1

Welcome to the 2014

Beam-Halo Monitoring Workshop

SLAC2014 September 19

Alan FisherSLAC National Accelerator Laboratory

2Fisher – Beam-Halo Workshop

Your Hosts

Program Committee Alan Fisher, SLAC Rhodri Jones, CERN

Administration Deborah Lilly, SLAC

3Fisher – Beam-Halo Workshop

Topics

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)

4Fisher – Beam-Halo Workshop

Layout of LCLS-2 at SLAC

Replace 1st 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

5Fisher – Beam-Halo Workshop

LCLS-2 Linac Design

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 consideringCornell DC Gun

Gulliford, et al.PRSTAB 16073401 (2013)

6Fisher – Beam-Halo Workshop

Halo in LCLS-2

Beam loss in undulators can change field of permanent magnets. To operate for 10 years (same as LCLS), losses must be lowered by 104. 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?

7Fisher – Beam-Halo Workshop

Concepts for Halo Monitoring in the LHC

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

8Fisher – Beam-Halo Workshop

Digital Micro-Mirror Array (DMA)

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

9Fisher – Beam-Halo Workshop

SpectraCAM CID Camera

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 107

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

10Fisher – Beam-Halo Workshop

SpectraCAM Measurement of Laser Profile

C.P. Welsch, E. Bravin, B. Burel, T. Lefevre, T. Chapman and M.J. Pilon, Measurement Science and Technology 17 (2006) 2035

11Fisher – Beam-Halo Workshop 11

Photomultiplier with a Rastered Mask

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

12Fisher – Beam-Halo Workshop

Comparing CCD, CID, and PMT

C.P. Welsch, E. Bravin, B. Burel, T. Lefevre, T. Chapman and M.J. Pilon, Measurement Science and Technology 17 (2006) 2035

13Fisher – Beam-Halo Workshop

Does It Really Measure Halo?

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.

14Fisher – Beam-Halo Workshop

Removing Diffraction: Coronography

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 106 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 1010 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

15Fisher – Beam-Halo Workshop

Comparison Tests

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

16Fisher – Beam-Halo Workshop

Summary

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.