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Progress in accelerators and detectors at TRISTAN
Nuclear Instruments and Methods in Physics Research A279 (1989) 7-15 North-Holland, Amsterdam
7
PROGRESS IN ACCELERATORS AND DETECTORS AT TRISTAN Kazuhisa NAKAJIMA
National Laboratory for High Energy Physics, Oho, Tsukuba-shi, Ibaraki-ken, 305, Japan We have conducted the first year of operation and experiments of the TRISTAN e + e - collider since successful commissioning . The current status of accelerator performance and collider experiments at TRISTAN are presented along with future upgrading programs. Finally operational aspects on beam backgrounds and radiation problems are mentioned .
1 . Introduction The TRISTAN project [1] was approved and started in 1981, aiming at the construction of an e + e - collider with a center-of-mass energy of 60 GeV . An accumulation ring was constructed in two years . A main ring was commissioned with e + e - collisions at the world highest energy of 50 GeV in November 1986 and the first large angle Bhabha event was observed in the VENUS detector just five years after the ground breaking ceremony. The accelerator complex of TRISTAN is composed of a 2 .5 GeV injector linac for electrons and positrons, an accumulation ring (AR), and a main ring (MR) . A
Table 1 Basic parameters of TRISTAN Circumference (m) Bending radius (m) Long straight section (m) Total rf sections (m) Rf frequency (MHz) Injection energy (GeV) Maximum energy (GeV) Number of insertion regions
MR
AR
3018 .1 246 .5 4 x 194 .4 509 .4 508 .6 6-8 25-30 4
377 .0 23 .17 2 x (19 .5 + 19 .1) 38 .1 508 .6 2 .5 6-8 2
plan view of TRISTAN in the KEK site is shown in fig. 1 . The basic parameters of the TRISTAN MR and AR are represented in table 1 [2] . At each collision point of the main ring, there are four experimental halls named TSUKUBA, OHO, FUJI and NIKKO, to which four experiments, TOPAZ, AMY, VENUS and SHIP, are assigned, respectively . During the last year, we have performed colliding beam experiments for about 2300 = 50, 52, 55 and 56 GeV. hours at
f
2 . Overview of accelerator systems
200 MeV High Curt. e-uNAC " e'-source
Fig. 1 . A plan view of TRISTAN in the KEK site . 0168-9002/89/$03 .50 0 Elsevier Science Publishers B .V . (North-Holland Physics Publishing Division)
The total number of magnets deployed in the MR is 1472 ; 272 bending magnets each of which is 5 .86 m long, 392 quadrupole magnets, 240 sextupole magnets, 520 correction magnets, 24 skew magnets and 24 wiggler magnets. These magnets are installed very precisely . Especially the alignment of the quadrupole magnets is adjusted within an rms error of 0 .2 mm radially, 0.3 mm azimuthally and 0 .2 mm vertically [3]. The rf system plays an important role in an e + e storage ring in order to keep a high energy and a high luminosity . On both sides of four experimental halls in the MR, 80 sections are allocated to rf cavity installation or the beam injection system. At present 52 sec tions of these are occupied by room temperature rf I. FUTURE ACCELERATORS
K. Nakajima / Progress in accelerators and detectors at TRISTAN vacuum pump in the MR. After an improvement of the DIP the average pressure in the MR is about 10 -6 Pa with a beam current of 10 mA in the 28 GeV operation . One of the important operational tasks is a beam orbit control. A closed orbit in the MR can be measured with an accuracy of 50 lum by use of 400 position monitor pickups located at all quadrupole magnets. The closed orbit distortion can be corrected within _+ 1 nun horizontally and ±0.5 nun vertically by an on-line computer. The automatic tune measurement system for betatron oscillations and the synchrotron radiation monitor system giving a visual beam image are indispensable beam diagnostic tools [6] . The TRISTAN accelerator is controlled with 25 16bit minicomputers which are distributed around the accelerator and linked together with an optical fiber ring network . The accelerator devices are controlled with touch panels on the operator's console in the control center. Fig. 3 shows a schematic diagram of the TRISTAN control system [7] .
Fig. 2 . Accelerating unit of APS for the TRISTAN MR. cavities, the so-called alternating periodic structure (APS) unit with 18 accelerating cells shown in fig . 2 [4] . The shunt impedance of the APS cavity, which stands for the accelerating efficiency, is about 22 M2/m . The total impedance amounts to 6178 MS2 which is compared to 2300 MS2 at PETRA . The rf power is generated by 26 klystrons each of which outputs 1 MW in cw and drives four 9-cell cavity units . This room temperature rf system produces a maximum accelerating voltage of 330 MV . It is a distinctive feature of the TRISTAN vacuum system [5] that all the vacuum components are made of aluminum alloy materials . A special extrusion technique is used in manufacturing vacuum pipes and chambers in order to realize ultrahigh vacuum performance . A distributed sputter ion pump (DIP) works as a main
3. Colliding beam performance of TRISTAN The TRISTAN MR has been operated for about 4700 hours until March 1988 for the use of physics experiments and accelerator developments . The operation statistics of TRISTAN is given in fig . 4 . The operational developments have been devoted to increase the beam current and the luminosity . The linac injector can provide the AR with - 50 mA electron beam in a short pulse of 2 ns (FWHM) at a repetition rate of 25 Hz. A positron beam can be attained with a peak current of - 10 mA after a 200 KEK central computers
operator's console NIP -
OP0 OPI
OOOOOO El OO
OP2
IL
O P3
OP4
OP5
d
d
BMI
BTI
i
AL0
ALI
LBO
.control station (CST) o master station (MST)
d=-1
optical fiber ring network (10 Mbps) RF0
m
CA
BMO
BTO
MGO
VAO
Accumulation ring --
RFI
RF2
®1®E®
MG4
VA I
GPO
GPI
; -- Main ring S .C .C . control system
Fig . 3 . A schematic diagram of the TRISTAN control system .
K. Nakajima / Progress in accelerators and detectors at TRISTAN
Operation 0
500
of TRISTAN 1000
1500 hr
commissioning Nov . 19 . 1986 260 hr / 25 GeV
86 .11 12 87 . 1 2 3 4 5 6 7 8 9 10 i1 12 88 . 1 2 3
phys . exprimenis 518 hr/ 25 GeV 0 occ . developments 0 halts etc .
Fig. 4 . The operation statistics of TRISTAN . MeV high current electron beam of 10 A is converted by a tantalum target [2] . Typically a 20 mA beam is boosted from 2.5 - GeV injection energy to 7 .5 GeV 5
4
a E
3
U L
extraction energy in the AR and transferred to the MR with an efficiency of 60-70% . We have searched for optimum operating conditions of the MR surveying all controllable machine parameters . A distinctive feature of the TRISTAN operation is the use of wiggler magnets which are installed in each quadrant . They work effectively to stabilize beams and to decrease beam-beam effects so that no separation between electron and positron beams is necessary during the injection . The maximum current per bunch is limited to around 5 mA due to single beam instabilities at the injection . The total beam current of two electron and two positron bunches amounts to - 14 mA . The optimum parameters were found by hunting for working points on the betatron tune diagram, rf voltages and closed orbits. Fig. 5 shows the maximum bunch current as a function of the rf voltage at the injection . Ramping of the beam energy has been carried out without beam loss from the injection energy of 7 .5 GeV to the flat top energy of 28 GeV . The beta function at the collision point is tuned to 1 .8 m/2 .0 m horizontally and 10 cm vertically . The rf frequency is slightly shifted in order to reduce the transverse beam emittance . The rf frequency shift of +1 .8 kHz lowers the beam emittance by a factor of 40% at 28 GeV so that the luminosity gains by the same factor, although this procedure requires a 10% higher rf voltage. Another effective procedure increasing the luminosity is to reduce the verticalto-horizontal emittance ratio . This ratio can be lowered to - 1% by making a fine adjustment of the closed orbit . Fig . 6 shows luminosities and vertical tune shift parameters measured at 28 GeV as a function of the current per beam. The peak luminosity attains to 1 .4 x 10 31 cm -2 s -1 and the maximum beam-beam tune shift reaches 0 .035. The luminosity is still proportional to the beam current squared. The vertical tune shift parameter, which is a measure of beam-beam effects, increases linearly to the beam current . It indicates that the luminosity is not saturated at the beam-beam limit .
4 . Overview of detecter systems
m 2
40
50
60 70 80 90 100 RF Voltage (MV)
110
Fig. 5 . The maximum bunch current as a function of rf voltage at 7 .5 GeV .
There are three general purpose detectors, VENUS, TOPAZ and AMY, rolled in TRISTAN . The fourth experiment, SHIP, is a special purpose detector aimed at the search for highly ionizing particles . Fig . 7 shows a cross sectional view of the VENUS detector [8] ., The basic tracking device of VENUS is the inner drift chamber with 6 layers of anode and cathode readouts and the central drift chamber consisting of 20 layers of axial and 9 layers of stereo wires. They are contained in a very thin superconducting solenoid with a 0 .75 T magnetic field . The transition detector will be accommodated for particle identification outside the central drift chamber as a detector upgrade. Outside the l . FUTURE ACCELERATORS
10
K. Nakajima / Progress in accelerators anddetectors at TRISTAN
solenoid cryostat there are barrel streamer tubes and 5160 barrel lead glass counters for electromagnetic calorimetry. The end cap calorimeter of VENUS is constituted by liquid argon detectors with lead converters of 20 .5X0 (radiation lengths) . The muon tracking system consists of 6 interaction-length muon filters and 5422 multilayer drift tubes in the barrel and end cap part . Other detectors are 96 time-of-flight (TOF) counters and the luminosity monitor. The TOPAZ detector [9] is shown in fig. 8a. A principal feature of the TOPAZ detector is the time projection chamber (TPC) as a central tracking device. The TPC gives us two unique capabilities of three dimensional particle tracking and particle identification due to dE/dx measurement. Inside the TPC, there is accommodated the inner drift chamber consisting of 10 layers of anode wire and cathode strip readouts . 64 segments TOF counters are between the 1 T superconducting solenoide and the TPC. The solenoid coil is surrounded by 4 layers of the barrel drift chamber and the barrel calorimeter consists of 4320 lead glass counters . The end cap calorimeter is a gas sampling calorimeter consisting of a 18X0 thick lead/proportional tube sandwich . The outermost detector is 4 layers of muon drift chamber interleaved with muon filters with a total thickness of 1 m. A schematic drawing of the TPC is shown in fig. 8b . The TPC is enclosed by the FRP (fiber reinforced plastic) pressure vessel with dimensions 2.6 m in diameter and 3 m in length . The vessel is divided into two drift cells by the central membrane to which HV is applied. Drifting electrons are detected at both
I (mA) /Beam
3 4 I (mA) /Beam
5
6
7
8
910
Fig. 6. The luminosity and the vertical beam-beam tune shift parameter as a function of current per beam .
C
Transition Radiation Detector Outer Drift Tube Time-of-Flight Counter Superconducting Coil
~~~~"" ~ """H\'_N_11111111ÍI_IIIIlIHNN~~
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`J
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OCI Liquid Argon Counter Luminosity Monitor Magnet Return Yoke
`!3 '!4 í5
Chamber
Muon Chamber Muon Filter Equipment Transportation Cart
TOTAL WEIGHT -~ 20001 MAGNETIC FIELD 075 T 3
4
5m
Fig. 7. A cross-sectional view of the VENUS detector.
K. Nakajima / Progress in accelerators and detectors at TRISTAN
Coarse Field tage TPC (Fine Field Cage
b
OUTER FRP PRESSURE INNER FINE FIELD CAGE OUTER FINE FIELD CAGE SECT
VESSEL
-
- -
\
M
1101
\ )SM, 1R
00 \0~
,,
Z
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END PLATE
Fig. 8. (a) A cut-out view of the TOPAZ detector . (b) A schematic drawing of the TOPAZ TPC.
ends of the cylinder with 16 sectors of multiwire proportional chambers . Each sector contains 175 sense wires and 512 cathode pads . The TPC is operated at 3.5 atm of P10 gas with a drift field of 353 V/cm in a 1 T magnetic field. The AMY detector [10] is represented in fig. 9. One of the most characteristic features of AMY is a 3 T superconducting solenoid which provides precise momentum measurements in a compact volume . The tracking system of AMY consists of 4 layers of straw chambers, and the central tracking chamber consists of 25 layers of axial- and 15 layers of stereo wires. The
transverse momentum resolution is as good as 0 .7%p r. The main AMY calorimeter is the barrel shower counter which is a highly segmented gas calorimeter consisting of 20 layers of plastic tube chambers with cathode pads, interleaved with 5 Xo thick lead sheets . This shower calorimeter gives a good y/m° separation. The muon detection system of AMY is a thick iron absorber and 4 layers of muon chamber. For the purpose of electron identification, a synchrotron X-ray detector is installed outside the central drift chamber. The SHIP detector [111 consists of 12 modules of a CR-39 laminated stack set around the collision point, I. FUTURE ACCELERATORS
12
K Nakajima / Progress in accelerators and detectors at TRISTAN
n-
Beam Pipe Distributed Ion Pump Small Angle Luminosity Monitor Inner Tracking Chamber Pole Tip Counter Central Dritt Chamber Trigger Counter Ring Veto Counter Shower Counter Superconducting Solenoid Coil Magnet Yoke/Hadron Absorber
Fig. 9. A schematic view of the AMY detector. and UG-5 glass placed inside the vacuum pipe. These etchable track detectors cover a solid angle - 0.9 x 41T sr and are shown schematically in fig. 10 . 5. Status of TRISTAN experiments Physics goals of TRISTAN experiments are: given - Search for new flavors;
Search for new heavy leptons; QED test ; Electroweak theory test ; Study of isolated leptons in hadronic events ; Study of two-photon processes; Search for Dirac magnetic monopoles. The integrated luminosities delivered to physics experiments in the first year are summarized in table 2 at each center-of-mass energy. Fig. 11 shows the integrated luminosity per day accumulated in the TOPAZ detector. Events collected in e+ e- annihilations are -
-
e + e - -> e + e - , 1'y, w + Ir - , T + T - , hadrons. Fig. 12 shows a typical event of multihadron production at = 52 GeV, observed in the TOPAZ detector . A
r
Table 2 Energies and integrated luminosities in the first year of TRISTAN experiments [GeV] Run period
Fig. 10 . Schematic representation of CR-39 and UG-5 detector configuration of the SHIP detector.
50 52 55 56 Total
May-June 1987 June-July 1987 Oct.-Dec. 1987 Jan.-March 1988
Integrated luminosities [pb -1 1 VENUS TOPAZ AMY 0.7 0.5 0.7 2.9 3.6 4.0 2.5 2.9 3.3 5.3 5 .6 6.1 11 .4 12 .6 14 .1
SHIP 0.8 4.0 4.0 7.5 16 .3
13
K Nakajima / Progress in accelerators and detectors at TRISTAN Integrated
Luminosity /Day in TOPAZ t-
T 400 a c p 300 w
is=5OGeV
T
T--52GeV
C_2 F -
c
r=56 GeV
T=55 GeV
M
0
1
11 JUN 1987
21
1
11 JUL
21
1
11 NOV
21
1
11 DEC
Date
21
1 21 JAN 1988
11 21 FEB
1
11 MAR
Fig. 11 . The integraded luminosity per day accumulated in the TOPAZ detector in the first year operation. 111111111111
1
.
0
w il kkl2 ital l~ri»eAAa ,ql :l
Prom
~äll~
M-Nd'dl
~:
r
1111111111111111111 A typical event of multihadron production at v~s = 52 GeV observed in the TOPAZ detector . Fig. 12 . typical result of TRISTAN is represented by R values, the ratio of the hadronic cross section to the QED [r+[,- cross section, as shown in fig. 13 . The physics results obtained from TRISTAN are summarized [12] : 1) e +e- -> e+ e-, yy and + W- processes are con sistent with the standard theory ; 2) The quark fragmentation model describes many features of the hadronic final states ; 3) R values are consistent with the standard theory of five flavor production ; 4) The top quark was not observed up to ~rs_ = 55 GeV. 5) More study is required on the existence of a charge - -' heavy quark. 6) No positive evidence was observed for the copious production of isolated leptons as was found by MARK-J group. 7) The lower mass bound for sequential leptons was given to be 26.8 GeV . 8) The upper limit on the cross section for the production of the Dirac monopole is 0.3 pb at 95% CL with a mass of 26 .1 GeV/c2.
R ratio 6Eiio - (JADE
w
AMY
MARK-J
+
TOPAZ
TASSO
X
VENUS
6 flavours
m= = 92.5Gev/cz sinZB, = 0.226 40
45 50 Center of Mass Energy (GeV)
55
60
Fig. 13 . The ratio R resulting from the TRISTAN experiments and plotted together with the data from PETRA [161 . The solid curve shows the standard model prediction without the top quark. The dashed curve is the ratio R expected for the top quark. I. FUTURE ACCELERATORS
14
K. Nakajima / Progress in accelerators and detectors at TRISTAN
6. Upgrading programs of TRISTAN Upgrading programs [13] for energy and luminosity are in progress . The energy upgrade of the MR can be carried out by the installation of superconducting rf cavities into the NIKKO rf sections . In addition to the present conventional rf cavities, 16 superconducting cavities will be installed in October 1988 to increase the beam energy up to 31 GeV. Further 16 cavities will be deployed in April 1989 so that the total accelerating voltage should reach as high as 570 MV which gives a maximum beam energy of 33 GeV. The superconducting 5-cell cavity made of niobium sheets has been developed successfully in KEK as a consequence of R&D work over many years. We have already achieved a quality factor higher than 2 x 10 9 at 5 MV/m and a maximum accelerating field higher than 10 MV/m as shown in fig. 14 . Actual beam tests have been done in the AR to demonstrate stable operation with high field and high beam current. The liquid helium cryogenic systems consist of 16 cryostats each of which contains two 5-cell cavities, a 380 m long transfer line and a refrigerator system with 6.5 kW cooling power at 4.4 K. The luminosity upgrade is conceived as a minibeta scheme using superconducting quadrupoles (QCS) rather than the present low-beta scheme . The installation of QCS is scheduled in the spring 1990. The minibeta scheme allows us to reduce the beta functions up to 0.85 m horizontally and 4 cm vertically at the collision point, provided with 70 T/m field gradient of QCS. The luminosity should gain by a factor of 2.
radiation problems in high energy and high luminosity colliders. The beam background causes detectors to prevent a regular operation, to increase a trigger rate and a dead time, and to damage itself. In the straight section of the MR around each detector, the 4 fixedand 2 movable beam masks are installed to protect experiments from high synchrotron radiation fluxes and from off-momentum electron/positron backgrounds . Unfortunately, these masks were not always effective to reduce beam noise on the detector . According to our experience, a drastic improvement was brought about by expanding the diameter of the beam pipe inside the detector . The minimum diameter of the beam pipe should be 20 times larger than the maximum beam size around the collision point. Even the outermost muon chambers have suffered from beam backgrounds, probably caused by soft photons. This type of beam background has been decreased by one order of magnitude after hanging 2 mm thick lead sheets on the wall between the experimental hall and the accelerator tunnel . In a daily physics operation, the beam background depends on fine adjustments of the beam orbit, rf frequency shifts and improvement of the vacuum . An example of the beam background [14] is shown in fig. 15 . LUMI BKG RUN1882
2000 1500
1500
1000
1000
500
500
7. Operational aspects on beam backgrounds and radiations
06
The accelerator performance and experimental situation are inevitably affected by beam backgrounds and
10
Vertical
Test of 5-Cell
8 1L 0 I2 BEAM CURRENT (mA )
14
EVENT RATE (Hz) RUN1882
06 .
1
8 1I0 12 BEAM CURRENT (mA )
14
8 10 12 BEAM CURRENT (mA)
14
8 10 12 BEAM CURRENT(mA) (b)
14
EVENT RATE (HZ) RUN1934
to
e
(#3a,#3b) 2 0 6
3
60
2
. .0 0 . . 8 10 12 BEAM CURRENT (mA)
1 0.9 08 07 06
Fig. 14 . Q-values of superconducting five-cell cavities measured as a function of the accelerating field.
14
0
0 12 BEAM CURRENT (mA) (a)
DEAD TIME (Í) RUN1934
40 30 20
ó 00
8
.
6 60 50
0
20 10 06
2
DEAD TIME (%) RUN1882
50 40 30
0 O
Eacc (MV/m)
LUMI BKG RUN1934
2000
14
10 0 6
Fig. 15 . The background, trigger rate, and dead time observed before beam tuning (a) and after beam tuning (b), as a function of the beam current in daily experimental runs of VENUS.
K. Nakajima / Progress in accelerators and detectors at TRISTAN K .P Lomóert ef cl .
15
-2 s -1 at the center-of-mass luminosity of 1 .4 x 10 31 cm collision energy of 56 GeV . The TRISTAN experiments give us important results on lepton and quark physics. They will contribute to opening up a new generation of physics while TRISTAN upgrading programs are in progress. Although we had been faced with serious situations as increased energy and luminosity, problems should be solved and situations improved .
Acknowledgements
Fig. 16 . Offset voltage of the operational amplifier as a func tion of the radiation dose . The data are compared with ref . [17] . Excessive radiation causes serious damage to many accelerator components . Examples have been found on parts of magnets, vacuum chamber components, cables, electronics parts and so on in only six months . The radiation dose in the TRISTAN tunnel is 10 4-10 5 Gy/A h around the beam chamber with a lead shield and _ 10 6 Gy/A h on the chamber surface for 27 .5 GeV beam energy. The radiation damage on the aluminum vacuum components was caused by HN0 3 produced from N02 and H 2O . Connectors and cables were damaged by HF produced by decomposition of Teflon insulators. Cooling rubber hoses of magnets were hardened by radiation exposure . The radiation damage for electronics components was tested [15] in the TRISTAN MR . It was found that silicon transistors were relatively endurable up to several 10 4 Gy while CMOS RAM and EPROM were quite vulnerable to the radiation . These ICs were broken at an exposure of 700 Gy . Fig . 16 show the influence of radiation dose on typical operational amplifiers . In order to make high energy and high luminosity possible, it is urged that radiation resistive materials and electronics should be devised and applied to the accelerator, and detectors as well as completed radiation shields are made . 8 . Conclusion We started the operation of TRISTAN on schedule after a five-year construction . We have achieved a
The author would like to express his sincere thanks to Prof. S. Ozaki and Y . Kimura for their guidance and support . He is very much indebted to Prof . T. Kondo for kindly encouraging him to present this talk at this conference. Special thanks are given to all of TRISTAN accelerator, VENUS, TOPAZ, AMY and SHIP groups for their assistance and offer of the latest information on their work.
References [1] TRISTAN project group, KEK report 86-14 (1987) . [2] G . Horikoshi and Y . Kimura, Proc . IEEE Particle Accelerator Conf., Washington (1987) vol. 1, p. 34. [3] A . Kabe et al ., ibid., vol. 3, p. 1648 . [4] T. Higo et al ., ibid ., vol. 3, p. 1945 . [5] H . Ishimaru et al ., Proc . 6th Symp . on Accelerator Science and Technology Tokyo (1987) p . 138 . [6] T . Ieiri et a] ., ref. [2], vol . 2, p . 729 . [7] S . Kurokawa et al., ref. [5], p . 233. [8] VENUS collaboration, TRISTAN proposal (1983) TRISTAN-EXP-001 . [9] TOPAZ collaboration, TRISTAN proposal (1983) TRISTAN-EXP-002 . [10] AMY collaboration, TRISTAN proposal (1984) TRIS TAN-EXP-003 . [11] SHIP collaboration, TRISTAN proposal (1985) TRISTAN-EXP-004 . [12] F . Takasaki, presented at Int . Symp. on Lepton and Photon, Hamburg (1987) KEK preprint 87-92 . [13] S . Isagawa, presented at 23rd Recontre de Moriond on Electroweak Interactions and Unified Theories, Les Arcs (1988) KEK preprint 88-9. [14] S . Uehara, private communication . [15] T. Momose, Europ. Part . Accel. Conf., Rome (1988). [16] H.J. Berend et al ., Phys . Lett . B183 (1987) 400 . [17] K.P. Lambert et al., CERN 75-4(1975) .
I. FUTURE ACCELERATORS
7
PROGRESS IN ACCELERATORS AND DETECTORS AT TRISTAN Kazuhisa NAKAJIMA
National Laboratory for High Energy Physics, Oho, Tsukuba-shi, Ibaraki-ken, 305, Japan We have conducted the first year of operation and experiments of the TRISTAN e + e - collider since successful commissioning . The current status of accelerator performance and collider experiments at TRISTAN are presented along with future upgrading programs. Finally operational aspects on beam backgrounds and radiation problems are mentioned .
1 . Introduction The TRISTAN project [1] was approved and started in 1981, aiming at the construction of an e + e - collider with a center-of-mass energy of 60 GeV . An accumulation ring was constructed in two years . A main ring was commissioned with e + e - collisions at the world highest energy of 50 GeV in November 1986 and the first large angle Bhabha event was observed in the VENUS detector just five years after the ground breaking ceremony. The accelerator complex of TRISTAN is composed of a 2 .5 GeV injector linac for electrons and positrons, an accumulation ring (AR), and a main ring (MR) . A
Table 1 Basic parameters of TRISTAN Circumference (m) Bending radius (m) Long straight section (m) Total rf sections (m) Rf frequency (MHz) Injection energy (GeV) Maximum energy (GeV) Number of insertion regions
MR
AR
3018 .1 246 .5 4 x 194 .4 509 .4 508 .6 6-8 25-30 4
377 .0 23 .17 2 x (19 .5 + 19 .1) 38 .1 508 .6 2 .5 6-8 2
plan view of TRISTAN in the KEK site is shown in fig. 1 . The basic parameters of the TRISTAN MR and AR are represented in table 1 [2] . At each collision point of the main ring, there are four experimental halls named TSUKUBA, OHO, FUJI and NIKKO, to which four experiments, TOPAZ, AMY, VENUS and SHIP, are assigned, respectively . During the last year, we have performed colliding beam experiments for about 2300 = 50, 52, 55 and 56 GeV. hours at
f
2 . Overview of accelerator systems
200 MeV High Curt. e-uNAC " e'-source
Fig. 1 . A plan view of TRISTAN in the KEK site . 0168-9002/89/$03 .50 0 Elsevier Science Publishers B .V . (North-Holland Physics Publishing Division)
The total number of magnets deployed in the MR is 1472 ; 272 bending magnets each of which is 5 .86 m long, 392 quadrupole magnets, 240 sextupole magnets, 520 correction magnets, 24 skew magnets and 24 wiggler magnets. These magnets are installed very precisely . Especially the alignment of the quadrupole magnets is adjusted within an rms error of 0 .2 mm radially, 0.3 mm azimuthally and 0 .2 mm vertically [3]. The rf system plays an important role in an e + e storage ring in order to keep a high energy and a high luminosity . On both sides of four experimental halls in the MR, 80 sections are allocated to rf cavity installation or the beam injection system. At present 52 sec tions of these are occupied by room temperature rf I. FUTURE ACCELERATORS
K. Nakajima / Progress in accelerators and detectors at TRISTAN vacuum pump in the MR. After an improvement of the DIP the average pressure in the MR is about 10 -6 Pa with a beam current of 10 mA in the 28 GeV operation . One of the important operational tasks is a beam orbit control. A closed orbit in the MR can be measured with an accuracy of 50 lum by use of 400 position monitor pickups located at all quadrupole magnets. The closed orbit distortion can be corrected within _+ 1 nun horizontally and ±0.5 nun vertically by an on-line computer. The automatic tune measurement system for betatron oscillations and the synchrotron radiation monitor system giving a visual beam image are indispensable beam diagnostic tools [6] . The TRISTAN accelerator is controlled with 25 16bit minicomputers which are distributed around the accelerator and linked together with an optical fiber ring network . The accelerator devices are controlled with touch panels on the operator's console in the control center. Fig. 3 shows a schematic diagram of the TRISTAN control system [7] .
Fig. 2 . Accelerating unit of APS for the TRISTAN MR. cavities, the so-called alternating periodic structure (APS) unit with 18 accelerating cells shown in fig . 2 [4] . The shunt impedance of the APS cavity, which stands for the accelerating efficiency, is about 22 M2/m . The total impedance amounts to 6178 MS2 which is compared to 2300 MS2 at PETRA . The rf power is generated by 26 klystrons each of which outputs 1 MW in cw and drives four 9-cell cavity units . This room temperature rf system produces a maximum accelerating voltage of 330 MV . It is a distinctive feature of the TRISTAN vacuum system [5] that all the vacuum components are made of aluminum alloy materials . A special extrusion technique is used in manufacturing vacuum pipes and chambers in order to realize ultrahigh vacuum performance . A distributed sputter ion pump (DIP) works as a main
3. Colliding beam performance of TRISTAN The TRISTAN MR has been operated for about 4700 hours until March 1988 for the use of physics experiments and accelerator developments . The operation statistics of TRISTAN is given in fig . 4 . The operational developments have been devoted to increase the beam current and the luminosity . The linac injector can provide the AR with - 50 mA electron beam in a short pulse of 2 ns (FWHM) at a repetition rate of 25 Hz. A positron beam can be attained with a peak current of - 10 mA after a 200 KEK central computers
operator's console NIP -
OP0 OPI
OOOOOO El OO
OP2
IL
O P3
OP4
OP5
d
d
BMI
BTI
i
AL0
ALI
LBO
.control station (CST) o master station (MST)
d=-1
optical fiber ring network (10 Mbps) RF0
m
CA
BMO
BTO
MGO
VAO
Accumulation ring --
RFI
RF2
®1®E®
MG4
VA I
GPO
GPI
; -- Main ring S .C .C . control system
Fig . 3 . A schematic diagram of the TRISTAN control system .
K. Nakajima / Progress in accelerators and detectors at TRISTAN
Operation 0
500
of TRISTAN 1000
1500 hr
commissioning Nov . 19 . 1986 260 hr / 25 GeV
86 .11 12 87 . 1 2 3 4 5 6 7 8 9 10 i1 12 88 . 1 2 3
phys . exprimenis 518 hr/ 25 GeV 0 occ . developments 0 halts etc .
Fig. 4 . The operation statistics of TRISTAN . MeV high current electron beam of 10 A is converted by a tantalum target [2] . Typically a 20 mA beam is boosted from 2.5 - GeV injection energy to 7 .5 GeV 5
4
a E
3
U L
extraction energy in the AR and transferred to the MR with an efficiency of 60-70% . We have searched for optimum operating conditions of the MR surveying all controllable machine parameters . A distinctive feature of the TRISTAN operation is the use of wiggler magnets which are installed in each quadrant . They work effectively to stabilize beams and to decrease beam-beam effects so that no separation between electron and positron beams is necessary during the injection . The maximum current per bunch is limited to around 5 mA due to single beam instabilities at the injection . The total beam current of two electron and two positron bunches amounts to - 14 mA . The optimum parameters were found by hunting for working points on the betatron tune diagram, rf voltages and closed orbits. Fig. 5 shows the maximum bunch current as a function of the rf voltage at the injection . Ramping of the beam energy has been carried out without beam loss from the injection energy of 7 .5 GeV to the flat top energy of 28 GeV . The beta function at the collision point is tuned to 1 .8 m/2 .0 m horizontally and 10 cm vertically . The rf frequency is slightly shifted in order to reduce the transverse beam emittance . The rf frequency shift of +1 .8 kHz lowers the beam emittance by a factor of 40% at 28 GeV so that the luminosity gains by the same factor, although this procedure requires a 10% higher rf voltage. Another effective procedure increasing the luminosity is to reduce the verticalto-horizontal emittance ratio . This ratio can be lowered to - 1% by making a fine adjustment of the closed orbit . Fig . 6 shows luminosities and vertical tune shift parameters measured at 28 GeV as a function of the current per beam. The peak luminosity attains to 1 .4 x 10 31 cm -2 s -1 and the maximum beam-beam tune shift reaches 0 .035. The luminosity is still proportional to the beam current squared. The vertical tune shift parameter, which is a measure of beam-beam effects, increases linearly to the beam current . It indicates that the luminosity is not saturated at the beam-beam limit .
4 . Overview of detecter systems
m 2
40
50
60 70 80 90 100 RF Voltage (MV)
110
Fig. 5 . The maximum bunch current as a function of rf voltage at 7 .5 GeV .
There are three general purpose detectors, VENUS, TOPAZ and AMY, rolled in TRISTAN . The fourth experiment, SHIP, is a special purpose detector aimed at the search for highly ionizing particles . Fig . 7 shows a cross sectional view of the VENUS detector [8] ., The basic tracking device of VENUS is the inner drift chamber with 6 layers of anode and cathode readouts and the central drift chamber consisting of 20 layers of axial and 9 layers of stereo wires. They are contained in a very thin superconducting solenoid with a 0 .75 T magnetic field . The transition detector will be accommodated for particle identification outside the central drift chamber as a detector upgrade. Outside the l . FUTURE ACCELERATORS
10
K. Nakajima / Progress in accelerators anddetectors at TRISTAN
solenoid cryostat there are barrel streamer tubes and 5160 barrel lead glass counters for electromagnetic calorimetry. The end cap calorimeter of VENUS is constituted by liquid argon detectors with lead converters of 20 .5X0 (radiation lengths) . The muon tracking system consists of 6 interaction-length muon filters and 5422 multilayer drift tubes in the barrel and end cap part . Other detectors are 96 time-of-flight (TOF) counters and the luminosity monitor. The TOPAZ detector [9] is shown in fig. 8a. A principal feature of the TOPAZ detector is the time projection chamber (TPC) as a central tracking device. The TPC gives us two unique capabilities of three dimensional particle tracking and particle identification due to dE/dx measurement. Inside the TPC, there is accommodated the inner drift chamber consisting of 10 layers of anode wire and cathode strip readouts . 64 segments TOF counters are between the 1 T superconducting solenoide and the TPC. The solenoid coil is surrounded by 4 layers of the barrel drift chamber and the barrel calorimeter consists of 4320 lead glass counters . The end cap calorimeter is a gas sampling calorimeter consisting of a 18X0 thick lead/proportional tube sandwich . The outermost detector is 4 layers of muon drift chamber interleaved with muon filters with a total thickness of 1 m. A schematic drawing of the TPC is shown in fig. 8b . The TPC is enclosed by the FRP (fiber reinforced plastic) pressure vessel with dimensions 2.6 m in diameter and 3 m in length . The vessel is divided into two drift cells by the central membrane to which HV is applied. Drifting electrons are detected at both
I (mA) /Beam
3 4 I (mA) /Beam
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6
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Fig. 6. The luminosity and the vertical beam-beam tune shift parameter as a function of current per beam .
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Fig. 7. A cross-sectional view of the VENUS detector.
K. Nakajima / Progress in accelerators and detectors at TRISTAN
Coarse Field tage TPC (Fine Field Cage
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Fig. 8. (a) A cut-out view of the TOPAZ detector . (b) A schematic drawing of the TOPAZ TPC.
ends of the cylinder with 16 sectors of multiwire proportional chambers . Each sector contains 175 sense wires and 512 cathode pads . The TPC is operated at 3.5 atm of P10 gas with a drift field of 353 V/cm in a 1 T magnetic field. The AMY detector [10] is represented in fig. 9. One of the most characteristic features of AMY is a 3 T superconducting solenoid which provides precise momentum measurements in a compact volume . The tracking system of AMY consists of 4 layers of straw chambers, and the central tracking chamber consists of 25 layers of axial- and 15 layers of stereo wires. The
transverse momentum resolution is as good as 0 .7%p r. The main AMY calorimeter is the barrel shower counter which is a highly segmented gas calorimeter consisting of 20 layers of plastic tube chambers with cathode pads, interleaved with 5 Xo thick lead sheets . This shower calorimeter gives a good y/m° separation. The muon detection system of AMY is a thick iron absorber and 4 layers of muon chamber. For the purpose of electron identification, a synchrotron X-ray detector is installed outside the central drift chamber. The SHIP detector [111 consists of 12 modules of a CR-39 laminated stack set around the collision point, I. FUTURE ACCELERATORS
12
K Nakajima / Progress in accelerators and detectors at TRISTAN
n-
Beam Pipe Distributed Ion Pump Small Angle Luminosity Monitor Inner Tracking Chamber Pole Tip Counter Central Dritt Chamber Trigger Counter Ring Veto Counter Shower Counter Superconducting Solenoid Coil Magnet Yoke/Hadron Absorber
Fig. 9. A schematic view of the AMY detector. and UG-5 glass placed inside the vacuum pipe. These etchable track detectors cover a solid angle - 0.9 x 41T sr and are shown schematically in fig. 10 . 5. Status of TRISTAN experiments Physics goals of TRISTAN experiments are: given - Search for new flavors;
Search for new heavy leptons; QED test ; Electroweak theory test ; Study of isolated leptons in hadronic events ; Study of two-photon processes; Search for Dirac magnetic monopoles. The integrated luminosities delivered to physics experiments in the first year are summarized in table 2 at each center-of-mass energy. Fig. 11 shows the integrated luminosity per day accumulated in the TOPAZ detector. Events collected in e+ e- annihilations are -
-
e + e - -> e + e - , 1'y, w + Ir - , T + T - , hadrons. Fig. 12 shows a typical event of multihadron production at = 52 GeV, observed in the TOPAZ detector . A
r
Table 2 Energies and integrated luminosities in the first year of TRISTAN experiments [GeV] Run period
Fig. 10 . Schematic representation of CR-39 and UG-5 detector configuration of the SHIP detector.
50 52 55 56 Total
May-June 1987 June-July 1987 Oct.-Dec. 1987 Jan.-March 1988
Integrated luminosities [pb -1 1 VENUS TOPAZ AMY 0.7 0.5 0.7 2.9 3.6 4.0 2.5 2.9 3.3 5.3 5 .6 6.1 11 .4 12 .6 14 .1
SHIP 0.8 4.0 4.0 7.5 16 .3
13
K Nakajima / Progress in accelerators and detectors at TRISTAN Integrated
Luminosity /Day in TOPAZ t-
T 400 a c p 300 w
is=5OGeV
T
T--52GeV
C_2 F -
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11 JUN 1987
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11 JUL
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11 NOV
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11 DEC
Date
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1 21 JAN 1988
11 21 FEB
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Fig. 11 . The integraded luminosity per day accumulated in the TOPAZ detector in the first year operation. 111111111111
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.
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1111111111111111111 A typical event of multihadron production at v~s = 52 GeV observed in the TOPAZ detector . Fig. 12 . typical result of TRISTAN is represented by R values, the ratio of the hadronic cross section to the QED [r+[,- cross section, as shown in fig. 13 . The physics results obtained from TRISTAN are summarized [12] : 1) e +e- -> e+ e-, yy and + W- processes are con sistent with the standard theory ; 2) The quark fragmentation model describes many features of the hadronic final states ; 3) R values are consistent with the standard theory of five flavor production ; 4) The top quark was not observed up to ~rs_ = 55 GeV. 5) More study is required on the existence of a charge - -' heavy quark. 6) No positive evidence was observed for the copious production of isolated leptons as was found by MARK-J group. 7) The lower mass bound for sequential leptons was given to be 26.8 GeV . 8) The upper limit on the cross section for the production of the Dirac monopole is 0.3 pb at 95% CL with a mass of 26 .1 GeV/c2.
R ratio 6Eiio - (JADE
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m= = 92.5Gev/cz sinZB, = 0.226 40
45 50 Center of Mass Energy (GeV)
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60
Fig. 13 . The ratio R resulting from the TRISTAN experiments and plotted together with the data from PETRA [161 . The solid curve shows the standard model prediction without the top quark. The dashed curve is the ratio R expected for the top quark. I. FUTURE ACCELERATORS
14
K. Nakajima / Progress in accelerators and detectors at TRISTAN
6. Upgrading programs of TRISTAN Upgrading programs [13] for energy and luminosity are in progress . The energy upgrade of the MR can be carried out by the installation of superconducting rf cavities into the NIKKO rf sections . In addition to the present conventional rf cavities, 16 superconducting cavities will be installed in October 1988 to increase the beam energy up to 31 GeV. Further 16 cavities will be deployed in April 1989 so that the total accelerating voltage should reach as high as 570 MV which gives a maximum beam energy of 33 GeV. The superconducting 5-cell cavity made of niobium sheets has been developed successfully in KEK as a consequence of R&D work over many years. We have already achieved a quality factor higher than 2 x 10 9 at 5 MV/m and a maximum accelerating field higher than 10 MV/m as shown in fig. 14 . Actual beam tests have been done in the AR to demonstrate stable operation with high field and high beam current. The liquid helium cryogenic systems consist of 16 cryostats each of which contains two 5-cell cavities, a 380 m long transfer line and a refrigerator system with 6.5 kW cooling power at 4.4 K. The luminosity upgrade is conceived as a minibeta scheme using superconducting quadrupoles (QCS) rather than the present low-beta scheme . The installation of QCS is scheduled in the spring 1990. The minibeta scheme allows us to reduce the beta functions up to 0.85 m horizontally and 4 cm vertically at the collision point, provided with 70 T/m field gradient of QCS. The luminosity should gain by a factor of 2.
radiation problems in high energy and high luminosity colliders. The beam background causes detectors to prevent a regular operation, to increase a trigger rate and a dead time, and to damage itself. In the straight section of the MR around each detector, the 4 fixedand 2 movable beam masks are installed to protect experiments from high synchrotron radiation fluxes and from off-momentum electron/positron backgrounds . Unfortunately, these masks were not always effective to reduce beam noise on the detector . According to our experience, a drastic improvement was brought about by expanding the diameter of the beam pipe inside the detector . The minimum diameter of the beam pipe should be 20 times larger than the maximum beam size around the collision point. Even the outermost muon chambers have suffered from beam backgrounds, probably caused by soft photons. This type of beam background has been decreased by one order of magnitude after hanging 2 mm thick lead sheets on the wall between the experimental hall and the accelerator tunnel . In a daily physics operation, the beam background depends on fine adjustments of the beam orbit, rf frequency shifts and improvement of the vacuum . An example of the beam background [14] is shown in fig. 15 . LUMI BKG RUN1882
2000 1500
1500
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7. Operational aspects on beam backgrounds and radiations
06
The accelerator performance and experimental situation are inevitably affected by beam backgrounds and
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8 1L 0 I2 BEAM CURRENT (mA )
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Fig. 14 . Q-values of superconducting five-cell cavities measured as a function of the accelerating field.
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LUMI BKG RUN1934
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Fig. 15 . The background, trigger rate, and dead time observed before beam tuning (a) and after beam tuning (b), as a function of the beam current in daily experimental runs of VENUS.
K. Nakajima / Progress in accelerators and detectors at TRISTAN K .P Lomóert ef cl .
15
-2 s -1 at the center-of-mass luminosity of 1 .4 x 10 31 cm collision energy of 56 GeV . The TRISTAN experiments give us important results on lepton and quark physics. They will contribute to opening up a new generation of physics while TRISTAN upgrading programs are in progress. Although we had been faced with serious situations as increased energy and luminosity, problems should be solved and situations improved .
Acknowledgements
Fig. 16 . Offset voltage of the operational amplifier as a func tion of the radiation dose . The data are compared with ref . [17] . Excessive radiation causes serious damage to many accelerator components . Examples have been found on parts of magnets, vacuum chamber components, cables, electronics parts and so on in only six months . The radiation dose in the TRISTAN tunnel is 10 4-10 5 Gy/A h around the beam chamber with a lead shield and _ 10 6 Gy/A h on the chamber surface for 27 .5 GeV beam energy. The radiation damage on the aluminum vacuum components was caused by HN0 3 produced from N02 and H 2O . Connectors and cables were damaged by HF produced by decomposition of Teflon insulators. Cooling rubber hoses of magnets were hardened by radiation exposure . The radiation damage for electronics components was tested [15] in the TRISTAN MR . It was found that silicon transistors were relatively endurable up to several 10 4 Gy while CMOS RAM and EPROM were quite vulnerable to the radiation . These ICs were broken at an exposure of 700 Gy . Fig . 16 show the influence of radiation dose on typical operational amplifiers . In order to make high energy and high luminosity possible, it is urged that radiation resistive materials and electronics should be devised and applied to the accelerator, and detectors as well as completed radiation shields are made . 8 . Conclusion We started the operation of TRISTAN on schedule after a five-year construction . We have achieved a
The author would like to express his sincere thanks to Prof. S. Ozaki and Y . Kimura for their guidance and support . He is very much indebted to Prof . T. Kondo for kindly encouraging him to present this talk at this conference. Special thanks are given to all of TRISTAN accelerator, VENUS, TOPAZ, AMY and SHIP groups for their assistance and offer of the latest information on their work.
References [1] TRISTAN project group, KEK report 86-14 (1987) . [2] G . Horikoshi and Y . Kimura, Proc . IEEE Particle Accelerator Conf., Washington (1987) vol. 1, p. 34. [3] A . Kabe et al ., ibid., vol. 3, p. 1648 . [4] T. Higo et al ., ibid ., vol. 3, p. 1945 . [5] H . Ishimaru et al ., Proc . 6th Symp . on Accelerator Science and Technology Tokyo (1987) p . 138 . [6] T . Ieiri et a] ., ref. [2], vol . 2, p . 729 . [7] S . Kurokawa et al., ref. [5], p . 233. [8] VENUS collaboration, TRISTAN proposal (1983) TRISTAN-EXP-001 . [9] TOPAZ collaboration, TRISTAN proposal (1983) TRISTAN-EXP-002 . [10] AMY collaboration, TRISTAN proposal (1984) TRIS TAN-EXP-003 . [11] SHIP collaboration, TRISTAN proposal (1985) TRISTAN-EXP-004 . [12] F . Takasaki, presented at Int . Symp. on Lepton and Photon, Hamburg (1987) KEK preprint 87-92 . [13] S . Isagawa, presented at 23rd Recontre de Moriond on Electroweak Interactions and Unified Theories, Les Arcs (1988) KEK preprint 88-9. [14] S . Uehara, private communication . [15] T. Momose, Europ. Part . Accel. Conf., Rome (1988). [16] H.J. Berend et al ., Phys . Lett . B183 (1987) 400 . [17] K.P. Lambert et al., CERN 75-4(1975) .
I. FUTURE ACCELERATORS