Thomas Jefferson National Accelerator Facility

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Thomas Jefferson National Accelerator Facility
JLab logo white2.jpg
Motto “Exploring the nature of matter”
Established 1984
Research type Nuclear physics
Budget US$72 million (2004)
Director Hugh E. Montgomery
Staff 675
Location Newport News, Virginia, United States
Campus 214 acres (87 ha)
Operating agency
 Jefferson Science Associates, LLC
Website www.jlab.org

Thomas Jefferson National Accelerator Facility (TJNAF), commonly called Jefferson Lab or JLab, is a U.S. national laboratory located in Newport News, Virginia. Since June 1, 2006, it has been operated by Jefferson Science Associates, LLC, a limited liability company created by Southeastern Universities Research Association and PAE Applied Technologies. Until 1996 it was known as the Continuous Electron Beam Accelerator Facility (CEBAF); commonly, this name is still used for the main accelerator.

Aerial view of Jefferson Lab
12GeV upgrade, currently under construction

Founded in 1984, Jefferson Lab employs more than 675 people, and more than 2,000 scientists from around the world have conducted research using the facility. Its stated mission is "to provide forefront scientific facilities, opportunities and leadership essential for discovering the fundamental structure of nuclear matter; to partner in industry to apply its advanced technology; and to serve the nation and its communities through education and public outreach."[1]

Accelerator

The laboratory's main research facility is the CEBAF accelerator, which consists of a polarized electron source and injector and a pair of superconducting RF linear accelerators that are 7/8-mile (1400 m) in length and connected to each other by two arc sections that contain steering magnets. As the electron beam makes up to five successive orbits, its energy is increased up to a maximum of 6 GeV. This leads to a design that appears similar to a racetrack when compared to the classical ring-shaped accelerators found at sites such as CERN or Fermilab. Effectively, CEBAF is a linear accelerator, similar to SLAC at Stanford, that has been folded up to a tenth of its normal length.

The design of CEBAF allows the electron beam to be continuous rather than the pulsed beam typical of ring shaped accelerators. (There is some beam structure, but the pulses are very much shorter and closer together.) The electron beam is directed onto three potential targets (see below). One of the distinguishing features of Jefferson Lab is the continuous nature of the electron beam, with a bunch length of less than 1 picosecond. Another is Jefferson Lab's use of superconducting Radio Frequency (SRF) technology, which uses liquid helium to cool niobium to approximately 4 K (−452.5 °F), removing electrical resistance and allowing the most efficient transfer of energy to an electron. To achieve this, Jefferson Lab houses the world's largest liquid helium refrigerator, and it was one of the first large-scale implementations of SRF technology. The accelerator is built 8 meters below the Earth's surface, or approximately 25 feet, and the walls of the accelerator tunnels are 2 feet thick.

The beam ends in four experimental halls, labelled Hall A, Hall B, Hall C, and Hall D. Each hall contains specialized spectrometers to record the products of collisions between the electron beam or with real photons and a stationary target. This allows physicists to study the structure of the atomic nucleus, specifically the interaction of the quarks that make up protons and neutrons of the nucleus.

With each revolution around the accelerator, the beam passes through each of the two LINAC accelerators, but through a different set of bending magnets in semi-circular arcs at the ends of the linacs. The electrons make up to five passes through the linear accelerators.

When a nucleus in the target is hit by an electron from the beam, an "interaction", or "event", occurs, scattering particles into the hall. Each hall contains an array of particle detectors that track the physical properties of the particles produced by the event. The detectors generate electrical pulses that are converted into digital values by analog to digital converters (ADCs), time to digital converters (TDCs) and pulse counters (scalers).

This digital data is gathered and stored so that the physicist can later analyze the data and reconstruct the physics that occurred. The system of electronics and computers that perform this task is called a data acquisition system.

12 GeV upgrade

As of June 2010, construction had begun on a $338 million upgrade to add an end station, Hall D, on the opposite end of the accelerator from the other three halls, as well as to double beam energy to 12 GeV. Concurrently, an addition to the Test Lab, (where the SRF cavities used in CEBAF and other accelerators used worldwide are manufactured) is being constructed.

As of May 2014, the upgrade had reached the point where a new record for beam energy was achieved, at 10.5 GeV, delivering beam to Hall D.[2]

Physics program

Jefferson Lab conducts a broad program of research using the electromagnetic interaction to probe the structure of the nucleon (protons and neutrons), the production and decay of light mesons, and aspects of the interactions of nucleons in the atomic nucleus. The main tools are the scattering of electrons and the creation and utilization of high energy real photons. In addition, both electron and photon beams can be made highly polarized, allowing exploration of so-called spin degrees of freedom in investigations.

The four experimental halls have distinct but overlapping research goals, but with instrumentation unique to each.

Hall A

Matching high resolution spectrometers (HRS) have been used to study deep-inelastic electron scattering. Using very well controlled polarized electron beams, parity violation in electron scattering has been studied.

Hall B

The CLAS detector was the mainstay of the Hall B experimental program from 1998 to 2012. Physics Working Groups in the areas of Deep-Inelastic Interactions, Hadron Spectroscopy, and Nuclear Interactions exist. See the article related to the spectrometer itself and physics program at the link CLAS. Polarized real photons and electron beams were used. Physics targets included liquid hydrogen and deuterium, as well as massive nuclear materials.

In the era of 12 GeV beams at Jefferson Lab, the Hall B program has been restructured to include a new detector called CLAS12, as well as several other experiments using more specialized hardware.

Hall C

Multiple spectrometers and specialized equipment has been used to study, for example, parity-violating electron scattering to measure the weak charge of the proton and hypernuclear production with the electromagnetic interaction.

Hall D

This experimental hall was built for the beginning of the 12 GeV beam-energy program starting in 2014. This hall houses the GlueX experiment, which is designed to map out the light unflavored meson spectrum in detail in the search for explicit gluonic excitations in mesons.

Free electron laser

JLab houses the world's most powerful tunable free electron laser, with an output of over 14 kilowatts.

CODA

Since CEBAF has three complementary experiments running simultaneously, it was decided that the three data acquisition systems should be as similar as possible, so that physicists moving from one experiment to another would find a familiar environment. To that end, a group of specialist physicists was hired to form a data acquisition development group to develop a common system for all three halls. CODA, the CEBAF Online Data Acquisition system, was the result.[3]

CODA is a set of software tools and recommended hardware that facilitates a data acquisition system for nuclear physics experiments. In nuclear and particle physics experiments, the particle tracks are digitized by the data acquisition system, but the detectors are capable of generating a large number of possible measurements, or "data channels".

Typically, the ADC, TDC, and other digital electronics are large circuit boards with connectors at the front edge that provide input and output for digital signals, and a connector at the back that plugs into a backplane. A group of boards is plugged into a chassis, or "crate", that provides physical support, power, and cooling for the boards and backplane. This arrangement allows electronics capable of digitizing many hundreds of channels to be compressed into a single chassis.

In the CODA system, each chassis contains a board that is an intelligent controller for the rest of the chassis. This board, called a ReadOut Controller (ROC), configures each of the digitizing boards upon first receiving data, reads the data from the digitizers, and formats the data for later analysis.

References

  1. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ coda.jlab.org

External links

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