A Digital Micro-mirror array-based beam halo monitor

Blaine Lomberg University of Liverpool and The Cockcroft

Institute3rd OPAC Topical Workshop on Beam

Diagnostics20/04/23 1

B.Lomberg-3rd OPAC Topical Workshop on Beam Diagnostics: Beam Halo Talk- Vienna, Austria

20/04/23 2B.Lomberg-3rd OPAC Topical Workshop on Beam

Diagnostics: Beam Halo Talk- Vienna, Austria

• “Beam Halo” is associated with many negative effects for high Intensity Beams:

– Particle losses lead to nuclear activation,• Increase in secondary emission, space-charge.• Damage of the surrounding vacuum chamber.

• Using the monitor to observe the number of particles in the tail region of the beam distribution – and minimise particles at large radii (nσ: n=3-4 ) from beam

core.

– What makes this monitor so special ?– HDR can be extended easily

To demonstrate the operation of an optimized, programmable light filtering device and use it as a adaptable optical mask

that employs a digital micro-mirror array to produce an image of the halo of a beam with an enhanced dynamic range.



• Provide knowledge on beam losses which originate in the low-density halo that extend far from the beam core

Define Beam Halo: Beam Losses associated with a small fraction of particles surrounding a dense beam core. Difference between “tail” and “halo” (Beam losses?) Knowledge about the structure of halo

Depend: beam distribution or halo mechanism

Define it either by “geometrical characteristic” or “nature of mechanism”*

Formation due to specific halo Mechanisms** Provide understanding and possible

control/prevention20/04/23

B.Lomberg-3rd OPAC Topical Workshop on Beam Diagnostics: Beam Halo Talk- Vienna, Austria 4

*(ICFA Workshop “Halo 03”, Long Island 2003) **(ICFA Workshop “Halo and Scraping”,

Wisconsin 1999)

20/04/23B.Lomberg-3rd OPAC Topical Workshop on Beam

Diagnostics: Beam Halo Talk- Vienna, Austria 5

10.8 um

1920 x 1080 array Al micro-mirror [ Discovery 4100]

USB Interface (0.95” Chipset )

DMD dimensions 14.4x10.8 mm

high-speed port 64-bit @ 400 MHz for data transfer

up to 23.148 full array mirror patterns / sec (48 GBs)



48o

24o

4σ Beam Spot

Rough Halo

HALO MONITOR OPERATION

1. DMD micro-mirrors have three possible states: - all floating, flat state ( no power) - two independent assignable + or - 120

states (power on)

2. Rotation axis along diagonal of each micro-mirror, i.e. at 45 degrees wrt to HD-DMD row or column

- rays imaged onto the DMA plane are reflected with different optical path lengths at twice the angle of incidence

3. Diffraction effects: array acts like a 2D

grating producing diffraction patterns





Lens

Target

Incident Beam

24oReflected

Beam

Image on DMD

12o

DMD Mirror Pixel

The selection of 8 reference points





Adaptive light filtering for masking any beam shape

Information transfer from CCD to DMD

Define area

Set mask limit or size



DR with CCD only DR~>10-2 (i.e. ~400) Extended with HD-DMD





1mm

CCD exposure time=15ms, gain=1

Beam on, DMA all on Beam on, DMA all off



DMA all on (with Scheimplug compensation)

Texposure=15ms , Gain=1

I = 20k counts

DMA all floating(no compensation)

Texposure=15ms , Gain=1

INormal = 5k counts

Laser spot image (colour corresponds to pixel value) Masked(Smoothed image )

1D distributions; Fitted to the equation 2/exp cbxa

Vertical (95% confidence bounds) a = 1.045 (1.037, 1.052) b = 0.09277 (0.09229, 0.09326) c = 0.07938 (0.07869, 0.08007)

Full profile Masked profile

Horizontal (95% confidence bounds) a = 1.041 (1.035, 1..047)b = 0.1314 (0.1311, 0.1317) c = 0.05562 (0.05524, 0.056)

Vertical (95% confidence bounds) a = 0.2466 (0.239, 0.2542) b = 0.08099 (0.07766, 0.08432) c = 0.1315 (0.1268, 0.1362)

Horizontal (95% confidence bounds) a = 0.2366 (0.2298, 0.2433) b = 0.125 (0.1229, 0.1272) c = 0.09309 (0.09003, 0.09616)

Goodness of fit: SSE: 0.7308 R-square: 0.7682 Adjusted R-square: 0.7678 RMSE: 0.02436




1.5m

m

20/04/23 B.Lomberg-3rd OPAC Topical Workshop on Beam Diagnostics: Beam Halo Talk- Vienna, Austria

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From a geometrical perspective: Define the halo as the area that contains all particles

outside the Gaussian shaped beam core. To increase DR, mask needs to be larger and then exposure

time extended.20/04/23

B.Lomberg-3rd OPAC Topical Workshop on Beam Diagnostics: Beam Halo Talk- Vienna, Austria 17

Signs of charge overflow and could allow using the DMD to characterize a whole range of sensors.





The method is versatile and adaptable to any accelerator High or low intensity, energy or any particle species

Preliminary Results Adaptive mask method developed and use to measure halo of

laser High dynamic range measured (~ 10-4) Good filtering ~10-4

Limitations on dynamic range Beam intensity Screen property: efficiency, saturation, light filters Scattered light

Possible solution in accelerator higher intensity beam (LHC) More efficient screen, e.g. YAG, or use of OSR, OUR etc. improve optics (large aperture optics, Lyot stops)

Gas-jet monitor measurements at Cockcroft Institute

Other prospectsStudy halo propagation in the BSRT at

CERNExperiments at the ALICE facility at

Daresbury Labs





Acknowledgements: This work is supported by

the European Union under contract PITN-GA-2011-289485 and by STFC under the Cockcroft Institute Core Grant No. ST/G008248/1.

A Digital Micro-mirror array-based beam halo monitor

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