# Presentation: 140755

## Laser Beam Profile Development

**for SNS**

- Collaboration of ORNL, BNL, LANL,
- SLAC and FERMILAB

## December 2000, ORNL

- "We know it works we just don't know how to do it."

- March 2002

- "We know how to do it, but not very well."

- Then and now

## Laser Beam Profile Monitor

- First ionization potential for H- ions is 0.75eV.  Photons with<1500nm can remove an electron leaving neutral H plus electron.
- Laser can be used to mark of portion of beam by neutralization.  Detector is beam current transformer, BPM stripline or electron pickup.
- This method has been used at Los Alamos for transverse and longitudinal emittance measurements.
- System is completely non-invasive.  Beam can be monitored continuously with nothing in the beamline.
- Eliminates risk of damage to superconducting cavities.
- Two  experiments have been done on BNL linac.
- MEBT platform has been delivered to LBL.

## Laser neutralization cross section

- Calculated cross section for H-
- photoneutralization as a
- function of photon wavelength.*
- Nd:YAG laser has =1064 where
- the cross section is about 90% of
- the maximum.

- *J.T. Broad and W.P. Reinhardt,
-  Phys. Rev. A14 (6) (1976) 2159.

## Laser options

- We are exploring two laser options:
- 1.	Cheapest photon blaster is a flashlamp-pumped, Q-switched Nd:YAG.
- 	A.	10-20 Hz maximum flash rate.  One flash/machine cycle
- 	B.	50, 200, 400 mJ/pulse.  Can get >90% neutralization
- 	C.	Flash duration ~20 ns,  overlaps ~8 pulses on SNS
- 	D.	Good results with current transformer at 750 keV
- 	E.	Not-so-good results with striplines at 200 MeV
- 2.	Mode-locked laser
- 	A.	~10 ps pulses at subharmonic of beam (100MHz)
- 	B.	Maximum average power of ~10W gives 10-4 neutralization
- 	C.	Simulations show difficult signal processing from S/N

## 750 keV experiment, using BCT

- Diagram of experiment installed on BNL linear accelerator.  The laser is on the platform at the left.  The top-center mirror switches between vertical and horizontal scans.  Mirror at top-right scans horizontally and mirror at bottom left scans vertically.

## LPM on linac.  Laser is upper right

## Laser profile experiment on BNL linac

## Scope trace of current notch

- Ocilloscope trace of output of current
- transformer showing current notch
- created by laser.
- The signal is filtered with a 50 MHz low
- pass filter to remove the linac 200 MHz rf.
- Profile measurements were made by
- measuring the notch depth at each mirror
- position.
- S/N at beam center of 25dB.  Signal to
- ~2.5 .

**Scope trace of current notch**

## Beam Profiles Measured on BNL linac

- Horizontal (top) and vertical profiles of the BNL linac beam.  Measurements were made after the rfq with 750 keV beam.
- The markers are the data and the lines are gaussian fits.
- Widths of fits:
- 	x = 3.32 ± 0.05 mm
- 	y = 7.3 ± 0.6 mm
- Each data point is from averaging 15 pulses.
- Carbon wire was installed, but we were unable to get beam current turned down far enough to permit insertion.

##  MEBT LPM will use BCTs

## MEBT Laser Wire on  Beam Box

## Installation at 200 MeV

## Laser Wire Profile with 100uA 200MeV Polarized Beam

- =8.0±2. mm averaging 30 measurements
- Data are the notch depths on  the 400 MHz beam envelope.

## Beam Noise and Dynamic Range Problems

- For phase shifter plus  attenuator plus hybrid:
- Reasonable CMRR is 25 to 30dB
- Heroic CMRR is 40dB
- This looks like a BPM
- Reasonable CMRR is 40dB
- Heroic CMRR is 60 dB?

## Laser "notch" in stripline signal

- Scope was set on
- infinite persistance
- for several hundred
- beam pulses.  This
- is difference signal
- at 400 MHz from
- upstream and
- downstream BPMs.

## LPM laboratory

## Laser Wire Profile with 10mA 200MeV Beam

- Measurement conditions were much more difficult for the unpolarized beam. There were tens of MHz intensity modulations of ~25% at 200MHz, larger at 400MHz, due to characteristics of magnetron source. The modulations are not expected from the SNS source.
- Upper plot is carbon wire scanner and lower is LPM  measurement made one day later (Carbon wire opened).
- (wire)= 3.51 ± .34 mm
- (LPM)= 5.3 ± 1.3 mm
- Beam conditions varied significantly between these measurements.

## Signal/noise simulation, mode-locked laser

- 10 W mode-locked laser
- will neutralize ~10-4 of
- beam it hits.
- Pulse stream with 1%
- amplitude noise.  20dB
- CMR reduces this to 0.1%.
- This is result for laser overlapping
- half of beam and 100 k pulses.
- Neutralized beam strikes beam
- pipe downstream.  ~50W power in
- beam.

## Status

- 1.  Pulsed laser with BCT gave good signals.  S/N=25dB at beam center with
-      10-pulse averaging.
- 2.  Pulsed laser at 200 MeV with striplines very difficult to make measurement
- 	a.  Quiet 0.1 mA beam from polarized source "not too bad"
- 	b.  Noisy 10 mA beam from regular source very difficult.
- 3.  Mode-locked laser close but probably not enough punch.
- 4.  Tests planned on MEBT beam at LBL.
- 5.  We will try to get more 200 MeV measurements with LANL laser.
- 	a.  Better rf technique
- 	b.  Try again with current transformer
- 6.  System development is shifting to ORNL.
- 	a.  Saeed is coordinator
- 	b.  Warren will be doing laser specification and installation design.