CEBAF as a High Energy Fixed Target Machine:
An Alternate Possibility for Nuclear Physics
Beyond the 12 GeV Upgrade
Thomas Jefferson National Accelerator Facility
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Strategic Planning Exercise
Page 1
Talks so far have discussed:
• 12 GeV
(extremely well understood, exciting, and happily well
underway)
• A possible EIC as a follow-on
(a growing science program and developing machine
and experimental equipment designs; preliminary
case made to the larger community)
This talk will raise the possibility of an alternate
future based on a higher energy, fixed target
accelerator
Thomas Jefferson National Accelerator Facility
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Strategic Planning Exercise
Page 2
CEBAF as a High Energy Fixed Target
Machine: An Alternate Possibility for Nuclear
Physics Beyond the 12 GeV Upgrade
Bottom Line Question: Will a higher energy
fixed target machine deliver better physics for
comparable (or lower) cost?
With thanks to: G. A. Krafft, Y. Roblin, and Y. Zhang, who have
done much of the speculating and all of the performance
estimates
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Possible Future Cases Explored
• 24 GeV Recirculated Linac (push present tunnel to limit)
— Average current de-scoped by a factor of 2 (to 40 µA/hall max) due
to 1 MW dump limit (get to 100 µA with new, higher power dump?)
— Magnet layout and types roughly the same as 12 GeV, but 24 GeV
requires stronger focusing and a completely new set of magnets!
— Need 20 "C150s" and 10 C100s, and 20 C50s, yielding just over
2.5 GeV per linac pass (better than 50 total C100s)
• 50 GeV “Blue Sky Site Filler”
— Same dumps w/ 1 MW limit imply 20 µA/hall maximum;
for 100 µA either develop 5 MW dump or full (or partial) energy
recovery (interesting to explore possibility)
— Arcs are Theoretical Minimum Emittance (TME), Normal
Conducting
— Optimize shape, pass #, and cryomodule energy gain (C200s?) for
lowest cost, as done for CEBAF
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A Possible “Phase II”: 12  24 GeV
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CEBAF @ 50 GeV:
Five Passes through
Three 3.33 GeV Linacs
22 x C150
or 17 x C200
400 m long
400 m
radius
Thomas Jefferson National Accelerator Facility
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Strategic Planning Exercise
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What Beam Performance Appears to be Feasible?
The CASA folk have looked broadly at the question of
technical feasibility and provided rough estimates of
achievable beam properties
If we are seriously interested in either of these
possibilities, further work is highly desirable
Thomas Jefferson National Accelerator Facility
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Strategic Planning Exercise
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Expected Beam Parameters
Units
6 GeV
12 GeV
24 GeV
50 GeV @
24 GeV
50 GeV
Energy @ A, B, C
GeV
6
11
24
24
50
Energy @ D
GeV
n/a
12
27
n/a
n/a
CW
CW
CW
CW
CW
Mode
Total Current
µA
200
85
40
(80 mA with
2 MW
Dump)
Beam Power/Dump
MW
1
1
1
1
1
Emittance
(unnormalized,rms)
nmrad
<1
2.7
86
1
40
Relative Energy
Spread (rms)
10-3
0.025
0.2
1.1
0.4
2.2
%
0.5
2.4
26
10
110
Dilution Parameter
(𝛿𝜑/𝜋)
20
20
100 mA with
100 mA
5 MW
with 5 MW
Dump
Dump
(& ERL?)
(& ERL?)
Thomas Jefferson National Accelerator Facility
Spot Size (rms)
mm
0.2
0.2
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Strategic0.7
Planning Exercise
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0.2
0.6
Spin
Rotator
Fix Spin Dilution Problem by injecting w/
orientation vertical (along arc magnetic
fields) and rotating to longitudinal on the
way to the halls
CEBAF @ 50 GeV:
Five Passes through
Three 3.33 GeV Linacs
22 x C150
or 17 x C200
400 m long
400 m
radius
Thomas Jefferson National Accelerator Facility
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Strategic Planning Exercise
Page 9
Physics Example – Pion Form Factor
12 GeV Upgrade
EIC
24 GeV fixed target
50 GeV fixed target
Garth Huber
(private
Page 10 communication)
Thomas Jefferson National Accelerator
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Physics Example: DIS/DES Reach
12 GeV Upgrade
CEBAF @ 50 GeV
Harut Avakian
(private
Page 11
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communication)
ep→e’p+X Kinematic coverage
24 GeV
50 GeV
GeV
50
For a given luminosity (30min of runtime with L=1035cm-2s-1 ) and given bin in
hadron z and PT, higher energy provides higher counting rates and wider
coverage in x and Q2
Harut Avakian (private communication)
10
4
Counts
Counts
ep→e’p+X Kinematic coverage
0.9<PT<1.1 GeV
0.5<z<0.6
50 GeV
10
3
5
10
4
10
3
10
2
2
0
10
0.09<x<0.11 GeV
0.5<z<0.6
24 GeV
11 GeV
10
10
-2
10
1
1.2 1.4 1.6 1.8
2
PT2(GeV )
-1
Wider x range allow studies of transverse
distributions of sea quarks and gluons
0.2 0.4 0.6 0.8
x
Wider PT range will be important in
extraction of kT-dependences of PDFs
For a given luminosity (30 min of runtime with L=1035cm-2s-1) and given bin in hadron z
and PT, higher energy provides higher counting rates and wider coverage in x and PT to
allow studies of correlations between longitudinal and transverse degrees of freedom
Harut Avakian (private communication)
Counts
ep→e’p+X Kinematic coverage
10
4
10
3
10
2
0.29<x<0.31
0.4<PT<0.5
0.5<z<0.6
5
10
15
20
25
2
30
2
Q (GeV )
For a given luminosity (30min of runtime with L=1035cm-2s-1) ) and given bin in
hadron z and PT, higher energy provides higher counting rates and wider coverage in
Q2, allowing studies of Q2 evolution of 3D partonic distributions in a wide Q2 range.
Harut Avakian (private communication)
Physics Issues
• Is it worth investigating possible external
beam upgrades more seriously?
• Much has been done to develop the
physics that can be done with an Electron
Ion Collider, but what can be done with a
fixed target machine, and which machine
can do the physics we consider essential
better???
• The issue should be discussed, and
pursued if we consider the effort waranted
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Page 15
Another Topic for Consideration:
Positrons in CEBAF @ 12 GeV and Beyond
•
The PEPPo experiment (below), which will use polarization transfer to convert
an intense e- beam into a usable (nA level) e+ beam, is in preparation for
commissioning during the last 6 month run and then running at the start of
the 12 month shutdown
It aims to demonstrate that state-of-the-art mA e- beams could drive mA level
e+ sources, but will need continued encouragement to succeed
Such a source could support research ranging from GPD and TPE physics
(after acceleration to 12 GeV in CEBAF) to condensed matter studies (at 10100 keV energies)
•
•
Intensity controls
Energy measurement
(10-2)
Polarization measurement
(1.5%)
Laser
PEPPo
Variable energy
2-7.5 MeV
Polarization controls
Parallel Positron Source Concept
Parallel to Injector:
 Duplicate Gun  100MeV
portion of injector available
for low energy positron
operations (materials and/or
NP research) in parallel with
CEBAF in electron mode
 First CW polarized positron
source:

Beam currents up to
~microAmpere (1013
e+/s)

Polarization TBD,
calculations suggest it
will be as high as 70%

Injection/Extraction
energy tunable via RF.

Should it be included as part of our long range planning,
Thomas Jefferson National Accelerator Facility
implying resource allocations
over the next few years?
Page 17
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Strategic Planning Exercise
Conclusions
Efforts on MEIC collider continue and progress is being
made, but consideration should be given to alternatives:
• A 24 GeV CEBAF-like machine in the present tunnel
It would require further upgrades to the beam acceleration system and
a completely new (normal conducting) complement of magnets
• A 50 GeV fixed target machine
The CEBAF site could support such a machine, but it would be a
major construction effort (of the scale of MEIC). Polarization and
emittance dilution imply a completely new layout is required, and even
so, special optics (or transverse injection w/ rotation at 50 GeV) may
be needed to retain the polarization
• For both of these machines there are tradeoffs between beam power,
dump development, cost, etc.
In addition, should we consider developing a positron source
to add new physics capabilities at a relatively modest cost?
Thomas Jefferson National Accelerator Facility
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Strategic Planning Exercise
Page 18
Thomas Jefferson National Accelerator Facility
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Page 19
HALLD
ARCA
ARC9
ARC8
ARC7
ARC6
ARC5
ARC4
ARC3
ARC2
ARC1
From 6 GeV to 12 GeV has been studied in detail
Thomas Jefferson National Accelerator Facility
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Page 20
Transverse Emittance* and Energy Spread†
p/p
x
y
[x10-3]
[nm]
[nm]
Chicane
0.5
4.00
4.00
Arc 1
0.05
0.41
0.41
Arc 2
0.03
0.26
0.23
Arc 3
0.035
0.22
0.21
Arc 4
0.044
0.21
0.24
Arc 5
0.060
0.33
0.25
Arc 6
0.090
0.58
0.31
Arc 7
0.104
0.79
0.44
Arc 8
0.133
1.21
0.57
Arc 9
0.167
2.09
0.64
Arc 10
0.194
2.97
0.95
Hall D
0.18
2.70
1.03
Area
12 GeV Beam Properties
Double Bend Achromat
Design
Damping
Sync. Rad.
* Emittances are geometric
† Quantities are rms
Thomas Jefferson National Accelerator Facility
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Emittance Growth By Scaling
• Beam emittance growth over a section of beam line due to synchrotron
radiation (ODU Lectures)
2
55C  mc 2  5
H
 u 
  ds 3

64p 3
• For a FODO lattice
 ds
H
3


 3
lb   H  
lb , 0   3 
where α is the bending angle of the beam line, equals to π for a half
circle, ϑ= lb/ρ is bending angle of a dipole, lb/lb0 is the packing factor of
the FODO cell, [<H>/ρϑ3] only depends on phase advance of the FODO
cell
• In this case
 5 3  5 E 5
 u ~
~ 4 ~ 4



• Comparing the 12 GeV case and a potential 50 GeV accelerator: E
increased by a factor of 45/11=4.09 and ρ increased by a factor of 3,
then emittance growth scales by a factor of 4.095/34 ~ 14
Thomas Jefferson National Accelerator Facility
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Energy Spread by Scaling
• Energy spread after passing a section of circular beam line with
uniform 180 degree bend
E 2
E2
55re c 5 ds
55pre c  5

  3
2
2
2

48 3p mc
24 3p mc 
• Thus,
E
E
~
E5/ 2

• Comparing 12 GeV and 50 GeV CEBAF, after last arc
E increased by a factor of 45/11=4.09,
ρ increased by a factor of 3,
then
δE/E increased by a factor of 4.095/2/3=11.3
Thomas Jefferson National Accelerator Facility
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Strategic Planning Exercise
Page 23
Polarization Loss
• Spin precession per 180 degree bend
g  2 Ebend
 
p
2
2 mc
• For 9 arcs
g  2 Elinac
  45
p
2
2 mc
• Polarization angle spread yields dilution
 

   o  E 
E 

E 
E 

2 
P  P0 cos 
 % correction to P o 

E 
E 


• Fixed By Clever Longitudinal Optics?
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Strategic Planning Exercise
Page 24
2
Thomas Jefferson National Accelerator Facility
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Page 25
The 12 GeV Upgrade is Well Underway
Thomas Jefferson National Accelerator Facility
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It’s Science Case is Well Developed and Growing
• The Hadron spectra as probes of QCD
(GluEx and heavy baryon and meson spectroscopy)
• The transverse structure of the hadrons
(Elastic and transition Form Factors)
• The longitudinal structure of the hadrons
(Unpolarized and polarized parton distribution
functions)
• The 3D structure of the hadrons
(Generalized Parton Distributions and Transverse
Momentum Distributions)
• Hadrons and cold nuclear matter
(Medium modification of the nucleons, quark
hadronization, N-N correlations, hypernuclear
spectroscopy, few-body experiments)
• Low-energy tests of the Standard Model
and Fundamental Symmetries
(Møller, PVDIS, PRIMEX, …..)
And other science
we can’t foresee
The EIC is an Evolving Candidate to Follow 12 GeV
ELIC @ JLab: add figure-8 hadron & electron rings to CEBAF
medium energy IP
Three compact rings:
• 3 to 11 GeV electron
• Up to 20 GeV/c proton (warm)
• Up to 100 GeV/c proton (cold)
low energy IP
With an evolving science case
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Page 28
There is Still Work to be Done on the Science Case
Bob McKeown at the JLab
S&T “Visit” this year
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Page 29