# ![](media/image1.wmf){width="8.625in" height="1.0027777777777778in"}Summary Report on Laser Wires for the SNS Linac

## Tom Shea, Alexander Aleksandrov, Saeed Assadi, Peter Cameron, Roger Connolly, Mike Plum, Robert Shafer, Jim Stovall

## August 8, 2001

## **Abstract**

## In the past 10 years, significant advances have been made in the performance, size, and cost of lasers. These developments have made it more practical to use Nd:YAG lasers in beam diagnostics for high current H^-^ beams. In the past, Los Alamos and Brookhaven National labs have successfully implemented lasers on their accelerators to measure transverse and longitudinal beam parameters. In this report, we discuss a laser-based transverse profile monitor (LPM) for the SNS linac. The following issues will be addressed: 

-   *What has to be developed in addition to what is available at
    present?*

-   *What is the cost to ASD for (a) R&D, (b) construction on large
    scale?*

-   *What is the risk to the project by choosing Laser wire instead of
    conventional wire for SCL?*

##### Current Status

Several paper studies have shown that a laser profile monitor should be
capable of measuring the SNS beam (ref. 1,2,3). To date, our only
experimental study was performed at the low energy (750 keV) end of the
BNL linac. With this test, the BNL team demonstrated profile measurement
out to 2.5 sigma (ref. 4).

Our baseline profile monitor for the SCL is a conventional wire scanner
that went through its conceptual design review on July 17, 2001. Bob
Shafer has prepared a comparison between this device and a laser device
(ref. 5). In this comparison, several open issues are identified. Others
have raised additional concerns about the viability of a laser profile
monitor for commissioning and machine studies (Stovall, Plum, private
communication). The following list summarizes the primary issues that
must be addressed during further development efforts:

-   Signal to noise: measurement of low current beams and beam halo

-   Lifetime/maintenance problems due to radiation damage

-   Accuracy of measurement, especially with small beams

-   Viability as a commissioning diagnostic

###### R&D plan

The proposed R&D program meets the following criteria:

-   addresses the primary issues listed above

-   is complete several months before the "drop dead date" of April 2002
    (last date to continue effort on the conventional wire scanner
    without schedule disruption)

-   results in a reviewed conceptual design

####### Description

Existing hardware from the 750keV test will be installed in the high
energy (HEBT, 200MeV) line of the linac during August 2001. The location
is in the old linac-AGS transfer line which is not used for beam
transport so we will have more control over the beam current and pulse
length than we had in the low-energy test.

In the low-energy test no lenses were used between the laser the ion
beam. For the HEBT test, the laser beam will be focused by two
cylindrical lenses. A 300mm focal length lens creates a very broad 1mm
waist transverse to the ion beam and a 50 mm lens spreads this line
focus along the ion beam direction to reduce power density on the
graphite beam stop. This combination of lenses has been tested and gives
a line focus 1mm wide and 25 mm long at the beam center and 3mm x 50 mm
at the beam stop.

For the low-energy test the signal detector was a Pearson current
transformer with a permanent dipole magnet to remove the stripped
electrons from the ion beam. In addition to the current transformer we
are installing two single-plane beam position monitors, one in front of
the laser and the other after the transformer. The signals from the two
striplines in each BPM will be combined, one combined signal will be
inverted and the two will be added together. This is to test the
feasibility of using the beam-current signal from BPM\'s as the LPM
detector.

We will investigate the possibility of using difference signals from the
BPMs for halo measurement. We will also consider the possibility of
adding an electron detector to the beambox at a later date.

The LPM location is 7m downstream from an X-Y wire harp profile monitor.
Between the wire monitor and LPM are two quadrupoles. We will not be
able to compare absolute profiles from these two instruments. However we
will be able to adjust the optics to cause changes in the beam widths.
We will check that relative changes track between the two instruments.
In addition, a carbon wire will be installed in the beambox, permitting
direct comparison of profiles.

This test should be a crude test for laser lifetime in an accelerator
radiation environment. The plan is to place an integrating dosimeter at
the laser to give a lifetime measurement.

*Open Issues that will be addressed by the proposed R&D*:

1.  Signal to Noise for several detection methods:

```{=html}
<!-- -->
```
a.  single BCM or sum signal from BPM

b.  differential BCM or BPM

c.  possibly electron detector

d.  possibly loss monitor

```{=html}
<!-- -->
```
2.  Radiation hardness

```{=html}
<!-- -->
```
a.  measured effect of accumulated dose

b.  paper study -- tech note will be written, in collaboration with
    partner labs

```{=html}
<!-- -->
```
3.  Quality of profile measurement - an evaluation will be accomplished,
    based on comparison with harp and wire scanner data. The laser beam
    will be characterized with a commercial imaging system.

4.  Halo measurement -- this will be a part of the S/N program mentioned
    above

5.  Use during commissioning will be addressed as part of the conceptual
    design

*Personnel*

Peter Cameron -- one man month

Roger Connolly -- two man months

Marty Kesselman -- one man month

Bob Sikora -- one man month

John Cupolo -- one man month

###  ORNL-ASD staff (Tom, Saeed, Sasha, Bob Shafer) - three man months total

AGS will provide motion control and required quad supplies at no charge
to SNS.

####### Equipment

All hardware to accomplish the R&D program is in house.

#### Schedule

1 Sep -- Hardware installation complete, begin profile measurement

15 Sep -- preliminary measurement results available (in time for ASAC)

1 Dec - laser wire Conceptual Design Review

1 Feb -- measurement program complete

**Cost for production units**

Direct cost should be between \$7K and \$10K more per unit than Carbon
wire. Some significant amount of EDIA has been completed for MEBT laser
wire, and the process of mounting the experiment at 200MeV will
significantly reduce the amount of EDIA required. An estimate of an
additional \$100K to \$200K for EDIA is probably reasonable. Since we
are still in the conceptual design phase, at least 30% contingency
should be allocated.

###### Risk Assessment

We can mange the risk of deploying laser wires in SCL by pursuing the
proposed R&D program and then making a decision (laser vs. conventional)
after the December CDR. The cost of this approach is outlined above. In
any case, we plan to deploy a few laser wires in MEBT and HEBT so that
profiles may be measured during full power operation.

If we stay with conventional wires in SCL , we live with the risk of
cavity damage. The detailed design effort will address this issue via
qualification testing and MPS integration. There is a potential for cost
overrun if higher QA level is required. When running full beam power,
the operational limitations are significant.

If lasers are selected, the risk to commissioning and machine studies is
currently significant, but may be reduced after the R&D is complete. The
risk to operations is primarily driven by unknown sensitivity to
radiation, which could result in high maintenance costs. Again, the R&D
program should address this operational issue. The production cost is
estimated above and risk in this area is managed by the 30% contingency.

######## References and supporting documents

Please note that these documents were prepared concurrently and some
details may be inconsistent.

1)  Laser wire neutralization simulation at 1000 Mev, R.E. Shafer,
    August 2001.

2)  Preliminary Signal to noise estimations, M. Kesselman, August 2001.

3)  Note on different Schemes of Laser Wire Technique, Implementation
    for SNS. A.V. Aleksandrov, July 2001.

4)  Laser Profile Measurement of an H^-^ Beam, R. Connolly, P.
    Cameron, J. Cupolo, M. Grau, M. Kesselman, C-J. Liaw and R. Sikora,
    PAC 2001.

5)  Comparison of conventional and Laser Wire scanner in the SRF LINAC,
    R.E. Shafer (prepared for this report), August 2001.