2. Photon Beam
2.2 High Intensity LEP Beam (E<3 GeV)
The LEPS experiment at BL33LEP has produced two kinds of LEP beam using Ar laser (351 nm,
6.5 W, CW) and deep UV laser (257 nm, 1.2 W, CW), which result in the maximum LEP energies of
2.4 GeV and 3.0 GeV, respectively. Intensities of these LEP beam has reached 800 K/sec and 150
K/sec, respectively. In the year 2006, the Ar laser was replaced to two sets of solid state lasers (355
nm, 8 W) in order to achieve a higher intensity LEP beam with simultaneous injection of them into
the storage ring (Fig. 2.2.1). Because the new solid state laser is pulsed with 80 MHz frequency, no
interferences are expected at the same focus points even with the multi-laser injection. In addition, it
works as a quasi-CW laser to hit electron bunches efficiently. This scheme has succeeded resulting in
a LEP intensity of 2 M/sec. Since aperture of the BL33LEP is narrow due to structures of the
beamline, only 1.4 region of Gaussian beam cross section can be passed to the focus point. (See Fig.
2.2.1.) It was hard to increase number of lasers to be injected. A new beamline with large aperture is
therefore desired to increase LEP intensity.
aperture [mm]
BL33LEP
crotch absorber : 10.0□ @28.76m
pre-mask : 19.2□ @17.9m
x 28.5
 1 mm (1/e2)
BM chamber : ±10.0(y) @29.4~32.2m
additional prism for 4-laser injection
Distance from 1st mirror [m]
Figure 2.2.1 Two laser injection at BL33LEP. The right panel shows
the structure of the beamline.
Injecting 4 sets of solid state or deep UV lasers is planned at the new beamline (BL31ID). Two
knife-edge prisms, each of which merges two beam axes, will be placed in parallel just before the
entrance port into the storage ring. Expanders to focus the beams at the 40-70 m downstream straight
section will be set between the prisms and lasers. In order to control beam directions and positions,
at least two mirrors will be installed in the individual beam axis. In the construction of large aperture
beamline, there are geometrical limitations at quadru-pole magnet, crotch absorber, and bending
magnet as shown in Fig. 2.2.2. Beam pipes will be enlarged to maximumly allowed diameters at
these structures. If the lasers are focused at the nearer part of the straight section, peripheral region
of Gaussian beam will not be interfered by those structures even in case that a laser cross section is
expanded twice by beam shaping, explained below. The shorter focus has also an advantage in terms
of better resolutions of tagger energy measurement. (See Fig. 2.2.4.)
crotch
Q-magnet absorber
first mirror
0m
25 m
28.5 m
H 80 mm (for beam shaping)
H 11 mm
H ＋6 mm
At least 40 mm (beam core)
 30 mm
＋30 mm
Figure 2.2.2
bending
magnet
straight section
31 m
(40.25 – 70.25 m)
H 5 mm
 20 mm (bottle neck)
Aperture limitations of BL31ID and their possible modifications.
An electron beam in the storage ring has a horizontally wide shape (W:400 m x H:10 m), which
differs from a laser beam shape. It causes efficiency loss of backward Compton scattering. By
compressing the laser shape in the vertical direction, energy density of the beam becomes twice so
that the collision efficiency would be increased. Technically this is achievable by putting a beam
shape transformer to make an elliptical beam before an expander. Since a vertical size of the laser at
a focus point is compacted to be half, twice of the expander size is necessary in the vertical direction.
The laser beam shaping is rather easier than the electron beam shaping, which tends to make electron
beam divergence worse and make the length of straight section shorter by adding beamline optics.
Table 2.2.1 summarizes expected LEP intensities for the two kinds of lasers. In both cases, factors
of 4 and 2 will be gained by injecting 4 lasers and making energy density twice with laser shaping,
respectively. Another factor of 2 may be gained by twice power of the solid state laser, which is
under development at the producer company (Coherent Inc.) and may be available in a few years.
Additional factor may be obtained for deep UV laser by rotating the laser cross section with mirrors
because it has a vertically long shape due to second harmonic generation of the laser light through a
BBO crystal. The current estimation of increased LEP intensities are 8-16 M/sec and 1.2 M/sec for
solid state and deep UV lasers, respectively.

2.4-GeV LEP with Ar laser [351 nm, 6.5 W, CW] : ~800 Kcps
 Paladin (Solid state & 80 MHz pulsed laser) [355 nm, 8 W]
- 4-laser injection w/ large aperture beamline
x4
- Paladin 16 W model may be available in future.
(x2)
- Twice energy density by laser beam shaping
in vertical direction
x2

 In total 8-16 times more intensity relative to Ar laser
(Note: 2 Mcps has been achieved by 2-laser injection at BL33LEP.)
3.0-GeV LEP with Deep-UV laser [257 nm, 1-1.5 W, CW] : ~150 Kcps
- 4-laser injection (4-different focus points)
- laser beam shaping
x4
x2
- vertically long beam shape because of SHG  horizontally long
shape (like electron beam) by mirrors
[additional factor]
 In total 8+ times more intensity
Table 2.2.1 Expected LEP intensities for solid state and deep UV lasers at BL31ID.
Right panels of Figure 2.2.3 show horizontal divergence of an electron beam at BL33LEP and
BL31ID. The divergence at BL31ID is 14 rad, which is much better than BL33LEP, because there
are many optics in a long straight section. This results in smaller spread of the LEP beam at BL31ID,
so that a large volume detector system can be placed in the position far from the storage ring. Left
panels of Figure2.2.3 shows scattering angles and beam spread of LEP, which are kinematically
calculated depending on energy. The black lines indicate cone angles of backwardly scattered LEP’s.
Effects of electron beam divergences at BL31ID and BL33LEP are shown by green and yellow
hutches, respectively. RMS’s of the cone projection onto the horizontal plane are also shown by
colored lines. In case of BL31ID, the LEP beam spread is suppressed to be ~5 mm at the detector
position which is 150 m apart from the straight section. Since the electron beam divergence is small
at BL31ID, LEP energy can be identified by scattering angles, or vertex positions of photo-reactions,
in addition to the tagger measurement. In a similar way, recoil electron spread at the tagger can be
calculated. Since the bending magnet gives momentum difference of 30 MeV/c in each 1 mm at the
tagger, this spread can be translated to an achievable energy resolution. Figure 2.2.4 shows recoil
electron spread as a function of distance between the tagger and the scattered point. Thanks to good
divergence of the electron beam, a tagger energy resolution of BL31ID will be better than BL33LEP
in case that lasers are focused in the nearer part of the straight section.
8 GeV electron
scattering angle
<x’ >=14 rad
BL31ID
tagger
laser
LEP beam
30 m
~150 m
scattering angle
(3D cone)
BL33B2 (RMS)
BL33B2
<x’ >=58 rad
BL31ID (RMS)
8m
RMS of scattering
angle (x-projection)
tagged
BL33B2 (RMS)
BL31ID (RMS)
Angle measurement will also give photon energy.
Figure 2.2.3
Scattering angles of LEP beam and its spread for 150 m
distance depending on LEP energy (left panels). A comparison of electron
beam divergence at BL31ID and BL33B2 is also shown (right panels).
BL33LEP
20 MeV
~12 MeV
BL31ID
6 MeV
Figure 2.2.4 Recoil electron spread as a function of distance between
the tagger and the scattered point (E=2 GeV). Corresponding energy
resolutions are also indicated.
Another advantage of the long straight section is a capability to have several beam waists at
different focus points. In case of CW laser like the deep UV laser, interference of multi-injected
lasers is problematic. This can be avoided by shifting a focus relative to each other. Since the deep
UV laser has a ~4 m-long waist, the 4-laser injection needs 16 m or more length for the straight
section. BL31ID is suitable for this purpose.
3. Detector
3.2 Forward Spectrometer
The detector system based on E949 solenoid gives a worse momentum resolution when a charged
particle is directed to a small polar angle, which is roughly less than 10 degree. Although the solid
angle of this region is small, some part of events have a charged track in the extremely forward
acceptance resulting in worse resolutions of a missing mass and an invariant mass. Nevertheless,
most of photo-reactions can be well identified with techniques of a kinematical fit, a coplanarity
requirement, and so on thanks to particle detections in a large solid angle. We consider that only the
spectrometer inside the solenoid would be enough at the first stage of the experiment.
An example of experiments which need good momentum resolutions in the most forward region is
a search for mesic nuclei. Chiral symmetry broken in vacuum is theoretically expected to be restored
partially at nuclear densities. Medium modification of a meson potential results in mass reduction or
a nuclear bound state. In case of ω-mesic nuclei, a 2.75 GeV photon reacts with a nucleon producing
a ω meson at rest and emitting a high momentum proton to the extremely forward direction. (See
Fig.3.2.1.) A signal of the mesic nuclei can be examined in the lower tail of a missing mass
distribution calculated based on the forward proton or an invariant mass distribution of decay
products from a bound meson. In the former case, it is essential to measure proton momentum with
good accuracy.
Figure 3.2.1 Upper panels show momenta and kinetic energies of photoproduced
 mesons as a function of photon energy. Lower panels show them as a function
of proton polar angle.
There are several possibilities in order to achieve good momentum resolutions in the forward
acceptance. One way is to place a forward spectrometer with a dipole magnet just downstream of the
E949 solenoid. In case that the LEPS dipole magnet is used, forward acceptance up to ~6 degree can
be covered by the gap size of 1350 mm (W) x 550 mm (H). It may be also possible to construct a
new spectrometer with a large gap dipole magnet. In this case, the downstream end plate of the
solenoid must be modified to fit the new dipole magnet. It may be another option to do a separate
experiment at the same beamline with the current LEPS spectrometer setup using a target which is
additionally set downstream of the E949 solenoid in order to concentrate forward physics. Details
about possibilities of the forward spectrometer are under discussions.
6. Organization and Infrastructure
6.1 Laser Hutch
Laser hutch, which contains instruments related to LEP beam production inside radiation shields
of lead and steel walls, will be placed just outside the storage ring tunnel. The location is about 55 m
downstream from the center of straight section. Figure 6.1.1 shows a schematic layout of the laser
hutch. Four sets of solid state lasers (=355 nm) will be set on a two stage support, and their lights
will be simultaneously injected by being reflected at prisms. An expander to make a focus at the
straight section, a wave plate to control polarization, and mirrors to control a beam path will be
installed on the individual beam axis. Four sets of deep UV lasers (=257 nm) will be also set up
with a layout similar to the solid state lasers. The above lasers and optical elements will be placed on
a 5 m (L) x 2.6 m (W) surface plate.
storage ring
sweep magnet & pair spectrometer
solid-state laser
LEP beam
4m
beam dump
5.5 m
2m
2.6 m
2.5 m
power supplies and chillers
deep-UV laser
4m
space for x-ray
Hutch Height : 3.3 m
injection test
hutch door
5m
18 m
side view of solid state lasers
side view of deep-UV lasers
Figure 6.1.1 A schematic layout of laser hutch at the new beamline.
Electron and positrons are pair-created at thin lead plates, which must be set to decrease high
intensity x-rays from synchrotron radiation. Therefore, a sweep magnet is necessary to remove
contaminations of such charged particles in the LEP beam. At the BL33LEP, a 1 m-long and 6
cm-wide Nd-Fe-B magnet has been used inside the laser hutch giving a magnetic field of 0.5 Tesla.
A similar magnet will be installed on the LEP beam axis at the new beamline. In addition, a
spectrometer which utilizes the sweep magnet is under considerations in order to monitor LEP
energy resolution by measuring momenta of electron and positron pairs. Such a setup, called pair
spectrometer, has been operated at J-Lab. It is useful at the new beamline by the following reasons:
(1) Photon energy resolution can be independently measured with resolutions better than the tagger
or the detector system with the E949 solenoid. (2) Photon energy resolution can be monitored while
it changes in the range of 6-20 MeV as a function of distance between the tagger and the laser focus
point, which cannot be directly measured. The left panel of Figure 6.1.2 shows an example of the
spectrometer setup with a large sweep magnet. In the right panel, bending track positions at two drift
chambers and momentum resolutions expected with 150 m position resolution are shown
depending on momentum of electron or positron. Momentum resolution of ~10-3 is achievable so
that the pair spectrometer can measure energy resolution better than 6 MeV for 1.5-3.0 GeV LEP
beam. Angle measurement by two drift chambers is necessary because emission angle of electrons or
positrons are not negligible (<>~mec2/E~0.25 mrad). Effects by multiple scattering must be
minimized, for example, by using He gas for gap spaces. The large sweep magnet is also effective
for high energy photon beam by x-ray injection; 7 GeV/c electrons/positrons are sweeped at 4.3 cm
by 2 m-long magnet. A size of the pair spectrometer is estimated to be about 4 m (L) x 2 m (W)
including the sweep magnet.
In the most upstream part of the laser hutch, a space to test x-ray injection for high energy -ray
production will be prepared. It is considered to make a test chamber for inserting a x-ray mirror on
the vacuum beam pipe. Actual x-ray injection system may be constructed inside the storage ring
tunnel, but the laser injection system may be replaced in case that the x-ray injection system is
constructed inside the laser hutch. In summary, a size of the new laser hutch is expected to be about
18 m (L) x 4 m (W) while the laser hutch at BL33LEP is about 12 m (L) x 2 m (W).
Drift Chamber (Resolution~150 um)
Trigger Counter
DC2
2m
DC2
2m
He gas
DC1
DC1
Return Yoke
2m
0.5 Tesla
Nd-Fe-B magnet
He gas
DC1
30 cm
50 um W wire (Yield~10-2 x #photon)
Veto Counter
Figure 6.1.2 A setup of pair spectrometer. Bending track positions at drift
chambers and expected momentum resolutions are also shown
as a function of momentum.