1. 1 Welcome to the 2014 Beam-Halo Monitoring Workshop SLAC 2014 September 19 Alan Fisher SLAC National Accelerator Laboratory

2. 2 Your Hosts Program Committee  Alan Fisher, SLAC  Rhodri Jones, CERN Administration  Deborah Lilly, SLAC Fisher – Beam-Halo Workshop

3. 3 Topics Fisher – Beam-Halo Workshop  The program is divided into three sessions:  Invasive techniques  Non-invasive techniques with gas or electrons  Optical techniques  We’ll consider halo measurements in:  Electron linacs (x-ray FELs)  Electron storage rings  Proton storage rings (LHC)

4. 4 Layout of LCLS-2 at SLAC Replace 1 st km of 3-km copper linac with superconducting linac.  CW RF will allow 1-MHz bunch rate (compared to 120 Hz in LCLS-1)  Install new Injector, SCRF linac, and extension in sectors 0-10  Re-use existing bypass line from Sector 10 to beam switchyard (BSY)  Re-use existing high power dump in BSY  Add RF spreader to direct beams to dump, SXR or HXR undulators  Install new variable gap HXR (replacing LCLS-1) and SXR  Re-use existing transfer line (LTU) to HXR; modify HXR dump  Construct new LTU to SXR and new dump line  Modify existing LCLS-1 x-ray optics and build new SXR x-ray line Fisher – Beam-Halo Workshop

5. 5 LCLS-2 Linac Design Fisher – Beam-Halo Workshop K. Baptiste, et al, NIM A 599, 9 (2009) J. Staples, F. Sannibale, S. Virostek, CBP Tech Note 366, Oct. 2006 Sannibale, et al. MOPRI054(IPAC2014) Wells, et al. MOPRI056(IPAC2014) Also considering Cornell DC Gun Gulliford, et al. PRSTAB 16 073401 (2013)

6. 6 Halo in LCLS-2 Fisher – Beam-Halo Workshop  Beam loss in undulators can change field of permanent magnets.  To operate for 10 years (same as LCLS), losses must be lowered by 10 4.  Better collimation, helped by halo monitoring  Main source is dark current in RF cavities  Dark current from linac will have lower energy than beam  Overbent in first bend of compressor chicane  Monitor electrons hitting chamber wall to get loss spectrum  Dark current from RF gun will have the same energy as the beam  Could deflect electrons in empty RF buckets and at wrong phase  Electrons emitted off-axis may be stopped at collimators  Optical monitor after linac, at final dog-leg?

7. 7 Concepts for Halo Monitoring in the LHC Fisher – Beam-Halo Workshop  Several concepts have been discussed for monitoring proton halo in the high-luminosity upgrade planned for the LHC  Fixed mask with zoom optics  Digital micro-mirror array (DMA)  See Jeff Corbett’s talk  CID camera with independent integration time for each pixel  Raster scan of small aperture in front of PMT, with gain controlled by servo loop

8. 8 Digital Micro-Mirror Array (DMA) Fisher – Beam-Halo Workshop 1024  768 grid of 13.68-µm square pixels Pixel tilts about the diagonal, toggling from −12° to +12° Mirror array is mounted on a control board, tilted by 45° so that reflections are horizontal

9. 9 SpectraCAM CID Camera Fisher – Beam-Halo Workshop  If you read a CCD fast enough to avoid saturating the brightest pixels, read noise will dominate the dim pixels.  Sum of dim-pixel values is not equivalent to a long integration.  A CID (charge injection device) can check the level of charge integrated in any one pixel, without clearing it.  Clear bright pixels imaging core of beam frequently  Integrate halo pixels for long periods  Incur read noise only once, even for dim pixels  Thermo Scientific’s SpectraCAM CID camera  Dynamic range of 10 7  2048 by 2048 pixels with non-destructive readout of selectable regions  Random-access (pixel-selective) integration, and no blooming  Three-stage thermoelectric cooling for low noise  Gigabit Ethernet output

10. 10 SpectraCAM Measurement of Laser Profile Fisher – Beam-Halo Workshop C.P. Welsch, E. Bravin, B. Burel, T. Lefevre, T. Chapman and M.J. Pilon, Measurement Science and Technology 17 (2006) 2035

11. 11 Photomultiplier with a Rastered Mask Fisher – Beam-Halo Workshop  Raster scan a mask with a small hole across an optical image of the beam.  Measure the transmitted light with a photomultiplier.  Servo the PMT high voltage to maintain a constant signal.  PMT gain varies as a power of the voltage over orders of magnitude  Or just insert attenuation as needed. Hamamatsu R1828-01

12. 12 Comparing CCD, CID, and PMT Fisher – Beam-Halo Workshop C.P. Welsch, E. Bravin, B. Burel, T. Lefevre, T. Chapman and M.J. Pilon, Measurement Science and Technology 17 (2006) 2035

13. 13 Does It Really Measure Halo? Fisher – Beam-Halo Workshop  These ideas can measure over a large dynamic range.  But…will they measure halo, or will a measurement be dominated by diffracted and scattered light from the core?  Deconvolution with the point-spread function (transmission pattern of a point source) can correct some of this, but:  Only if measured with the same optics (the LHC light monitor)  Each lens or mirror will have unique scatter  The beam is not a point source. Its halo is included in any PSF measured with the beam. An independent point source would be needed.  Also, in a real machine, stray light from bends reflected along the inside of the beampipe will look like halo.  A thorough arrangement of baffles to restrict the source region can help.

14. 14 Removing Diffraction: Coronography Fisher – Beam-Halo Workshop  We want to image a dim beam halo surrounding a bright bunch  Compare to viewing the sun’s corona without the benefit of a solar eclipse  First observed by Bernard Lyot in the 1930s  Mask B blocks the image of the solar disk, but not the Airy diffraction rings.  The sun is about 10 6 times brighter than the corona. Even the rings are too bright.  Mask D, on the Fourier-transform plane of the masked image, removes the rings.  Similar techniques, with up to 10 10 dynamic range, are being developed to observe exoplanets (planets around other stars).  See the final talk by Sandrine Thomas. Bernard Lyot, Monthly Notices of the Royal Astronomical Society 99 (1939) 580

15. 15 Comparison Tests Fisher – Beam-Halo Workshop  Test bench with controllable “halo”  Use various sources of controllable bright and dim light (lasers, fibers, bulbs, LEDs), along with adjustable attenuation  Carefully placed black baffles to reduce stray light  A Lyot stop may be needed  Test at an electron ring (SPEAR3)  Does halo change with collimation?  Test on the replica of the optical tables in the LHC tunnel

16. 16 Summary Fisher – Beam-Halo Workshop  Beam halo needs to be measured over a large dynamic range.  In LCLS-2, lost particles may be useful to find sources of halo.  Optical techniques are subject to scatter and diffraction.  Astronomers have a lot of experience with this problem.