1. Undulator X-Ray Diagnostics, R&D Plans
Bingxin Yang
Argonne National Lab
My name is Bingxin Yang. I will discuss the R & D plans for the Undulator X-ray diagnostics.

2. Undulator X-Ray Diagnostics Scope
X-ray optics / detector systems (low power) to aid tuning of undulator systems, and maximizing the FEL gain.
Contents
Past conceptual developments
Far-field x-ray diagnostics R & D plan and Argonne’s participation
Intra-undulator diagnostics R & D
As a part of the x-ray beam transport system, a set of diagnostics is already planned to characterize the x-ray beam being delivered to the user experiments. It is not the x-ray diagnostics I will discuss here.
I WILL discuss x-ray optics / detector systems specifically designed to provide information of the electron and x-ray beams. AND the information is used for tuning the free electron laser, and for maximizing its gain. To avoid duplication with the user x-ray beam diagnostics, the emphasis here is on the low power x-ray optics, corresponding to the initial stage of FEL gain process, the start up and linear region.
I will cover the conceptual development in the past two years, how we came to our current view of these diagnostics; the R & D plans for far-field x-ray diagnostics, and the remaining work in the intra-undulator region, the long breaks.

3. Evolution of the x-ray diagnostics plans
CDR (Apr. 2002)
(ANL) Diagnostics for FEL start up in the undulator
(LLNL) Diagnostics for x-ray beam out of the undulator
Re-examination
(Jan. 2004, UCLA) Undulator commissioning workshop
(Feb. 2004, SLAC) X-ray diagnostics planning meeting
Major issues
Beam damage of optical components
Getting sufficient information for FEL tuning?
In the original Conceptual Design Report, the x-ray diagnostics is divided between Argonne and Livermore: Argonne is responsible for diagnostics to start the FEL and to optimize it. Livermore is responsible for transport and diagnostics after the undulator. Chapter 9, written by Livermore, is mostly about proper handling and characterizing an x-ray beam with very high peak power.
That plan was re-examined in January 2004 in UCLA undulator commissioning workshop. Major issues about diagnostics came up: how are we to deal with beam damage to optical components? And more pointedly, FEL physicists asked the question: Are we getting sufficient information from these diagnostics to tune up the FEL?
Upon Galayda’s request, John Arthur called a follow up meeting to discuss plans for the x-ray diagnostics R&D on February 10.

4. Major issues at UCLA workshop
Beam damage of optical components
Example from Marc Ross’ coupon test, LINAC 2000
Saturated FEL beam deposit higher energy density
Desirable information
Trajectory accuracy (Dx~1mm)
Effective K (DK/K ~ 1.5×10-4)
Relative phase (Df~10º)
Intensity gain (DE/E~0.1%, z-)
Undulator field quality
The damage to the intra-undulator optics may be inflicted by two beams:
The electron beam can blow through the optics in a single shot, as shown by Marc Ross in a 1999 experiment. At FFTB, he made a 10 micron diameter tunnel in a 1.4-mm thick copper plate by passing a single bunch of 1.2 nC electrons through it. The LCLS beam cross section will have 10 times the area compared to these experiments, but no one can be sure how much better we really are.
The x-ray beam, at saturation, deposits 10 to 1000 times more power density than the electron beam. But due to exponential attenuation into the material, the optics are likely peeled off in layers.
On the second issue: Instead of saying what information can be provided by various diagnostics, people at the planning meeting turned around and asked what information would really be useful for starting up the FEL process. Here are five important beam characteristics we would like to know.
We would like to know that the trajectory is accurate to within 1 micron, or direction to 0.25 micro-radian, the K is accurate to 5 x 10^-4, relative phase of adjacent undulator segments to 10 degrees. We would like to see clearly that intensity is increasing as a FEL process sets in. Some information about undulator field quality would also be useful since BPMs give electron coordinates at only discrete points.

5. Rethink x-ray diagnostics (Galayda)
Intra-undulator diagnostics
Electron beam position monitor (BPM)
Electron beam profiler (OTR & wire scanner)
Low power x-ray Intensity measurements (R&D)
Far-field low-power x-ray diagnostics (R&D)
Clean signature from spontaneous radiation
Space for larger optics / detectors
Single set advantage (consistency, cost)
Goal = obtain “desirable information”
Based on the discussions of the workshop, John made a crucial decision: the x-ray diagnostics for the FEL start-up is removed from the list of construction items. It is now an R & D project.
Between the undulators, in the long breaks, we will only retain electron beam profile measurements, and low-power x-ray beam intensity measurements as a function of distance, z.
The majority of the start-up x-ray diagnostics is moved to the end of undulator. What do we gain by doing that? In the far-field, the signatures of the radiation pattern is better understood. We have more space for larger optical components or detectors to do a decent job of characterizing the beam. A single set of diagnostics improves consistency of the measurements, and reduces the cost.
The march order John gave was to develop diagnostics to provide the information desired for tuning up FEL, we translated it into the information I listed in the last viewgraph.

6. Two Essential Elements for Far-Field Measurements
Roll away undulators
Spontaneous radiation is most useful when background is clean, with each undulator rolled in individually.
Adequate Far-field X-ray Diagnostics
Adequate x-ray diagnostics extracts the beam / undulator information:
Electron trajectory inside the undulator (mm / mrad accuracy)
Undulator K-value (DK/K ~ 1.5 × 10-4)
Relative phase of undulators (Df ~ 10°)
X-ray intensity measurements (DE/E ~ 0.1%, z-dependent)
Micro-bunching measurements (z-dependent)
This is a shortened version of a viewgraph I presented at the February planning meeting. I consider two components essential for far-field x-ray diagnostics to succeed: the Roll-away undulators, and an adequate set of x-ray optics.
From our experience of APS diagnostics undulators, the desired accuracy of the measurements here are very high for beam-based measurements, especially so since LINAC beam is not ideal Gaussian in shape, or in divergence, or in energy. Inference on one undulator is often made based on subtle features of the radiation pattern. It would be impossible if the radiation pattern is overlaid with 32 others simultaneously.
Adequate x-ray optics / detectors are also necessary. Their wavelength region, resolution, sensitivity, dynamic range, should be optimized for the start-up diagnostics. They all depend on the spatial-spectral feature of the undulator radiation pattern selected for the diagnostics.
The five properties are basically a repeat of what I listed before, except the micro-bunch measurement, which is complementary to the intensity measurements to characterize the FEL process.
It is not possible to go into the details of each of the measurements here. In the next five viewgraphs, I will give an overview of our current plan on how to perform some of the measurements.

7. Far-Field measurement of x-ray beam centroid
Use center of the far-field pattern to determine e-beam trajectory and slope (x, x’) inside the undulator.
Need relative accuracy 0.25 mrad or better.
For an electron to travel on the magnetic axis of an undulator, it needs to come in at a specific angle of the order of a micro-radian, and with a specific offset of a fraction of micron. The sub-micron offset is negligibly small, but the angle deviation can be important since it strongly impacts the phase difference between x-ray and electron beam.
The proposed measurement is based on the spontaneous radiation pattern at the far-field, and for a single undulator segment. At the APS, we routinely obtain beam direction from undulator far-field measurements with sub-micro-radian resolution. The non-Gaussian distribution of the LCLS electron beam means that the on-energy, single peak pattern is not good for characterizing beam direction. At lower photon energy, the radiation pattern becomes concentric rings. With sufficient energy offset, the thickness of the rings are dominate by the e-beam divergence, which is 1.5 micro-radian or higher. With adequate data processing program, we should have no problem for getting beam average direction inside the undulator at resolution of 0.25 micro-radian or much better.

8. Far-Field measurement of Undulator K-value
Use angle-integrated spectrum to set all undulator to same K (or to a known taper).
Needed relative accuracy DK~ (DK/K ~ 1.5×10-4).
Issues: e-beam energy spread and jitter DE/E=2~5×10-4.
The angle-integrated intensity spectra can be used to extract effective K information of a single undulator segment.
The required accuracy, DK/K ~ 1.5 x 10^-4, is reachable but near the resolution limit of the technique. The main issues here are proper handling of beam effect. The e-beam has a specified energy spread sE/E ~ 5 x 10^-4, and energy jitter of DE/E ~ 2 x 10^-4. Their effect doubles in the photon spectrum to 10 x 10^-4 and 4 x 10^-4 respectively.
Simulation will be the next step to show the beam effect. We will also look for other features to use for the diagnostics.

9. Far-field measurement of relative phase of undulators
Use interference of radiation from two undulators to tune their phase differences
Relative accuracy ~ 10 degrees or better
Reformulate the question for distributed phase shift?
Using far-field radiation pattern to set the relative phase of two adjacent undulators has been briefly studies by a BESY group (FEL2000). The radiation pattern is sensitive to the phase difference.
We would like to test the approach with numeric simulations under the LCLS operating parameter space.
The BESY study was for two identical undulators with different longitudinal phase. One intriguing question is for our current undulator scheme: we are adjusting the K-value of one of the two undulators to obtain the best match of the average phase!

10. Far-field measurement of FEL gain (z)
Measure monochromatic x-ray beam intensity as undulator segments are added, characterize the FEL start up and early gain process
Wide bandwidth monochromator (DE/E ~ 0.1%)
Multilayer reflectors
Mosaic or asymmetrically-cut crystals
Large dynamic range detector(s)
Low power only (before saturation)
To show that electrons are starting to micro-bunch, we could show that the x-ray beam intensity is increasing faster than the length of the undulator, z.
Here we do not need to search the spectral features to study: We need to measure the total x-ray beam intensity near the fundamental photon energy as a function of distance into the undulator, z.
In the UCLA workshop, Claudio Pellegrini pointed out the mismatch between the narrow bandwidth of a simple crystal monochromators and the broad stimulated FEL radiation.
Hence for the FEL intensity gain measurements, the core development task is a broadband x-ray monochromator. It can be realized with several optical components. We will evaluate experimentally the suitability of multilayer reflector, mosaic crystals, or asymmetrically cut crystals.
Other development tasks include x-ray detectors with large dynamic range, and wide spectral response.

11. More x-ray diagnostics of FEL physics?
Take single shot spectrum (DE/E ~ 0.5%, dE/E ~ 0.01%, z-dependent)
Measurement of electron beam micro-bunching (z-dependent)
Several other diagnostics related FEL physics also came up. Their priorities are not high at this point. We will try to address them when we are comfortable with answering John Galayda’s march orders.

12. FY04 effort in far-field x-ray diagnostics
In the new R&D plan, Argonne is a part of the (SLAC/LLNL/ANL) collaboration on x-ray diagnostics: concept development, performance simulation, and system design.
Develop concept from lessons learned from APS diagnostic undulator
Design numeric tools for simulation of far-field x-ray diagnostics
In the new structure for the x-ray diagnostics, Argonne is a part of the collaboration among three labs.
Our tasks have been to develop concepts for the far-field x-ray diagnostics, based, somewhat, on our 6 year’s operating experience of the APS diagnostics undulator. Some of the promising concept are shown in the previous viewgraphs.
We will also need new tools for simulation of these diagnostics, especially in the LCLS operation parameter space.

13. Remaining intra-undulator diagnostics
Location: every long break (905 mm)
Diagnostics chamber length: 425 mm
Functional components
RF BPM, Cherenkov detector, OTR profiler, wire scanner, x-ray (intensity) diagnostics
Now we look at what diagnostics remain between undulator segments:
There is one long break between every three undulator segments, 10 between a total of 33 undulators. The length of the breaks are 905 mm between the pole faces of the near by undulators.
There is one RF BPM, one focusing quad, one Cherenkov detector, an OTR e-beam profiler, a wire scanner, and a set of x-ray intensity diagnostics.
>> Go through the components on the picture:
RF BPM, Quad, Cherenkov detector
Horizontal sliding platform, beam tube,
OTR screen and camera module
Wire scanner module
First x-ray optics, second x-ray optics, imaging detector, integration detector.

14. FY04 accomplishments Layout of diagnostics chamber OTR profiler
Camera module designed
Wire scanner
Scanner design in progress
Wire card adapt SLAC design
X-ray diagnostics design
Beam intensity: double crystal
Beam profile: imaging detector
The main tasks in FY04 are to design prototypes.
We managed to squeeze all desired diagnostics in a chamber with 425 mm insertion length, which I am mostly proud of.
We designed the optical transition radiation profiler. Parts of the prototype camera module are being procured now. We expect to assemble it by the end of FY04.
We have designed a compact version of wire scanner prototype, based on the SLAC design we were given. Its procurements and fabrication will be governed by the fiscal reality outside of our control.
Current plan for x-ray diagnostics include a double crystal monochromator, a dual detector system that measures the x-ray beam intensity as well as its profile.
The final design of the x-ray diagnostics will depend on the evaluation of commercial scanning stages, which is also subject to our financial boundary conditions, meaning we are waiting for money to become available to start our evaluation of the components.

15. Conclusions and current status
Plan for start-up x-ray diagnostics has been restructured, driven by the need of FEL tuning and existing experimental limitations. Center of gravity shifts significantly towards the end of undulators. Goals are clearly specified.
With roll away undulators, we have, at least conceptually, a good handle on relative measurements of trajectory direction (field quality), undulator K-value, and x-ray intensity gain.
Concept for undulator phasing and micro-bunching measurements need further development
Intra-undulator diagnostics development is nearly on-target.
Read out the text with minor explanations.

16. R&D plan for x-ray diagnostics in FY05
Wide bandwidth monochromator (DE/E ~ 0.1%)
Critical to FEL diagnostics inside / end-of undulator
Test multi-layer optics and asymmetrically cut crystals
Search for mosaic crystal (with APS/XFD/Optics)
Far-field undulator radiation diagnostics
Identify suitable spatial-spectral features
Simulation with non-ideal beam and non-ideal field
Estimate realistic measurement accuracy
Develop x-ray optics / detector requirements
Test core optical components
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