LEP status and plans by LiamMessam

VIEWS: 16 PAGES: 5

									                                             LEP STATUS AND PLANS
                               S. Myers (for the LEP team), CERN, Geneva, Switzerland

                            Abstract
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A description is given of the present performance of LEP. The
major factors related to the limitations are discussed as are the                            60
measures currently used to overcome them.                                                                                                                        1994
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   The results from the “pretzel” scheme operation in 1994 are




                                                                     Integrated Luminosity
presented as well as a brief description of the new bunch train                                                                                                                                  40 pb-1
                                                                                             40
scheme. This scheme should ultimately allow the luminosity to
                                                                                                                                                                  1993                                  28.6 pb-1
be doubled by increasing the number of bunches per beam to six-                              30
teen (four trains of four bunches).
   The results and limitations from the energy calibration by res-                           20                                                                    1992
                                                                                                                                                                                                        17.3 pb-1

onant depolarization are summarized.                                                                                                                                          1991
   A review is then given of the performance limitations, hard-                              10
                                                                                                                                                                                     7.6 pb-1
ware requirements, and machine studies associated with oper-                                                                                                            1990                 1989 = 1.73 pb-1
ation of LEP at energies above the W threshold. Finally the                                  0




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                                                                                                  1
present plans for the LEP2 timescale are given.                                                                                           Days of Operation



                   I. INTRODUCTION                                                                              Figure. 1. Daily Integrated Luminosity
   The CERN Large Electron Positron (LEP) collider is a 26.6km
circumference e+ e storage ring which has, until the end of             2. Optimization of the beam-beam tune shift by emittance
1994, operated with 4 and 8 bunches per beam in an energy range             control, and
of 20 to 50 GeV (see previous conference reports, [1],[2],[3],[4]).     3. Increasing the number of bunches by the use of the pretzel
   LEP obtained its first circulating beam in July 1989 and per-             scheme.
formed collisions one month later in August. Since then, opera-
tion has been a mixture of physics data taking around the Z0 en- A. Improvements in the Operational Efficiency
ergy (45.6 GeV) and machine studies aimed at performance im-
provement, beam energy calibration, and future upgrades. Dur-          The operational efficiency in LEP (defined as the ratio of the
ing 1994, LEP was operated for physics with a pretzel scheme actual hours spent in physics to the scheduled hours) has steadily
and 8 bunches per beam and it is foreseen to further increase increased from about 40% in the first year of operation to about
the number of bunches during 1995 by the use of a bunch train 61% during 1994. It should be noted that due to the finite refill
scheme [5].                                                         time the efficiency can never be 100% (a realistic upper limit is
   For the second phase (LEP2) the collider will be operated at  85%), and that the efficiencies of all injectors in the LEP in-
an energy of about 90 GeV with an expected luminosity 7  jection chain are included in this figure. The improvement in the
1031 cm 2 s 1 to produce W pairs. This will be made possible by operational efficiency is due to two main components
the addition of 224 superconducting cavities giving a total volt-      1. Improvements in the reliability of all hardware. This has
age of more than 2.2 GV. A total RF-power of about 30 MW will            been done by identifying the least reliable components and
be available for the beam, which, with an energy loss of about           improving their design as well as doing preventative main-
1.9 GeV per turn, will be sufficient to store a current of 8 mA          tenance.
per beam.                                                              2. Reduction of the refill time. The accumulation time has
                                                                         been reduced by improving the injection efficiency by bet-
             II. PRESENT PERFORMANCE                                     ter diagnostics and by using synchrotron injection [6] [7] .
                                                                         In addition the applications software has undergone enor-
   For LEP1 the most critical parameters are the integrated lumi-
nosity, which dictates the number of Z0 s, and the precision with
                                                                         mous improvements which greatly speed up all “measure
                                                                         and correct” manipulations.
which the beam energy can be calibrated, which determines the
mass and the width of the Z0 interaction. Figure 1 shows the in-
                                                                    B. Beam-beam Optimization
crease in the daily integrated luminosity over the past 6 years.
The integrated luminosity has been increasing by about 50% per         The luminosity (L) is directly related to the vertical beam-
year over the past 3 years. Since there has been no significant in- beam tune shift (y ) i.e.
crease in the current per bunch this increase is due to three main
factors.                                                                                           
 kbib y
                                                                                            L=                                   (1)
    1. Improvements in the operational efficiency,                                                2re e y
                                                                                                    09/01/95   11.59
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                                               22.5   1.0            3.5       6.0            8.5   11                                                        AAAAAAAAAAAAAAAAAAAA
                                                                                                                                                               AAAAAAAAAAAAAAAAAAA
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                                                                                                                                                                                                                                            AAAAA AAA AAA
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                                                                                                                                                                                                                                                                                              AAAAAAAAAAAAAAAAAA                          AAA AAA AAAA
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                                                                                                                                                                                                                                                                                              AAAAAAAAAAAAAAAAAA                          AAA AAAAAAAA
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                                                                  Daytime in hours                                                                       0     AAAAAAA AAAAAAAAAAAA
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                                                                                                                                                                                                             AAA           AAAAA AAA
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                                                                                                                                                                                                                                              AAA               AAAAAAAAAA
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                                                                                                                                                                                                                                                                                                                                        AAAAAAAAAAAAA AAA   0




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Figure. 2. Evolution of beam parameters during a typical
physics run.
                                                                                                                                                          Figure. 3. Daily and Hourly Luminosity during 1994.
   where 
 is the relative energy, kb the number of bunches per
beam, and ib the bunch current. The beam-beam strength param-
                                                                                                                                       more difficult with the pretzel scheme and required very care-
eter is approximately given by
                                                                                                                                       ful control of the closed orbits, the chromaticities, the horizon-
                                                                             ib
                                                                    x = 2
efre
                                                                                                                                       tal and vertical separations, and the tune splits between electrons
                                                             y                                                               (2)      and positrons [9].
                                                                                         rev "x
   where "x is the horizontal emittance.                                                                                                                                                III. ENERGY CALIBRATION
   Clearly in order to maximize the integrated luminosity dur-
ing a physics run, it is necessary to maintain the y at its max-                                                                         The precise measurement of the mass and width of the Z0 res-
imum value independent of the bunch current which decays nat-                                                                          onance requires a high integrated luminosity and a very accu-
urally with time. Consequently the natural emittance must be suf-                                                                      rate absolute calibration of the beam energy. The average beam
ficiently small so that the beam-beam “limit” can still be reached                                                                      energy can be measured by resonant depolarization to a relative
with the low bunch currents at the end of the run. The natural                                                                         accuracy of around 10 5[10]. During the physics scan of the
horizontal emittance is approximately given by                                                                                         Z0 in 1993, all energy calibrations were performed at the end
                                                                                                                                       of the fills, usually with a single beam, and usually with about
                                                                                     2
                                                                            cq R

                                                                                                                                       12 days between calibrations. In order to estimate the average
                                                                      "x
                                                                              Q3
                                                                                                                              (3)      beam energy on physics fills between calibrations it was neces-
                                                                                 x                                                     sary to monitor all possible energy changes during the time inter-
   where R is the average radius,  the bending radius, and Qx                                                                         val between these calibrations. The average beam energy can be
the horizontal tune value.                                                                                                             changed either by a modification to the integrated bending field
   In LEP the small emittances are obtained by the use of a 900                                                                        or by a change in the length of the central orbit. The latter phe-
phase advance per cell lattice instead of the original design of                                                                       nomenon results from the fact that the beam path length is fixed
600. It is also clear from equation (2) that, in order to maintain                                                                     by the frequency of the RF system which can be held constant to
y constant, larger emittances are needed at the higher currents                                                                       around 10 10 . In principle, variations in the bending field should
associated with the beginning of the run. To this end emittance                                                                        be seen by the reference magnet which is connected in series with
wiggler magnets are excited at the beginning of the run and pro-                                                                       the dipoles and equipped with flip-coil and NMR field measuring
gressively reduced during the course of the run. The evolution                                                                         devices. However due to the large dimensions of LEP, there can
of the beam parameters, during a typical run in 1994, are shown                                                                        be differences in the environment between the reference magnet
in Figure 2.                                                                                                                           and the main dipoles (magnet temperatures etc.) which must be
                                                                                                                                       carefully monitored and used in correcting the energy estimates
C. Pretzel Operation                                                                                                                   from the reference magnet system. In addition, about once ev-
   A horizontal pretzel scheme was developed during machine                                                                            ery two weeks a measurement of the total integrated dipolar field
studies periods in 1993 and brought into operation near the end                                                                        (“flux-loop”) is done in order to cross-correlate with the resonant
of 1993 [8]. The same scheme was used throughout 1994 for op-                                                                          depolarization measurement. The length of the central orbit is
eration for physics and after an initial learning period resulted in                                                                   regularly measured by changing the revolution frequency until
an increase in luminosity which corresponded to the increase in                                                                        the beam is centered in all sextupoles. The observable for this
number of bunches. This can be demonstrated in Figure 3 where                                                                          centering technique is the change in the tune value when large
the reduction in luminosity results from reverting to 4 bunch op-                                                                      chromaticities are applied with the sextupoles.
eration at the end of the year. Fine tuning of the machine was                                                                            In LEP the beam energy at each interaction point is different
from the average beam energy as measured by the techniques                The number of bunches was doubled to 8, by the use of the
described previously. This difference results from a combina- pretzel scheme at the end of 1993 and throughout 1994. Although
tion of the energy “sawtoothing” and the geometrical alignment the pretzel scheme could have been extended to a larger num-
and properties of the RF accelerating system. RF phase errors ber of bunches, the detector electronics imposed a maximum of 8
and voltage asymmetries produce changes in the energies of the equally spaced bunches. Consequently two proposals were stud-
two beams at the collision points. The status of the RF system ied to used trains of bunches. The first, which had a horizontal
is an important ingredient in the final calibration of the colliding crossing angle was abandoned because of the large background
beams.                                                                 created by passing off center in the low  quadrupoles near the
   Despite all these correcting algorithms, considerable varia- interaction points. The second proposal involved separating the
tions in the LEP energy were observed during the course of the beams at the unwanted collision points using electro-static sep-
1993 scan. The path length variation has been identified with arators upstream and downstream of the interaction point. This
tidal effects and other variations on a longer time scale. More scheme has been studied extensively during machine study peri-
recently, other subtle effects have been investigated, such as the ods [5]. The limitation to the number of bunches in the train came
influence [11] of the water level in Lake Geneva and the effect from the detector electronics requirement that the total length of
of small values of dispersion at the collision points.                 the train be not greater than 750 ns. The minimum bunch spacing
                                                                       is given by the distance from the first separator to the interaction
                IV. LIMITATIONS FOR LEP1                               point, which sets an upper limit of 4 bunches per train. It is fore-
                                                                       seen to operate LEP1 during 1995 with 4 trains of 4 bunches per
A. Beam-beam, Background and Aperture                                  beam unless some unexpected critical problems are encountered
   The luminosity of LEP1 is limited by the interplay between with this scheme.
beam-beam effects, background in the detectors, and the aperture
                                                                       C. Summary of LEP1 Performance
as set by the collimator system. Low background conditions in
physics are ensured by a large number of collimators which de-            Table 1 gives a comparison of the LEP design parameters with
fine the LEP physical aperture, shield from synchrotron radiation those achieved in physics or in machine study sessions.
and from off momentum particles generated by beam gas inter-                     Table 1. Comparison of Achieved with Design
actions. For geometric reasons, it is not possible to collimate a
beam with a horizontal emittance greater than 45 ! 50 nm. The
                                                                              Parameter           Design Achieved            Units
                                                                                                                        1030cm 2 s 1
natural emittance at Z0 energies is around 13nm, which, at high
                                                                          Peak Luminosity           13          24
                                                                          Luminosity/day           480       1000      nb 1
currents is increased to around 40 nm by the use of emittance wig-
                                                                          Beam beam y             .030       .049
gler magnets.
                                                                          Bunch Current             750        820      A
   From equation (1), with a constant value of y it appears that           
                                                                          y                        .07       .037      m
the luminosity may be increased linearly with the bunch current
                                                                          Total Current             6.0       10.0      mA
(ib). It is also clear from equation (2) that for constant x the hor-
izontal emittance ("x ) increases linearly with current and for LEP       Energy calibration       25         1.7      MeV
reaches around 45nm with a bunch current of 350A. Increas-
                                                                                      V. FUTURE PLANS – LEP2
ing the bunch current beyond this value would require retrac-
tion of the collimators to avoid reductions in the lifetime. This A. Beam Energy Limitations
has been attempted on several occasions and always produced               The most crucial parameters for the physics to be performed
an increase in the background rates. Consequently the maximum above the W threshold are the maximum beam energy and the
bunch currents which can be collided at Z0 energy has been lim- integrated luminosity. The beam energy is determined by the
ited to around 350A for several years.                                total installed RF voltage needed to replenish the losses due to
   When operating at high values of  , the beam parameters synchrotron radiation and to provide an “RF bucket” sufficiently
(tunes, chromaticities, closed orbits, bunch current inequalities) large to provide a quantum lifetime of around 15 hours. Since
must be adjusted within very tight tolerances (.003 in tune the radiation losses increase with E4 the required RF voltage in-
and 1 in Q0 ) in order to avoid non Gaussian tails [12]. The creases dramatically with the beam energy needing more than
creation of these tails reduces dramatically the lifetime due to 2000 MV/turn needed at beam energies of  90 GeV (see Figure
the aperture reasons given above.                                      4).
                          
B. Number of Bunches and y                                           B. Luminosity Limitations
    For LEP1, when operating at the beam-beam limit, it is clear         Combining equations (2) and (3) gives
from equation (1) that the only remaining “free” parameters are                                               3
  
      
the y and the number of bunches (kb). The design value for the                               y    x / ib Qx Jx
                                                                                                             
3
                                                                                                                                       (4)
y was 7cm and LEP is currently operated with 5cm. On trial
          
runs the y has been reduced to 3.7 cm but without any increase          Consequently, for all other parameters constant, to maintain
in the luminosity and resulted in much greater sensitivity of the     identical beam-beam conditions at 90 GeV as at 45 GeV would
beam to small perturbations due to the very large  values at the     require an increase in the bunch current by a factor of 8. It is
first two insertion quadrupoles.                                       therefore unlikely that LEP2 will be strongly beam-beam limited
                3500
                                                                                                       require substantial changes to the transfer lines and an up-
                                                                                                       grade of the radiation shielding in the SPS machine.
                3000
                                                                                                     Reduction of the transverse impedance (k?). The major
                2500
                                                                                                       source of transverse impedance in LEP comes from the
                                                                                                       120 room temperature cavities (47% of the present total
                                                                                                       impedance) which could be replaced by about 32 SC cav-
RF Volts (MV)




                2000
                                                                                                       ities which have a much smaller transverse impedance
                1500                                                                                   (1.5%) due to their large bore diameter. This replacement,
                                                                                                       along with the other mentioned improvements should allow
                1000                                                                                   bunch currents of 0.8 mA at the LEP2 collision energies.
                                                                                                       Under these conditions and with 8 bunches per beam, a
                 500                                                                                   peak luminosity of  7 1031 cm 2 s 1 is predicted at 90
                                                                                                       GeV.
                   0                                                                                Further optimization of the luminosity may result from a re-
                    50.0        60.0            70.0              80.0          90.0    100.0
                                                                                                duction in the beam size at the IP (see equation (5)). This can be
                                                  Beam Energy (GeV)
                                                                                                achieved with a higher phase advance per cell lattice which re-
                       Figure. 4. RF Voltage as Function of Beam Energy.                        duces the horizontal emittance (see equation (3)). A lattice with
                                                                                                phase advances of 1080 and 600 (H and V) has been tested with
                                                                                                beam [16] with very encouraging results. If operation is possible
as is the current operation of LEP1. In this case the luminosity                                with this lattice and the high bunch currents described previously
is better written as                                                                            it is conceivable that LEP2 may produce beam-beam tune shifts
                                                                                                of .03 with a luminosity of  1032 cm 2 s 1 .
                                              i2               i2
                                       L / kb b         =     total
                                                                                      (5)
                                             xy             kbx y                           C. LEP2 Hardware
  In LEP2 the total current will be limited by the required beam                                   The hardware conversion to re-configure LEP1 to LEP2 has
power which is transmitted from the klystrons i.e.                                              been going on for several years with the final modifications car-
                                                                                                ried out during the last winter shutdown which ended in April
                           Pklystron = Pbeam / 2itotal U0                           4
                                                                         / 2itotal Eb   (6)     1995. These modifications included
                                                                                                    civil engineering for the new klystron galleries around IP4
   e.g. 30 MW of klystron power allows 8 mA of beam current                                           and IP8.
at 90 GeV. It is evident from equation (5) that with the total cur-                                 complete re-arrangement of the 8 RF straight sections to
rent being limited, the luminosity is maximized by storing this                                       physically accommodate the new SC cavities.
current in the minimum number of bunches kb , or in other words                                     installation of 4 new 12kW cryogenic plants
maximizing the bunch current. The current per bunch in LEP is                                       replacement of the superconducting low  quadrupoles
limited at injection energy by the Transverse Mode Coupling In-                                     and, an upgrade of power converters for higher energy
stability (TMCI). The approximate threshold for this instability                                   The final stage of the energy upgrade will be the installation of
is given by [13]                                                                                more than 200 SC cavities with their associated couplers, waveg-

                                                   P ik?i s
                                                                                                uides, klystrons and control electronics. This work will be car-
                                                  2Qs Eb frev
                                        ith =                                           (7)     ried out progressively until the spring of 1997 with several mile-
                                                 e               (       )
                                                                                                stones (see later).
   where Qs is the synchrotron tune, Eb the beam energy, and i
the betatron amplitude function at the location of the transverse                               D. Superconducting Cavities
loss factor k?i which decreases with increasing bunch length                                       For the LEP2 SC cavity system three aspects can be identified
(s). In order to maximize the current per bunch for LEP2 oper-                                 as being critical.
ation, several schemes have been proposed and tested.                                              1. Cavity production and module (4 cavities) assembly (for
    The “High Qs ” Scheme [14], in which the synchrotron tune                                       details see contribution to this conference [17] .
     is increased at injection energy and reduced in steps as the                                  2. Design and performance of the main coupler which must
     energy is increased. The highest bunch current reported in                                      be capable of passing 125kW to the beams (for details see
     Table 1 was obtained [15] using a Qs of .0125 at injection                                      contribution to this conference [18].
     energy. The main anticipated problem with this scheme is                                      3. Higher order mode couplers. The initial design of the out-
     the possible beam loss due to traversing (at higher energies)                                   put lines from these couplers imposed a current limitation
     low order synchro-betatron resonances with high beam in-                                        for LEP especially when operation was foreseen with bunch
     tensities.                                                                                      trains. Recent improvements in the design of the output
    Increasing the injection energy from 20 to 22 GeV. Two new                                      lines have allowed the HOM power per cavity to be in-
     superconducting bi-modules have been installed in the SPS                                       creased from the 400W “design” to 1700W which is beyond
     which will allow the extraction energy to be raised to 22                                       any powers foreseen with a reasonable set of parameters.
     GeV. This should increase the threshold current by 10% and                                    At present there are 7 SC modules (28 cavities) installed in
     the luminosity by 20%. Energies higher than 22 GeV would                                   LEP. Two of these modules were installed in Autumn 1994 and
  2000
                                                                     [4] S. Myers; Proc. of the Second European Particle Accelera-
                                                                          tor Conference, held in Nice from 12 till 16 June, 1990. (pp
                                                                          13– 17), (1990)
  1500                                                               [5] E. Keil, W. Herr, et al.;“Experiments with Bunch Trains in
                   Total MV                                               LEP”, accepted for oral presentation at this conference.
                                                                     [6] S. Myers; LEP Notes 334 and 344 (1981)
  1000
                                                                     [7] P. Collier; “Synchrotron Phase Space Injection into LEP”,
                                                                          accepted to this conference.
   500                                                               [8] R. Bailey et al., “LEP Operation in 1993 with the Pretzel
                                                                          Scheme”, Proc. of the Fourth European Particle Accelera-
                                                                          tor Conference, held in London from 27 June until 1 July,
    0                                                                     1994. (pp 439–441), (1994)
          Dec-94




                     Apr-95




                                Jun-95




                                         Oct-95




                                                  May-96




                                                           Oct-96
                                                                     [9] P. Collier et al.; “Operational Procedures to Obtain High
                                                                          Beam-Beam Tune Shifts in Pretzel Operation”, accepted to
                                                                          this conference.
             Figure. 5. Planned Available RF Voltage.                [10] R. Assmann et al.; “Energy Calibration with Resonant De-
                                                                          polarization in LEP”, Proc. of the Fourth European Particle
                                                                          Accelerator Conference, held in London from 27 June until
could be tested with beam during the last running period of this          1 July, 1994. (pp 935– 937), (1994)
year. One of these modules was successfully operated for more        [11] J. Wenninger; “Radial Deformations of the LEP ring”, SL
than 70 hours at its design gradient of 6MV/m and in the presence         Note 95-21 (OP), Feb. 1995
of the normal operating intensities for LEP1.                        [12] K. Cornelis et al.; “Tail Distributions due to Beam-Beam in
                                                                          LEP”, accepted to this conference.
E. Installation Plans                                                [13] B. Zotter: CERN-SL 92-29 p193
                                                                     [14] S. Myers; “Synchro-Betatron Resonance Excitation in
   The planned installation (see Figure 5) of SC cavities should          LEP”, Proc. of the IEEE Particle Accelerator Conference
allow a physics data period with 70 GeV per beam at the end              held in Washington D.C., March 16–19, 1987, p. 1325.
of 1995 followed by a run at 80.5 GeV in the first part of 1996       [15] D. Brandt: CERN SL/94-06 (DI) p149, March 1994.
which will allow a precision measurement of the mass of the W.       [16] A. Hofmann et al.; “Low Emittance Lattice for LEP”, ac-
In October 1996 the increased RF voltage should allow phyics              cepted for this conference.
data taking at energies significantly above the threshold of W       [17] E. Chiaveri et al.; “Analysis and Results of the Indus-
production. In 1997 production physics runs will be started at the        trial Production of the SC Nb/Cu Cavities for the LEP2
maximum possible energy .                                                 Project”, accepted for this conference.
                                                                              u
                                                                     [18] J. T¨ ckmantel et al.; “Improvements to Power Couplers for
F. Summary of Plans                                                       the LEP2 SC cavities”, accepted for this conference.

   In 1995 LEP1 will continue operation at Z0 energies but using
a bunch train scheme which should significantly increase the lu-
minosity. During the first part of 1996 it is planned to perform
physics data taking at 80.5 GeV/beam for precise measurement
of the mass of the W. Following a shutdown in Autumn 1996,
LEP2 will be operated for the first time at energies above the W
threshold. Following the 1996–1997 winter shutdown LEP2 will
start an exciting physics programme at high energies which will
extend beyond the year 2000.

                              References
[1] A. Hofmann; “Performance Limitations in LEP”, Proc. of
    the Fourth European Particle Accelerator Conference, held
    in London from 27 June until 1 July, 1994. (pp 73– 77),
    (1994)
[2] L. Evans; “LEP Status and Future Plans”, Proc. of the 1993
    Particle Accelerator Conference, Washington, p. 1983
    (1993)
[3] S. Myers; “LEP Performance and Plans”, Proc. 15th
    Int. Conf. on High Energy Accelerators, Hamburg 1992;
    HEACC92, p. 66, (1992).

								
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