LEReC_RHIC_Retreat_2019

1. Low Energy RHIC electron
Cooling (LEReC):
Commissioning Summary and Plans
Alexei Fedotov
on behalf of the LEReC team
RHIC Retreat 2019
July 31, 2019

2. LEReC Project Overview
LEReC is world first electron cooler based on the RF acceleration of
bunched electron beam (all previous coolers used DC beams)
All key elements and experimental demonstrations required for this new
approach were successfully achieved:
 Building and commissioning of new state of the art electron
accelerator 
 Produce electron bunches with beam quality suitable for cooling
 RF acceleration and transport maintaining required beam quality
 Achieve required beam parameters in cooling sections
 Commissioning of bunched electron beam cooling
 Commissioning of electron cooling in a collider 
2

3. LEReC inside RHIC tunnel at
Interaction Region @ 2 o’clock (IR2)
Cooling sections
Injection Section
(DC photocathode Gun,
SRF Booster cavity)
Laser
Transport beamline
3

4. COOLING
in Blue RHIC ring
COOLING
in Yellow RHIC ring
High-Power
Beam
Dump
Extraction
beam line
DC
e- Gun
704 MHz
SRF
Booster
Cavity
2.1 GHz
Cu Cavity
9 MHz
Cu Cavity
704 MHz
Cu Cavity
Injection
beam dump
RF Diagnostic
Beamline
RHIC TRIPLET RHIC DX
180°
Bending
Magnet
e-
* NOT to scale
Cathode
loading
system
LF solenoid
HF solenoid
Transport solenoid
ERL solenoid
Quad corrector
Trim corrector
Corrector 3.8 ID
Corrector 6.0 ID
BPM 2.4 ID
BPM 4.8 ID
Bellows
Ion pump
Merger
Beamline
Transport
Beamline
704 MHz
Cu Deflector Cavity
LEReC electron accelerator
(100 meters of beamlines with the DC Gun, high-power fiber laser, 5 RF
systems, including one SRF, many magnets and instrumentation)
4

5. Alexei Fedotov, RHIC Retreat, July 31, 2019
LEReC cooling sections

6. Alexei Fedotov, RHIC Retreat, July 31, 2019
LEReC electron beam parameters
Electron beam requirement for cooling
Kinetic energy, MeV 1.6 2 2.6
Cooling section length, m 20 20 20
Electron bunch (704MHz) charge, pC 130 170 200
Effective charge used for cooling 100 130 150
Bunches per macrobunch (9 MHz) 30 30 24-30
Charge in macrobunch, nC 4 5 5-6
RMS normalized emittance, um < 2.5 < 2.5 < 2.5
Average current, mA 36 47 45-55
RMS energy spread < 5e-4 < 5e-4 < 5e-4
RMS angular spread <150 urad <150 urad <150 urad
6
Two energies commissioned

7. Alexei Fedotov, RHIC Retreat, July 31, 2019
LEReC bunched electron beam cooling
• In order to be accelerated to high energy by the RF cavities electron
beam has to be bunched (thus “bunched electron beam cooling”).
• Bunches are generated by illuminating a photocathode inside the high-
voltage Gun with green light laser (high-brightness in 3D: both
emittances and energy spread). Electron beam properties resulting
from acceleration of bunched beam are fundamentally different from
those obtained in standard DC beam coolers.
• The 704MHz high-power fiber laser produces required modulations to
overlap ion bunches at 9MHz frequency with laser pulse temporal
profile shaping using crystal stacking.
• RF gymnastics (several RF cavities) is employed to accelerate electron
beam and to achieve energy spread required for cooling. Electron
beams of required quality are delivered to cooling sections.
• Electron bunches overlap only small portion of ion bunch. All ion
amplitudes are cooled as a result of synchrotron oscillations of ions.
7

8. Alexei Fedotov, RHIC Retreat, July 31, 2019
LEReC beam structure in cooling section
LEReC Beam Structure
200 150 100 50 0 50 100 150 200
0
0.1
0.2
0.3
0.4
time, nsec
I,
A
LEReC Beam Structure
60 40 20 0 20 40 60
0
0.1
0.2
0.3
0.4
time, nsec
I,
A
Electron Beam profile
2 1 0 1 2
0
0.1
0.2
0.3
0.4
time, nsec
I,
A
110
nsec,
f=9
MHz
Ions structure:
120 bunches
f_rep=120x75.8347 kHz=9.1 MHz
N_ion=5e8, I_peak=0.24 A
Rms length=3 meters
1.42 nsec
30 electron
bunches per
ion bunch
Electrons:
f_SRF=704 MHz
Q_e=100 pC, I_peak=0.4 A
Rms length=3 cm
9 MHz bunch structure
Ion bunches
with new 9MHz RF
Electron Macro-bunch
8

9. Alexei Fedotov, RHIC Retreat, July 31, 2019
Attainment of “cold” electron beam suitable for
cooling
• LEReC is based on the state-of-the-art accelerator physics and
technology:
- Photocathodes: production and delivery system
- High power fiber laser and transport
- Laser beam shaping to produce electron bunches of required quality
- Operation of DC gun at high voltages (around 400kV) with high
charge and high average current
- RF gymnastics using several RF cavities and stability control
- Energy stability and control
- Instrumentation and controls
9

10. Transverse phase space measurements of electron beam
Movable slit and downstream
beam profile monitors are
installed at the beginning of
each cooling section.
Bunch charge 75 pC

11. Longitudinal phase space measurement of electron beam
1 macro-bunch of electrons (total charge 3nC)
6 macro-bunches, 3 nC each.
704MHz RF vertically
deflecting cavity
• First dogleg merger dipole is off
• Beam goes to RF diagnostic line
• 20 degree dipole produces
dispersion
• Deflecting cavity produces time
dependent vertical kick
In pulsed mode, subsequent electron macro-bunches
have lower energy due to beam loading in RF cavities. 11

12. LEReC: First observation of electron cooling using
bunched electron beam, April 5, 2019
Ion bunch #2 is being cooled
Bunch length time evolution
for two Au ion bunches in RHIC
Energy of electrons and ions matched
Ion bunch #4 which is not being cooled
Longitudinal profiles
of six ion bunches

13. First cooling observation: April 5, 2019
BNL, CAD Main Control Room

14. Simultaneous cooling in Yellow and Blue rings (76kHz mode, 6 ion
bunches: bunch #1 is being cooled; bunch #6 does not see electrons)
14
cooled and non-interacting bunches
Yellow
Blue
Blue
Yellow
Cooled, heated and non-interacting bunches
Blue
Yellow
bunch length
bunch intensity
loss rate
Longitudinal bunch profiles
cooled
cooled cooled

15. Cooling of 111x111 ion bunches at 3.85GeV (1.6MeV 9MHz CW electrons)
Cooled vs uncooled store (+/-0.7m trigger)
Electron cooling in a collider:
Cooling of hadron beams in collisions.

16. Cooling using 2MeV electron beam (ions at 4.6GeV)
16
Cooling on
OFF
No cooling
Cooling on
Longitudinal cooling: ion bunch length
Transverse cooling: vertical beam size

17. Cooling of 111x111 ion bunches at 4.6GeV (2MeV 9MHz CW electrons)
17
25 min into the store rate became
essentially constant, which should
allow for longer stores with cooling
and save time spent on refills.

18. Potential benefits from cooling
18
lifetime
bunch length
lifetime
bunch length
• Cooled bunch is kept shorter, more useful
events within trigger window
• Minimize ion beam de-bunching and
losses from the RF bucket
• Peak current significantly higher for
cooled bunch
• Transverse cooling can help with
reduction of beta*
Cooled
Non-interacting
Cooled
Non-interacting
Ion bunch profiles
Ion lifetime
Cooled bunch
Ion bunch profiles
Ion lifetime
Cooled bunch
BLUE
YELLOW

19. Alexei Fedotov, RHIC Retreat, July 31, 2019
Observed issues and limitations
1. Electron beam optics has to account for additional focusing from ions. Optics
matching between two rings needs to take this into account for both cooling
sections, and allow high-current CW running of e-beam without ions at the same
time.
2. Over focusing of electrons by ions. Different focusing on electrons distributed
at different longitudinal slices of ion bunch. The higher ion bunch intensity the
stronger are over focusing effects.
3. Observed strong “heating” effects of electrons on ions (which are coherent
space-charge beam-beam kicks from electron bunches on ions).
4. Ion lifetime limitations at low energy: physical aperture, dynamics aperture,
beam-beam, space charge and IBS.
19

20. Alexei Fedotov, RHIC Retreat, July 31, 2019
Blue cooling section optimization
• e-beam optics with negative alpha_x,y (diverging e-beam without ions) to
account for focusing from ions in both cooling sections.
20
Bunch length
Ions lifetime
cooled
uncooled

21. Alexei Fedotov, RHIC Retreat, July 31, 2019
Over focusing from ion beam
• To reduce observed strong “heating” effects of electrons on ions (which are
space-charge beam-beam kicks from e-beam on ions), we moved to smaller
ion beta-functions in cooling sections. This increased over focusing effects
from ions.
• To reduce observed “heating” effects, we have to operate with lower than
design electron bunch charge. This increased over focusing effects from ions.
• Ion bunch intensity is higher than expected, which increased over focusing
effects from ions.
21

22. Alexei Fedotov, RHIC Retreat, July 31, 2019
LEReC RHIC Beam Structure
200
- 100
- 0 100 200
0
0.05
0.1
0.15
0.2
time, nsec
I,
A
LEReC Beam Structure
60
- 40
- 20
- 0 20 40 60
0
0.05
0.1
0.15
0.2
time, nsec
I,
A
LEReC – electron and ion beam parameters (2019)
110
nsec,
f=9
MHz
Ions structure:
f_rep=9.1 MHz
N_ion=6e8, I_peak=0.3 A
Rms length=3.2 m
30 electron
bunches per
ion bunch
Electron bunches:
f_SRF=703.5 MHz
Q_e=50 pC, I_peak=0.13 A
FWHM length =400 psec
9 MHz bunch structure
Long ion bunches
with 9MHz RF
Electron Macro-bunch
22
With small ion beam size in
cooling sections and higher
ion peak current, stronger
over focusing of electron
beam by ions

23. Alexei Fedotov, RHIC Retreat, July 31, 2019
Effects of ions on e-beam (low intensity of ions, N_ions=3e8,
Q_electrons=55pC)
23
e-beam envelopes for groups
of electrons at ion bunch center,
1 and 2 longitudinal sigma
of ion bunch.
e-beam angles
with ions
Cooled ion bunch length
Peak current
Ions lifetime
cooled
non-interacting

24. Alexei Fedotov, RHIC Retreat, July 31, 2019
Effects of ions on e-beam (high intensity of ions, N_ions=6e8)
24
Without ions
e-beam envelope
e-beam angles
e-beam envelopes for groups
of electrons at ion bunch center,
1 and 2 sigma.
15 middle bunches within electron
Macro-Bunch (30 bunches) are over
focused and are not effective for cooling.

25. Alexei Fedotov, RHIC Retreat, July 31, 2019
Effects of electrons on ions: “heating” (March 2019)
• Even before cooling was established we observed strong “heating” effects of
electrons on ions (which are space-charge beam-beam kicks from e-beam on ions).
• These “heating” effects were reduced by going to smaller ion beta-function in
cooling section and by finding better working point in tune space.
25
Electrons and ions energies
are NOT matched

26. Alexei Fedotov, RHIC Retreat, July 31, 2019
Heating studies: loss rate of ions (April 2019)
beta@cooler=50m beta@cooler=30m
cooled bunch
Non-interacting
bunch
better working point
beta@cooler=30m
26
w.p. scan

27. Alexei Fedotov, RHIC Retreat, July 31, 2019
May 2015: LEReC project approved by DOE for construction
December 2016: DC gun successfully conditioned in RHIC IR2
February 2017: Gun test beamline installed in RHIC
April-Aug., 2017: Gun tests with beam
July-Dec., 2017: Installation of full LEReC accelerator
Jan.-Feb., 2018: Systems commissioning (RF, SRF, Cryogenics, Instrumentation,
Controls, etc.)
March-Sept. 2018: Commissioning of full LEReC accelerator with e-beam
Sept 2018 : All project Key Performance Parameters achieved
Oct.-Dec., 2018: Scheduled upgrades and modifications
December, 2018: Gun Conditioning and HVPS tests without e-beam
Jan.-Feb., 2019: Restart operation with electron beam. Achieved e-beam quality suitable
for cooling
March 2019: Start commissioning with Au ion beams
April 2019: First cooling demonstration. Cooling in both RHIC rings using e-bunches
at 76kHz frequency.
May 2019: Simultaneous cooling of many ion bunches using 9MHz CW e-beam
June 2019: Cooling optimization at 1.6MeV, cooling of beams in collisions (3.85GeV ions)
July 2019: Cooling commissioned at higher electron energy of 2MeV (4.6GeV ions)
27
LEReC project timeline

28. Alexei Fedotov, RHIC Retreat, July 31, 2019
• All LEReC cooling commissioning milestones were successfully achieved.
• First in the world electron cooling of hadron beams based on RF acceleration of
electron bunches was demonstrated. Such cooling approach is new (all previous
coolers used DC beam) and opens the possibility of using this technique to high
beam energies.
• First electron cooling using electron beam without any magnetization on the cathode
or cooling section, “non-magnetized” cooling, was demonstrated (all previous
coolers used magnetization on the cathode).
• First electron cooling of ion bunches in two different hadron rings simultaneously
using the same electron beam.
• First electron cooling in a collider (cooling of ion beams in collisions with various
effects impacting beam lifetime).
• Cooling was commissioned at electron energy of 1.6MeV (ion energy 3.85GeV)
• Cooling was commissioned at one more energy of 2MeV (ion energy of 4.6GeV)
• Cooling of full RHIC stores with 111 ion bunches in both RHIC rings was
demonstrated, both at 3.85 and 4.6 GeV ion beam energies.
• The next step will be to maximize collision rates with cooling in next year’s RHIC
low-energy collisions.
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Summary

29. Alexei Fedotov, RHIC Retreat, July 31, 2019
Acknowledgement
LEReC project greatly benefits from help and expertise of many
people from various groups of the Collider-Accelerator and other
Departments of the BNL.
As well as FNAL, ANL, JLAB and Cornell University.
29
Thank you!

30. More pictures and a story (June 5, 2019):
https://www.bnl.gov/newsroom/news.php?a=215585 30