Scattered electrons as possible probes for beam halo diagnostics.
P. Thieberger, C. Chasman, W. Fischer, D. Gassner, X. Gu, M. Minty, A. Pikin
Speculations based on:
THE ELECTRON BACKSCATTERING DETECTOR (EBSD), A NEW TOOL FOR THE PRECISE
MUTUAL ALIGNMENT OF THE ELECTRON AND ION BEAMS IN ELECTRON LENSES*
P. Thieberger#, F. Z. Altinbas, C. Carlson, C. Chasman, M. Costanzo, C. Degen, A. Drees, W.
Fischer, D. Gassner, X. Gu, K. Hamdi, J. Hock, Y. Luo, A. Marusic, T. Miller, M. Minty, C.
Montag, A. Pikin and S. White
OUTLINE
 Planned use of backscattered electrons for aligning the
electron beams with the proton beams in the RHIC electron
lenses.
 The recent successful commissioning of the electron backscattering detectors (eBSDs) with gold and 3He beams in RHIC.
 Speculations about possible applications to halo monitoring
using hollow electron beams.
 Other possibilities.
 Conclusions.
RHIC e-lens showing the location of the electron
back-scattering detector (eBSD)
3
 The beam-beam effect limiting RHIC
luminosity will be mitigated with two
electron lenses
 The 2 m long electron and proton
beams propagating in opposite
direction in two ~6T solenoids have a
width of ~300 m rms.
 The misalignment must be < 30 m for
the lenses to work properly.
 Misaligned can do more harm than
good.
 BPMs are not quite good enough to
guarantee satisfactory alignment.
 We will use electrons backscattered by
the protons as the “luminosity signal to
achieve alignment
 The system was recently successfully
commissioned with 100 GeV/amu Au
and 3He beams
Schematic of the detector and of the electron trajectories
The electrons, scattered in small impact parameter collisions with
the ions, reach the detector located close to the gun after
traversing a thin vacuum window
~1 to ~20 MeV
The upward
trajectory drift
is due to the
horizontal
bend.
ION BEAM
(~5 keV)
End of ~2 m long,
6 T solenoid
4
5 keV electron
beam envelope
Schematic diagram of the installed hardware
5
Cutaway view of the eBSD insertable housing .
6
Coulomb scattering calculations
Small deflections in the ion frame leads to
large deflections in the lab.
7
The classical Rutherford scattering equation with quantum and recoil corrections is
used to calculate the cross sections s in the ion frame of reference. Transformation to
the lab. frame yields the results shown next. Radiative corrections have not been
included but may be small – to be verified.
5 keV electrons in axial fields,
back-scattered by high energy
protons. The curves shown
were calculated using 250 GeV
protons but the results are
almost independent of this
energy.
Radiative corrections are not
included but may perhaps be
small (see next slide)
Electron-gold eBSD “luminosity” scans obtained by steering the
electron beam
Date:
4/15/2014
Ion Beam:
Gold
Beam energy:
100 GeV/u
Bunch intensity: 7*108
# of bunches:
2
Solenoid Field:
2T
Electron energy: 6 keV
e-beam current: 0.565 A
The electron beam is steered while
the ion beam is stationary.
9
These widths reflect the
widths of the electron
and the ion beam added
in quadrature. The
electron beam width is
known so the ion beam
width can be measured.
Electron-gold eBSD horizontal and vertical “luminosity” scans obtained by
steering the 100 GeV/amu gold beam
These scans are done with
an automated optimization
program developed for the
RHIC experiments. This
system will be used here
too.
eBSD counting-rate as function of detector position
obtained with a 100 GeV/amu 3He beam
3He
energy:
100 GeV/u
# of 3He bunches : 93
Bunch intensity: 4.7E10
e- beam energy: 6 keV
e-beam current: 88 mA
The counting rates with
protons will be similar
Time-resolved eBSD counts
Ion Beam:
Gold
Beam energy:
GeV/u
100
Bunch intensity:
1.1*109
# of bunches:
2
Electron energy:
5 keV
e-beam current:
0.15 A
Electron
beam OFF
Electron
beam ON
The background is
due to electrons
from the residual gas
( ~~ 5E-10 Torr at
pump )
Here the signal to
background ratio was
~20 in spite of the
low e-beam current
and passible
misalignment. Several
orders of magnitude
can be gained as
discussed next.
Hollow electron beams as possible halo probes
A current density of 1 A/mm2 of 5 keV
electrons has an electron density equal to
the electron density in 2.15×10-6 Torr of H2 .
(useful for rough background estimates).
A hollow electron beam seems ideal as a
halo probe.
But: Some residual gas electrons
backscattered by the intense ion beam
core will be counted too.
Countermeasures:
 Improve the vacuum. The best way to
improve the vacuum would be to use a
cold-bore solenoid.
 Pulse the electron beam
Cartoons of possible hollow beam configurations
A coaxial design seems
attractive but the gun
may be difficult to
implement.
The more conventional
design would have a
significant background
contribution from the
beam crossing.
Halo detection efficiency
would be azimuthally
asymmetric.
Possible compromise solution
 An annular electron collector surrounding the ion
beam shouldn’t be difficult to design.
 There is no background-producing beam crossing.
 The halo detection efficiency can be made
azimuthally symmetric
Rough order-of-magnitude sensitivity limit
A current density of 1 A/mm2 of 5 keV electrons has an electron density equal to the
electron density in 2.15×10-6 Torr of H2 .
Assumption 1: The cross sectional areas of the ion beam and of the electron annular
beam are roughly equal.
Assumption 2: We can generate a 5 keV electron annular beam with a current
density of 2 A / mm2
Assumption 3: The scattering cross section for the gas electrons is roughly the same
as for the electron beam electrons.
Assumption 4: We can reach a cryogenic vacuum of 5×10-12 Torr
Then a signal-to-background ratio of 1 is reached when the halo is
½ × 5 × 10-12/2.15 × 10-6 = 1.2 × 10-6 of the total ion intensity.
Modulating the electron intensity can then be used to correct for the background.
Idea for a possible Coulomb Scattering Electron Wire (CSEW) beam profile monitor




A ribbon shaped magnetized electron beam intersects the ion beam.
Some of the scattered electrons trajectories are intercepted by the detector.
The ribbon is steered to map the ion to measure the profile of the ion beam, perhaps including the halo
The three sets of solenoids form a closed ion orbit bump.
SUMMARY
 Scattered electrons are good probes for electron lens alignment.
 Hollow electron beam “lenses” may benefit from the same
technique for alignment.
 Hollow electron beams equipped with eBSDs could become good
halo intensity monitors for relativistic ion beams.
 To obtain maximum sensitivity for very low intensity halos, the
vacuum needs to be excellent. Cold-bore solenoids should
probably be used. Electron intensity modulation can be used to
separate the signal from the background.
CORONAGRAPH FOR LHC? NO SHOWSTOPPERS (SO FAR)