eV– Why?

UNDULATOR

3420 627

11528 mm

Horizontal Steering Coil

Vertical Steering Coil

Beam Position Monitor

852

X-Ray Diagnostics

Quadrupoles

Cell structure of the LCLS Undulator Line

Cell structure of the LCLS Undulator Line

33 Undulators ~ 130-m Overall Length

Pioneering Science andTechnology

Office of Science U.S. Department

of Energy

PRIOR WORKFOR INTRA-UNDULATOR DIAGNOSTICS (CDR April 2002)

Electron Beam Diagnostics (Section 8.11, Glenn Decker and Alex Lumpkin)Table 8.9 Undulator electron beam diagnostics

TypeQuantity Location BXY Notes

*Beam position monitor 48 All stations Cavity / Button combo

OTR imaging diagnostic 13 Every third station Thin aluminum foil screen

Wire scanners 4 Upstream of Undulator segments

*Cherenkov Detectors 33 All stations Fused silica / PMT combo

*Current Monitors 2 Upstream/downstream of undulator

Toroid transformer* Non-intercepting

X-ray Diagnostics (Section 8.13, Efim Gluskin and Petre Ilinski)

On-axis diagnostics Diamond (111), 4 – 9 keV, cooled silicon PIN diode.Off-axis diagnostics Hole crystal, 2 = 90, CCD detector.

PRIOR WORK (OBSERVATIONS)

Good starting point for non-interceptive diagnostics (BPM, Cherenkov detectors, current monitors, etc.)

Laid good foundation for low-power interceptive diagnostics (OTR, on-axis x-ray detectors, etc.)

Low photon energy (827 eV or 1.5 angstrom) x-ray

diagnostics was not addressed.

High-power interceptive diagnostics was not addressed.

IMPLICATION OF HIGH PEAK POWER (Extrapolation from CDR Section 9.1)

Table 9.1 Characteristics of the FEL x-ray beam (Adapted) 0.828 keV (4.54 GeV) 8.27 keV (14.35 GeV)

FEL Spontaneous FEL Spontaneous Energy per pulse (mJ) 3 1.4 2.5 22

Peak power (GW) 11 4.9 9 81 Divergence (rad FWHM) 9 780 1 250

Spot size at 50 m (m FWHM) 610 Aperture limited 130 Aperture limited Spot size in undulator (m FWHM) 75 66

Average power (W) 0.4 0.2 0.3 2.6 Average power density (W/mm2) 52 42

Table 9.2 Normal-incidence peak energy dose and damage to materials (Adapted) Dose at 50 m (eV/atom) Dose in undulator (eV/atom)

Material Melt

(eV/Atom) Evaporation*

(eV/atom) 827 eV 8.27 keV 827 eV 8.27 keV Li 0.1 1.8 0.02 0.0005 1.3 0.002 Be 0.3 3.8 0.08 0.001 5 0.004 B 0.5 4.8 0.2 0.003 13 0.01

C (graphite) 0.9 0.4 0.007 26 0.03 Al 0.2 3.9 0.4 0.2 26 0.8 Si 0.4 5.2 0.6 0.2 40 0.8 Cu 0.3 1.1 0.4 73 1.6

* Data from thermal measurements: fusion and evaporation latent heat, plus heating up with specific heat (25C).

Screen material is lost similar to laser ablation!

MAIN OBJECTIVES (in orders of priority)

(0) Measure the e-beam centroid position at all breaks To characterize electron trajectory (RF BPM)

(1A) Measure the x-ray beam profile and its overlap with e-beam at all long breaks

To characterize the amplifier in more details

(1B) Measure monochromatic x-ray beam intensity at all long breaks To characterize the FEL amplifier at chosen photon energy

(2) Measure the electron beam profile at all long breaks To characterize the electron beam transport

(3) Analyze the electron and x-ray beam in 6-dimensions (x, y, x’, y’, t) with adequate resolution

To study FEL physics

Three Groups of LCLS Intra-Undulator Photon Diagnostics

At saturation, the LCLS x-ray beam is capable of severe surface damages at 40 m downstream of the undulator exit (CDR Ch. 9). Inside the undulator, where the FEL beam is at or near saturation, the power intensity is 30 to 60 times higher than that on the user optics. No solid surfaces can avoid damage.

(1) Lower Power Diagnostics (LPD) Before amplified beam power exceeds spontaneous radiation power, conventional technology can be used with judicious choice of materials. Installation may cover the entire undulator during commissioning period. Reduce to about 50% (six to seven stations) at normal (full power) operations.

(2) High Power Diagnostics (HPD) Depending on where the amplified beam reaches saturation, in as much as fifty per cent of the undulator the beam power can be very damaging, especially at low photon energy end. A maximum 6 stations are needed to provide diagnostics without solid screens in the beam.

(3) End of Undulator Diagnostics (EOU) (Overlaps with LLNL and SLAC work)

End of Undulator Diagnostics

At the end of the undulator, the electron beam separates from the x-ray beam, and the spatial limit in the breaks no longer exists. We have a unique opportunity to perform more beam measurements.

(1) Gamma Ray diagnostics Gamma ray detector array: (use with wire scanner, beam loss monitor).

(2) Electron beam diagnostics Current monitor at the end of undulator and at the dump Bunch length (streak camera using OTR source, or electro-optic technique) Energy spread and emittance (characterize the extraction of beam energy)

(3) X-ray beam diagnostics: Intermediate power handling capabilities to complement user beamline diagnostics. Provide micro-bunching diagnostics for: During commissioning or machine study period, using the main x-ray beam,

before saturation is achieved. During full power x-ray operation, using one of the following sources in non-

intercepting mode: o Coherent dipole radiation, o Use a dipole to deflect the electron beam before the last undulator, and o Off-axis undulator radiation from the main beam?

Major Subsystems of the Low Power Diagnostics (LPD)

o Scanning wire assembly

o X-ray intensity monitor

o Optical transition radiation (OTR) imaging station

o X-ray monochromator / imaging detector

Low Power Diagnostics (LPD) Summary

Physics issues:

(1) Determine what screen is safe in the beam, for the undulator and itself (Scattering calculations and power load calculations).

(2) Select suitable monochromator elements covering 800 eV to 8 keV (Search for materials, techniques and vendors).

(3) Find linear detectors covering 800 eV to 8 keV (Explore detection techniques and vendors).

Expect to use quantitative analysis to put the proposed approaches on solid footing. Prototypes are to be used to experimentally verify the models.

Engineering issues:

(1) It is a challenge to accommodate all mechanisms under the stringent spatial constraint.

(2) All LPD instruments in the same chamber need to be designed “together” to minimize difficulties and design time.

Mechanical / vacuum design can start when an engineer is available and ready.

Major Subsystems of the High Power Diagnostics (HPD)

o X-ray intensity monitor (gas phase or disposable solid surfaces)

o Laser wire assembly

o Optical diffraction radiation (ODR) or off-axis undulator radiation monitor

The high power region is defined as where no solid surface is safe from damage (or total evaporation) by the x-ray beam. Hence the interceptive diagnostics use gas phase media or expendable solid surfaces. Some of the diagnostics in this region are totally non-interceptive. To date, we do not have a reasonably viable approach for characterizing the overlap of the electron and photon beam. Approaches considered were:

o Focused ion beam (in magnetic field free region EOU) o Molecular beam (spatial resolution) o Gas phase fluorescence imaging

High Power Diagnostics (HPD) Summary

Physics issues:

(1) The approach to x-ray intensity measurements needed to be studied (gas cell / jet proposed) with numeric modeling.

(2) The approach to optical e-beam divergence measurements needed to be identified (ODR imaging proposed) with numeric modeling.

(3) Laser wire represents significant investment in cost and maintenance commitment, although its resolution should be adequate. Alternative needs to be studied.

Major analysis work has to be performed to justify the proposed approaches. We have not proposed a technique to measure x-ray beam size with confidence.

Engineering issues:

(0) Gas cell / jets put heavy load on the vacuum / pumping system.

(1) Laser wire requires line-of-sight transport, which should be built in the concrete support base being designed.

Mechanical / vacuum design cannot start earnestly until major physics issues are resolved. Integration of final physics approaches could prove challenging.

High Power Diagnostics Strategy

It can be seen that the main development effort is in the high-power segments. The following strategies have been proposed to manage (delay or avoid) the tasks.

(1) Start with all lower power diagnostics (written in WBS)

Initial LPD installation covers the entire undulator. Replace stations at high-power end as the commissioning progresses. Gain time for development for high-power stations.

(2) Tune beam / undulator at short x-ray wavelengths (discussed with Galayda, no conclusion) Since absorption is roughly proportional to the third power of photon energy, the absorbed power is dramatically reduced at the high photon energy end. Do not need high-power diagnostics.

(3) Tune beam / undulator at low charge (SLAC Pat Krejcik) Use interceptive diagnostics only at low charge. Do not need high-power diagnostics.

Commissioning Workshop will formalize these strategies!

ENTRANCE SECTION

Gamma Ray Detector

Issues

• Quad Fixed to the Undulator– Has implications on using the cant for tuning

• Requires separate motion

• Tunnel Temperature Control– Would like better than +/- 0.2 Degrees C.

• Supports will probably need this or better

• 800 eV– Makes life very difficult for the x-ray diagnostics– Would prefer to start a > 2 KeV

Issues

• Motion Capability– We need some for K tuning but…– Do we need the ability to completely remove

the undulators?• Aside: 0.1 G deflects the 14 GeV beam by ~1.5 um

in 3.7 m.

• High Power Damage– Can we even make a diagnostic that is both

useful and can handle the power densities of the FEL?

Issues

• Radiation Damage– We will use NdFeB– We must protect the undulator from radiation

• This has implications on intraundulator interceptive diagnostics

• Canted Poles– BBA

• There will be some arbitrary offset and anle through the undulator

• How does this impact the K tuning?