| Discoveries
at the Relativistic Heavy Ion Collider (RHIC) have captured worldwide
attention. They’ve also raised compelling new questions about
the theory that describes the interactions of the smallest known
components of the atomic nucleus. To address these questions, we
need to study rare processes and thus to increase the collider’s
luminosity, or the rate at which ions collide inside the accelerator.
The BNL Collider-Accelerator Department is pursuing various upgrades,
including the investigation of a luminosity upgrade through electron
cooling of RHIC.
The electron cooled RHIC, known as RHIC-II, would use low emittance
(read cool), energetic and high charge bunches of electrons to cool
the ion bunches. This would increase the ion bunch density and lead
to a higher luminosity. Achieving the necessary electron bunch characteristics
will require advanced accelerator techniques, such as a high-brightness,
high-current energy recovery linac. Such a linac may have other
applications in eRHIC (energetic electron ion collider at RHIC)
and future light sources.
As RHIC operates, the luminosity goes down. This is due mostly to
Intra Beam Scattering (IBS) which causes the gold ion bunches to
increase their longitudinal emittance and, through dispersion, also
their transverse emittance, thus “heat” up and become
more diffuse. Emittance growth can be induced by a variety of mechanisms
besides IBS, including instabilities of the ions’ motion,
mechanical vibration of the magnets, and the collisions themselves.
More diffuse beams produce lower luminosity and fewer collisions.
To improve luminosity, RHIC accelerator physicists aim to eliminate
or reduce the buildup of heat within bunches through a process called
electron cooling.
Electron cooling was invented in Russia by Gersh Itzkovich Budker
of the Institute of Nuclear Physics in Novosibirsk in 1966 and since
has been applied at numerous storage rings around the world. The
idea behind electron cooling is very intuitive - bring cold electrons
into contact with the ions so that heat can flow from the warmer
ions to the colder electrons. The cold electrons are produced by
an electron source, then accelerated to match precisely the speed
of the ions in a straight section of the ring. There the two beams
would overlap and have a chance to exchange heat. The electrons
would be discarded after one pass and replaced by fresh electrons
to continue the cooling process. In RHIC, which has a circumference
of 3800 m per ring, this straight section will be over a 100 meters
long. There are other differences between RHIC and previous electron
cooled rings. RHIC will be the first collider to be cooled during
collisions, and will be the first cooler to use bunched electron
beams.
 |
| Fig. 1. A graphic showing a possible layout
of the electron cooler of RHIC at the 2 O’clock IP. The
cooling will take place in a 100 meter straight section located
in the RHIC tunnel between two superconducting RHIC quadrupoles.
The electron beam, generated by a 54 MeV superconducting RF
Energy Recovery Linac (shown below the center of the graphic)
will travel first with the Yellow (Counter-clockwise) Ring beam,
then loop back and travel with the Blue (clockwise) Ring beam,
to cool both rings. |
To gain confidence in the calculated performance of the RHIC electron
cooler, a large effort was made to develop dependable simulation
techniques and benchmark them in experiments. It is beyond the scope
of this article to cover this work even in minimal detail, but perhaps
this is a good opportunity to thank the many institutes that helped
us in this challenge: The Budker Institute at Novosibirsk, The Joint
Institute of Nuclear Research, Tech-X Corporation, Jefferson Laboratory,
Fermi National Accelerator Laboratory, and the Svedberg Laboratory.
The last two institutes also helped in benchmarking experiments
on their electron cooler.
One of the big challenges in cooling RHIC is its high energy —
about ten times higher than any previous electron cooler (54 MeV
electron energy for RHIC’s 100 GeV per nucleon gold ions).
This slows down the electron cooling, since the cooling time is
proportional approximately to the energy cubed, thus requiring an
electron beam that has a high energy, a high current and must cool
over a long straight section. So the conventional DC electron accelerator
cannot be used for cooling RHIC. Thus we adopted an Energy Recovery
Linac (ERL) electron accelerator to produce high-charge (about 5
nC) electron bunches with a low emittance, under 3 micrometer normalized
rms, and high energy of 54 MeV. Precisely matching the electrons
to the ions in position, speed and angular deviation is another
challenge. Figure 1 shows a possible layout of an electron cooler
at RHIC.
Even more difficult is the task of producing such low-emittance
and high charge bunches (or high-brightness) electrons. The Brookhaven
team is now working on a laser-photocathode superconducting radiofrequency
source to continuously produce a high-brightness electron beam,
capable of about 0.1 ampere (the design aims at 0.5 ampere continuous
average current). To make the ERL work without beam breakup, a superconducting
accelerator cavity was developed, capable of a very high current
ERL (over 3 amperes without beam-breakup) as well as other technologies
for accelerating a very high current very efficiently.
Following several years of intensive R&D, we are confident that
these techniques will increase the luminosity at RHIC according
to our calculations, allowing more sensitive, precision studies
of the substructure of matter. Figure 2 shows an ERL superconducting
cavity and the results of a cooling simulation.
The accelerator technologies that we are developing may also have
applications at Brookhaven beyond the RHIC-II upgrade, for example,
in the eRHIC upgrade, which would add electrons from an Energy Recovery
Linac to collide with the ion beams of RHIC, and possibly also at
future “light source” facilities using very high brightness
X-rays to study the properties of materials and biological samples.
More information about the Collider-Accelerator Department’s
electron cooling group can be found on the web at http://www.bnl.gov/cad/ecooling.
 |
| A photograph of the 703.75 MHz ampere-class
ERL superconducting cavity, and a plot of a simulation of the
luminosity of gold-gold collisions at 100 GeV/A per beam over
a 4 hours store, shown without and with electron cooling. The
5-cell cavity, developed by C-AD and built by local industry
(Advanced Energy Systems) is the first dedicated ERL cavity
to be developed. It will serve at the RHIC II electron cooler,
at eRHIC and various other applications. The cavity, which has
undergone chemistry and testing at Jefferson Laboratory, demonstrated
20 MeV acceleration at low power investment, and represents
one aspect of BNL’s entries into the area of Superconducting
RF particle accelerators. |
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