Someone asked me about the nuclear accelerator near my home. Jefferson
Lab is a nuclear physics research facility operated for the Office of
Science of the US Department of Energy by the Southeastern
Universities Research Association (SURA).
By smashing electrons into atoms in JLab's Continuous Electron Beam
Accelerator Facility (CEBAF) the scientists have already
mapped the structure of the proton to an new level.
Superconducting electron accelerating technology makes Jefferson Lab
unique. The electron beam recirculates up to five times through two
linear accelerators to reach energies up to 6 billion electron volts
(GeV). The beam is then split for simultaneous use in three experiment
rooms. To realize new research opportunities that are beyond Jefferson
Lab's present capabilities, the accelerator facility and its
experimental equipment are now in the process of upgrading to new
components that will double the energy of the electrons to 12 GeV. The
upgrade includes adding accelerating modules,
constructing a new experimental room and upgrading
the present research equipment.
Accelerator Physics
Inside the accelerator, a stream of electrons races around the
racetrack. A billion times per second, magnets steer and focus the
electrons into a beam the width of a human hair. The accelerator is
controlled and monitored by more than one hundred computers. Together,
they track, manage
and respond to more than 240,000 simultaneous signals and 40,000
hardware control points. Eventually, the electron beam is funneled
into three experiment halls where the high-speed particles are slammed
into target materials. Scientists from around the world use the
electron beam to perform experiments which refine theories about
how "quarks", the small particles that combine to
form protons and neutrons, behave in the nucleus.
By measuring the properties of scattered particles
after the electron beam collides with a target nucleus,
scientists learn how quarks and the forces that hold them together
interact and form the ordinary matter in the universe.
Jefferson Lab Upgrade
Doubling the energy of Jefferson Lab's electron beam to 12 GeV will
enable scientists to search systematically for glue-rich particles. In
addition, the upgrade will allow in-depth study of the proton, the
balance of forces within the nucleus and quark matter in the first
second
of the universe.
The new experimental "hall" includes approximately 18,000 Flash* A/D
Converter channels (ADC) and 4,300 Time-to-Digital Converter channels
(TDC). These ADC and TDC channels are connected to the readout
electronic modules that reside
in a standard VME card enclosure. The card enclosures are known as
"crates" and the new detectors proposed for the experimental hall will
need approximately 70 to 80 crates. The crates provide power to the
individual modules and establish a high-speed data acquisition path
using defined standards for data bus protocol.
Data acquisition electronics for nuclear physics applications have
taken advantage of a number of standards that define the mechanical
and electrical specifications for instrumentation modules and powered
card enclosures. Some of these standards date back to the late 1970's
and are, for all practical purposes, obsolete. New instrumentation
modules have been designed by Jefferson Lab and the industry to take
advantage of higher performance standards such as VME, VME64x and VXS
which are all standards created and supported by VITA, the VME
International Trade Association. The new instrumentation modules will
replace the
aging and obsolete modules that use the older
data bus standards.
Pipelined Data Acquisition
The new experimental apparatus, constructed with
various particle detectors, requires that the front-end
electronic modules support a high trigger rate.
Digitization of the input signals for analog pulse
information and timing must be buffered for
several microseconds, while the overall trigger
is formed from all the detector sections.
Since multiple signals could not be digitized and
stored in the front end modules, the older instrumentation standards
did not allow for pipelined data acquisition techniques. The overall
data acquisition trigger rate was limited by these non-pipelined
modules and remained well below 3 kHz.
With pipeline design techniques and improvements in FPGAs, the
digitization process can be increased at the front end and the
information can be stored until the trigger processing is completed.
The design goals for the new experiment are a 200 kHz trigger rate
with approximately 5 kbytes of event data. This corresponds to an
overall data acquisition rate of 1 Gigabyte per second. Creating the
Trigger
The instrumentation modules connected to the detector sections that
form the trigger, must transfer information from each module in the
crate to a central collection module. Each module produces a digital
sum of the signals that are present on each input channel. The sum
signal from each
module is collected and added to the other modules
in the crate. The crate sum is the first level of
trigger processing for the overall experiment trigger.
All the crates that collect signals from detector sections responsible
for the trigger are ultimately summed again to create the "Physics
Event" trigger signal. This final trigger signal is distributed to
all
the front-end instrumentation modules and initiates
the readout of stored data in the modules.
Designing with VXS The VITA 41 VXS specification
provides the capability for connecting each module
in a data acquisition crate with a central collection
module. The VXS standard defines high speed,
multigigabit serial connections from each VME64x
module slots to a central collection module.
The Electronics and Data Acquisition groups at
Jefferson Lab have embarked on a design that takes
advantage of the high speed backplane designs that
the VXS standard has created. This standard allows
JLab to use legacy VME/VME64x modules in a VXS crate, to develop
modules that will be used in the final design of the data acquisition
system for the detector systems constructed for the 12 GeV upgrade.
Commercial crate and backplane designs to support the VXS
specification have been researched at JLab. To support the plans for
designing front end
electronic modules and testing vendor backplanes,
JLab purchased the Pentek Model 6822 Dual Channel 215 MHz, 12-bit ADC
VXS board.
Commercialization
Through its technology commercialization programs,
SURA is collaborating with other national labs,
universities and the private sector to find practical
applications for these technologies. Exciting examples include
medical imaging for early detection of cancer and security scanning
devices.
Through the active pursuit of applying research to meet the needs of
the commercial marketplace, SURA's technology transfer program seeks
to positively impact the economy and improve people's lives in ways
that have never been considered before.
Quarks and the Strong Force
Quarks and Gluons From the stars overhead to the
atoms in our own DNA, all matter is composed of
fundamental particles -- particles that cannot be
divided into smaller parts. Quarks are among the
few fundamental particles in the universe.
There are six types of quarks: up, down, charm,
strange, top, and bottom. The lightest quarks,
called up and down, are the most common.
Quarks are bound to each other by the strongest
force in the universe.
Called simply the "strong force", this enormous
force binds them so tightly that one quark
cannot exist by itself. Glued by the strong force,
quarks are the building blocks of matter as
we know it. Understanding quarks and the
strong force is fundamental to the universe,
our world, and us.
Quarks stick together and won't come apart.
When pried even one proton's width apart,
quarks experience ten tons of force pulling
them together. Quarks are so small that we
have not been able to measure their size;
they take up less than one billionth of the
space inside the proton, and make up only
a few percent of its mass.
So, what takes up the rest of the space and
gives protons the rest of their mass? The strong force itself, via
carrier particles called "gluons". This binding glue surrounds and
connects the quarks and generates 98% of the universe's visible mass.
It has been discovered that nature builds particles in hundreds of
ways from these tiny quarks and forceful gluons. Here is research
going on using high density electron streams to split otherwise stable
atoms.
Darrell Lakin
3174 South Shore Drive
Smithfield, VA 23430
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