DELPHI was a general purpose detector for physics at the Large Electron-Positron Collider (LEP) on and above the Z0, offering three-dimensional information on curvature and energy deposition with fine spatial granularity as well as identification of leptons and hadrons over most of the solid angle. A superconducting coil provided a 1.2 T solenoidal field of high uniformity. Tracking relied on the silicon vertex detector, the inner detector, the Time Projection Chamber (TPC), the outer detector and forward drift chambers. Electromagnetic showers were measured in the barrel with high granularity by the High Density Projection Chamber (HPC) and in the endcaps by projective towers composed of lead glass as active material and phototriode read-out. Hadron identification was provided mainly by liquid and gas Ring Imaging Counters (RICH). The instrumented magnet yoke served as hadron calorimetry and as filter for muons, which were identified in two drift chamber layers. In addition, scintillator systems were implemented in the barrel and forward regions, as well as a Scintillation TIle Calorimeter (STIC) and a Very Small Angle Tagger (VSAT) for luminosity determination, a 3-layer micro vertex silicon detector for high precision vertex and lifetime measurements and a very Forward Silicon Tracker (VFT) for improved tracking and hermeticity at small polar angles.
The Large Electron-Positron Collider (LEP) is the largest particle accelerator
ever built. LEP had a circumference of 26.7 kilometers and was
built in a tunnel at a depth ranging from 50 m to 175 m
under the border of Switzerland and France. It was used from 1989 until year 2000.
When the LEP collider started operation in 1989 it accelerated the electrons and positrons
to an energy of 45-46 GeV to enable the production of Z bosons, which has a
mass of approximately 91 GeV. The accelerator was upgraded, starting in 1995, to
increase the energy and make it possible to produce pairs of W bosons,
each with a weight of approximately 80 GeV. The LEP collision energy continued to increase year to year
and eventually finished at 209 GeV at the end of 2000. This was done in order to search for
new physics processes. LEP was then shut down and
dismantled in order to make room in the tunnel for the construction of the
Large Hadron Collider (LHC).
The accelerator had eight 2.9 km long arcs and eight straight sections that
extended 210 meters on each side of the eight potential collision points.
Some 3400 dipole magnets were used to bend the particle beam trajectory
and 800 quadrupole, as well as 500 sextupole magnets, were used for focussing.
600 additional smaller magnets were used for orbit corrections.
The bending magnets had a field of only 0.135 Tesla and were conventional
warm magnets.
The electron and positron beams travelled in a pipe with a vacuum that was lower
than 10-11
torr. The vacuum was needed to reduce the probability of the beams
colliding with gas molecules in the beam pipe.
Each beam contained 4 bunches of particles at the start of the Z period
and they measured 0.15 x 0.01 x 10 mm along the bending radius,
perpendicular to the bending plane and along the beam direction, respectively.
This gave an approximate luminous volume of 0.3 x 0.06 x 2 mm at the interaction point.
During the Z running one increased the luminosity by increasing the number
of bunches. First to 8 and then to 12 in 1995. The peak luminosity was
~3.4 x 1031 cm-2s-1.
During the high energy running one instead increased the luminosity by increasing
the current in a 4-bunch configuration and one reached a peak luminosity of
~10 x 1031 cm-2s-1.
The challenge with an electron synchrotron is not to produce the required
magnetic bending field but to provide the accelerating voltage needed to
compensate for the energy lost due to synchrotron radiation (~3% of the
beam energy was lost in every turn at 209 GeV). At the start, LEP had 128 warm
five-cell copper cavities divided up into 8 units with 16 cavities each.
Each unit was powered by two 1 MW klystrons.
These were installed close to OPAL and L3. The system could deliver a maximum
accelerating voltage of 375 MV.
In the final configuration, LEP had
288 superconducting four-cell cavities in addition to 56 remaining warm copper cavities.
The superconducting cavities were powered by 36 0.6 MW klystrons.
The average voltage gradient of the super conducting cavities was 7 MV/m.
The RF system provided a total maxiumum accelerating voltage of 3630 MV at a 352 MHz frequency.
