Comparison of Conventional and Laser Wire scanners in the SRF Linac

R. E. Shafer 8/4/01

Conventional wire scanners

Laser wire scanner

Physics

The physics of wire signal (secondary electron emission) generation is
not well understood, and the signal amplitude is not easily calculated.
It depends on the velocity of the incident particle, the angle of
incidence, the condition of the wire surface, and the wire bias voltage.
For H- beams below several 100 MeV, the H- atomic electrons stop in the
wire, causing the wire signal to change polarity. This threshold energy
is not easily calculated, in part because of the very large range
fluctuations of the electrons and because the wires are round, not flat.

The ionizing radiation caused by the beam hitting the wire creates a
large beam-loss signal, because *all* of the beam hitting the wire is
stripped and eventually lost. This beam loss signal is significant above
100 MeV. In this mode, overlapping (crossed) wires cannot be used.

Physics

The photo-neutralization cross section varies with photon energy, and is
known to about +/- 10%. The neutralization yield depends on the H- beam
energy in a well-known way, and varies approximately as 1/*β.*

The most easily-detected signal is to separate the H- beam from the
neutralized beam using a dipole magnet, and measure the high energy
neutrals. The only competing signal is the neutral particle flux
resulting from residual-gas stripping, which is negligible for a
Q-switched laser wire.

When a dipole magnet is not available (as in the SRF linac), other
detection methods must be used, including measuring 1) a beam current
difference (using two beam current monitors), 2) a notch in the H- beam
current (measured in a single beam current monitor), or 3) the yield of
detached atomic electrons (540 keV for a 1-GeV H- beam). Other signals
(H^0^ 2p-1s atomic transitions) are too weak for this application.

Impact on normal operation

Major: Requires off-operation with 100-microsecond macropulses at low
rep rate.Use of wire scanner during normal beam-on-target operation will
damage wire and possibly contaminate SRF cavity.

Impact on normal operation

None. Several laser wires can be used simultaneously (during the same
macropulse) without impacting beam-on-target operation.

Profile characteristics

The measured profile represents the average profile during part or all
of short (off-normal) macropulses, taken at 1 or 2 Hz maximum pulse
rate.

Profile characteristics

The measured profile represents an average of many 12- to 20-ns samples
of normal 60-Hz, 1-ms macropulses, taken at 20 Hz.

Impact on SRF cavities and vacuum

Ablation from the wire (e.g., carbon wire), caused by high beam
currents, may contaminate the SRF cavity. Use of refractory metals
(e.g., tungsten wires) seems to minimize this. Because the wire scanner
actuator is a mechanical system with moving parts inside the vacuum
system, dirt from moving parts (e.g., bellows) could migrate into the
SRF cavities.

The relatively high surface area of the beam box and actuator leads to a
higher outgassing rate.

Impact on SRF cavities and vacuum

There is virtually no impact on the SRF cavities or vacuum, unless
ablation or outgassing from an internal laser beam stop is significant.
No beam box is required, only optical ports for the laser beam.

Beam loss radiation

Roughly 2% of the beam will be stripped by the wire and hit the quad
doublet sections between the SRF cavities. This is roughly 70 nA average
current (\~ 70 watts) for 100-us pulses at 1 Hz.

Beam loss radiation

Roughly 1 nA of beam current (\~ 1 watt) is lost in the quad doublet
sections between the SRF cavities (at 12-ns, 20-Hz laser operation).

Signal to noise and background

Not a problem.

Signal to noise and background.

Significant problem. Laser wire probably cannot be used at low peak beam
currents. Backgrounds include both thermal noise (kTB) and beam-related
signals (e.g., plasma oscillations). Needs further evaluation and
testing at high H^-^ beam energy (e.g., 200 MeV).

Measurement of halo

Possible, especially with stripper-flag attachment.

Measurement of halo

Very difficult because of signal to noise limitations.

Activation

Some. Mostly around quadrupole doublets.

Activation

None.

Setting and verification of wire "size".

Use micrometer before installation.

Setting and verification of wire "size".

Difficult to set and verify rms laser beam width. Needs development.
Also need to monitor focal point location.

Pulse to pulse variation in wire "thickness"

Not applicable. Beam current must be measured on a pulse-by-pulse basis
to normalize profile data.

Pulse to pulse variation in wire "thickness"

Laser pulse amplitude, as well as beam current, must be measured on a
pulse by pulse basis to normalize profile data.

Radiation hardness

Very rad hard.

Radiation hardness

Laser head includes laser rod, optics, Pockel cell, etc. Radiation
hardness is not well documented. Vacuum ports (windows) also must be rad
hard. Needs further evaluation.

Costs

\~ \$7k per unit (guess)

Costs

\~ \$7k to \$10k higher per unit (guess)

Major risks

Damage to wire could harm SRF cavities. Requires MPS protection to limit
macropulse length and rep rate when wire is not at full-out position.

Wire replacement would require opening vacuum system.

Major risks

Inability to measure beam profiles during commissioning due to
signal-to.noise limitations on profile measurement.

Radiation damage may limit lifetime.