# Laser wire neutralization simulations at 1000 MeV

R. E. Shafer 8/4/01

For a given rms H^-^ beam width, the width of the laser wire can be set
to a range of values. For a 200-mJ Nd:YAG laser, if the laser beam is
too narrow, saturation (depletion of the H^-^ beam) limits the
percentage yield of neutralized beam. Because of the low signal to noise
ratio, maximizing the neutralization yield is important. However, if the
laser beam is too wide, the measured profile of the H^-^ beam is
significantly wider than the actual H^-^ profile.

Linac beam simulations show that the expected rms width of the H^-^ beam
is about 1.7-mm at 1 GeV, for both x and y, midway between the two
quadrupoles in the quad doublet. An H^-^ energy of 1 GeV was selected
for this analysis, because the photo-neutralization yield scales roughly
as 1/, and is lowest at 1GeV.

The following simulations are carried out with the simulation program
LASERY4A.exe, based on the laser wire paper in the 1998 Beam
Instrumentation Workshop (SLAC, 1998). The simulation includes
saturation effects, and assumes Gaussian shapes for both the laser and
beam pulse transverse profiles. A rectangular (not Gaussian)
longitudinal pulse shape is used for the laser pulse length. When
saturation is important, the number of neutralized H^-^ atoms will vary,
depending on the longitudinal pulse shape used. The photo-neutralization
cross section is probably accurate to about +/- 10%. In the simulations,
the H^-^ beam is assumed to be an unmodulated (i.e., dc), 38 mA beam.
All the simulations are "single-pass" of the laser beam through the H^-^
beam, at 90 to the beam in the lab reference frame.

The following table shows the percentage yield and measured width, for a
200-mJ Q-switched laser wire and a 2.0-mm-rms wide, 1 000-MeV H^-^ beam.

Laser wire percentage measured ratio to

#  rms width yield width actual width

0.1 mm 7.4% 2.003 mm 1.002

0.2 11.1 2.012 1.006

0.3 13.0 2.026 1.01

0.4 14.2 2.045 1.02

0.5 14.9 2.069 1.03

0.6 15.3 2.097 1.05

0.7 15.5 2.129 1.06

0.8 15.6 2.166 1.08

0.9 15.6 2.206 1.10

1.0 15.5 2.251 1.12

Figures 1, 2, and 3 show the profiles for the incident beam (blue), the
neutralized beam (red), the un-neutralized H- beam (green), and the
measured fit (magenta points), for 0.1 mm, 0.5 mm, and 1.0 mm laser
wires. Specific parameters for the simulation are shown in the figures.
Saturation is very apparent for the 0.1-mm laser wire. The measured
width using the 1.0-mm laser wire is clearly a poor fit to the actual
width. From the table above, the maximum yield occurs with a laser wire
width of about 0.8 mm. Based on these simulations, the optimum laser
wire width for a beam profile diagnostic is about 0.5 mm.

The following table shows the expected neutralization yield vs. beam
energy in the SRF linac, using a 200 mJ laser wire with a 0.5-mm rms
width on a 2-mm-wide H- beam..

[Beam energy Yield]{.underline}

200 MeV 24.1%

500 19.5

> 1000 14.9

The simulations were done at an H^-^ current of 38 mA, the expected peak
current for normal operation. For a 0.5-mm wide laser wire, the total
yield of neutral H^0^'s is about 4.25E8, corresponding to about 70 pC.
If a 1:50 beam current transformer (BCT) is used, the charge across the
secondary termination is about 1.5 pC.

Figures 4 and 5 show the simulation results for the laser wire displaced
by 2 mm and 4 mm. The yields are about 8.7% and 2.0% respectively.

The percentage yield is roughly inversely proportional to the laser
pulse length, as seen in the following table. However, the number of
neutralized atoms is nearly independent of laser pulse length, as
expected. The reduction in the total number of neutralized atoms for
shorter pulse lengths is due to saturation effects.

Pulse Number of Percentage

[length neutralized atoms yield]{.underline}

12 ns 4.25E8 14.9%

24 4.83E8 8.5

48 5.17E8 4.5

Although the total number of neutralized atoms is slightly larger with
the longer pulse lengths, the shorter pulses with the higher % yield may
be easier to discriminate from background. This depends on the detection
method, however, because higher required bandwidths are often
accompanied with higher noise.

![](media/image1.png){width="6.004166666666666in"
height="3.7381944444444444in"}

Fig. 1. 0.1-mm rms width, 12-ns long laser beam. Maximum %
neutralization yield is 7.4%. Peak saturation is 98.2%. The profile fit
is very good.

![](media/image2.png){width="6.004166666666666in"
height="3.7111111111111112in"}

Fig. 2. Same as above, except laser beam 0.5 mm wide. Maximum %
neutralization yield is 14.9%. Peak saturation is 55.2%. The profile fit
is acceptable.

![](media/image3.png){width="6.004166666666666in" height="3.73125in"}

Fig. 3. Same as Figures 1 and 2, except laser wire is 1.0-mm wide.
Maximum % neutralization yield is 15.5%. Note that the measured width is
13% wider than the actual width of the H- beam. The profile fit (magenta
points) is not very good.

![](media/image4.png){width="6.004166666666666in"
height="2.865972222222222in"}

Fig. 4. Simulation of the 0.5-mm laser wire offset by 2 mm (1 ). The
yield is about 8.7%.

![](media/image5.png){width="6.004166666666666in"
height="2.9069444444444446in"}

Fig. 5. Simulation of the 0.5-mm laser wire offset by 4 mm (2 ). The
yield is about 2%.