SNS/ORNL Note-draft 07/22/2001

Note on Different Schemes of Laser Wire Technique Implementation for
SNS.

A.V.Aleksandrov.

1.  Introduction.

Laser wire technique for beam profile measurements was proposed for SNS
\[1\] and is being experimentally tested at BNL \[2\]. Current
implementation makes use of pulsed Q-switched Nd-YAG laser for
photo-detachment of electrons from H^-^ ions. Wide band measurements
downstream of the laser are used to detect reduction of the beam current
due to photo neutralization. While this scheme has certain advantage of
use of the cheapest way to obtain highest instant power thus providing
largest instant degree of photo-neutralization, there are some
disadvantages. First, efficiency of laser power use is quite low because
more than 99% of photons of 10ns laser pulse pass by H^-^ bunch of 20 ps
duration. Second, extremely high power laser beam requires more
complicated and expensive optics to handle it. Third, wide band
measurements are required to detect neutralization notch on the beam
current.

Note that choice of laser system is mainly dictated by signal detector
properties. Indeed, energy (i.e. number of photons) delivered for 1ms by
1mJ pulsed Nd-YAG, mode locked 1W Nd-YAG or 1W diode laser is the same
therefore total number of neutralized ions is the same. Beam current sag
produced by the pulsed laser has large amplitude over short time easily
observable by naked eye on an oscilloscope therefore this scheme is well
suited for initial "prove of principle" experiments. In contrast, CW
light sources produce photo neutralization of very small amplitude over
long time and require special tools for detection. Netherless CW mode
sources can provide significant advantages: possibility to measure
longitudinal profile in case of short pulse mode locked laser, low cost
and mechanical simplicity in case of laser diode. Photo-neutralization
yield for CW and mode locked laser wire is estimated and possible
detection schemes are discussed in this paper.

2.  Pulsed laser.

Short pulse (  10 ns) Nd-YAG laser ( = 1.064m) can deliver light
beam of very high instant power (250mJ energy at 10Hz repetition rate).
Such a beam when intercepting H- beam can neutralize substantial part of
ions. The result of neutralization can be observed on downstream current
monitor as sag in the current profile following shape of the laser beam.
BCM must have wide bandwidth (100 MHz) to measure narrow peak of
neutralization. Time windowing can help to reduce low frequency noise
but if BCM is located far downstream from laser wire, it can be
difficult to determine initial position of the window. Dependence of
estimated signal amplitude upon ion beam energy is shown in fig.1

![](media/image1.png){width="6.0in" height="4.5in"}

3\. Mode locked CW laser. Output of mode locked CW laser consists of
continues stream of short light pulses. Typical parameters of readily
available commercial laser used for further calculations are 2W of
average power, \<1ps pulse width, 50 MHz repetition rate at 1064 nm
wavelength. These parameters are flexible and its variation by order of
magnitude up or down doesn't affects price of the laser significantly
(for fixed wavelength, of course). The main advantage of this laser is
that light pulse can be shorter than ion pulse thus overlapping it
completely and providing 100% use of available laser power. Moreover by
scanning relative phase of light pulses relative to ion bunches one can
measure longitudinal distribution in the H^-^ micro bunch. Relative
instant photo-neutralization degree is shown in Table 1 for different
beam energies. It is rather small in comparison with possible 100% for
10ns pulsed laser but lasts for all beam pulse duration, therefore laser
output intensity can be modulated internally or by external optical
modulator and narrow band receiver used to detect modulation of the beam
current. Note that not only wide band current transformers can be used
in this case but also sum signal from BPM because spectral line of the
signal can be placed inside BPM receiver bandwidth around harmonic of
fundamental beam frequency by proper choice of laser repetition rate and
modulation frequency \[3\]. Electron detection as being discussed below
can make implementation of this diagnostics much simpler.

Table 1. Photo-neutralization output at different laser wire locations

(mode locked laser, τ\<1ps, P=2W, f=50MHz, λ=1064 nm, I~b~=38mA, )

  ----------------------------- ------------ ----------- ------------ ------------ ------------ ------------
  Beam energy, MeV              2.5          7.5         86           186          379          1000

  Instant neutralization degree 3.5⋅10^-5^   2⋅10^-5^    6.4⋅10^-6^   4.6⋅10^-6^   3.3⋅10^-6^   1.9⋅10^-6^

  Number of detached e^-^ for   2e4          1.2e4       3.8e3        2.7e3        2e3          1.1e3
  2.5 ns                                                                                        

  Number of detached e^-^ for   5.6⋅10^6^    3.3⋅10^6^   1.1⋅10^6^    7.5⋅10^5^    5.6⋅10^5^    3.1⋅10^5^
  945 ns                                                                                        

  Number of detached e^-^ for   8⋅10^9^      5⋅10^9^     2⋅10^9^      1⋅10^9^      8⋅10^8^      5⋅10^8^
  1.0 ms                                                                                        
  ----------------------------- ------------ ----------- ------------ ------------ ------------ ------------

3.  Diode Laser.

Output of diode laser is continues light flax, which can be easily
modulated by modulation of applied current. Typical DC output power of
several watts combined with large beam divergence results in much
smaller amplitude of instant photo-neutralization degree than for mode
locked laser as shown in Table 2. Problems associated with reliable
detection of so small signal can easily outweigh any other advantages of
this type of laser if modulation of beam current is used as observable.
But low cost and mechanical robustness of diode laser can justify use of
direct detection of photo-detached electrons as shown in fig.2.

Table 2. Photo-neutralization output at different laser wire locations

(diode laser, P=10W, λ=900 nm, I~b~=38mA, )

  ------------------------------ ---------- ---------- ---------- ---------- ---------- ----------
  Beam energy, MeV               2.5        7.5        86         186        379        1000

  Instant neutralization degree  2⋅10^-6^   1⋅10^-6^   3⋅10^-7^   2⋅10^-7^   1⋅10^-7^   8⋅10^-8^

  Number of detached e^-^ for    1⋅10^3^    7⋅10^2^    2⋅10^2^    166        120        68
  2.5 ns                                                                                

  Number of detached e^-^ for    3⋅10^5^    2⋅10^5^    6⋅10^4^    4⋅10^4^    3⋅10^4^    2⋅10^4^
  945 ns                                                                                

  Number of detached e^-^ for    4⋅10^8^    2⋅10^8^    6⋅10^7^    4⋅10^7^    3⋅10^7^    2⋅10^7^
  1.0 ms                                                                                
  ------------------------------ ---------- ---------- ---------- ---------- ---------- ----------

Electrons released in the process of photo-detachment have well defined
velocity along the beam. They can be easily deflected by uniform
magnetic field and collected. Bending radius for electrons in uniform
magnetic field is

$$R\lbrack\text{cm}\rbrack \simeq \frac{\text{1700}}{B\lbrack\text{Gs}\rbrack} \cdot \beta \cdot \gamma$$

For example in the MEBT $\beta \cdot \gamma \approx \text{.}\text{073}$
and magnetic field required to turn electrons with radius R is

$$B\lbrack\text{Gs}\rbrack \simeq \frac{\text{124}}{R\lbrack\text{cm}\rbrack}$$

It is useful to take $R = 2\text{cm}$ to guide stripped electrons just
outside the aperture of 1.5 cm. Then 62Gs magnetic field is appropriate
and total longitudinal size of the profilometer can be about 3cm, which
fits to the current MEBT wire scanner box. Note that quality of magnetic
field is not important and stray magnetic field of surrounding quads is
not an issue in contrast to beam imaging devices such as IPM. Single
detector can be used for horizontal, vertical etc. measurements.
Stripped electrons have energy of 1.25keV therefore no additional
accelerating voltage is needed inside the vacuum chamber. If phosphor
screen is placed at the exit of 90^0^ magnet then emitted light can be
collected by photomultiplier outside of the vacuum chamber.

References.

1.  B.Shafer, Laser Wire Profile Measurement Simulation.

2.  R. Connolly, et al., private communication.

3.  Proposed by T. Shea in private communication.