Barber, D P

Chojnacki, E

Derrick, Malcolm

Friedsam, H

Gorsky, A S

Hanuska, S

Jagger, J M

Krakauer, D A

Norem, J

Rotela, E R

Sen, T

Sharma, S

Teng, L

Thompson, K

CERN Document Server

AN ep COLLIDER WITH Ecm = 1 TeVIN A VLHC BOOSTER TUNNELM. Derrick, H. Friedsam, A. Gorski, S. Hanuska, J. Jagger, D. Krakauer,   J. Norem    , E. Rotela, S. Sharma, L. Teng, K. Thompson, ANL;  T. Sen, FNAL; E. Chojnacki, Cornell;   D. P. Barber, DESYAbstractThe low field option for the VLHC includes a 3 TeVproton booster with a circumference of 34 km.  We arestudying the option of an electron ring to fit in this tunnelwhich can produce ep collisions with a luminosity of 1 fb-1/yr with a center of mass energy of 1 TeV.  The machinewould utilize superconducting rf and small low fieldmagnets for the ~80 GeV electron beam.  We describe thevacuum chamber / magnet system, rf power supplyrequirements, vacuum chamber cooling, interactionregions and installation of the facility in the tunnel, aswell as provide preliminary estimates of beam stabilityand lifetimes.1 INTRODUCTIONThe present studies for the Very Large Hadron Collider(VLHC) [1] consider both high (10-14 T) and low field (2T) options for reaching 100 TeV in the center of mass.The low field option includes a 34 km circumferencetunnel for a 3 TeV booster.  We are considering an 80GeV electron ring in this tunnel to produce an ep colliderwhich could extend the operating range for ep collisions toÖ s = 1 TeV.  If this machine utilized existing detectorsand some of the 352 Mhz superconducting cavitiesavailable after LEP was decommissioned, the cost couldbe considerably reduced.  This machine could producephysics during the construction of the large VLHCcollider ring.We have assumed that the minimum requirements ofsuch a machine would have: 1) sufficient luminosity toproduce 1 fb-1/y, 2) the ability to collide e+ and e-, 3)useful polarization, 4) adequate beam lifetime, 5) detectoraccess to the maximum range of momentum transfer Q2,and, 6) Ö s ~ 1 TeV.  In addition, the priorities of thismachine seem to imply operation with or before, ratherthan after, the LHC.We have looked at issues which would affect the costand performance of the machine with the aim ofdetermining its feasibility.2 BASIC PARAMETERSThe parameters of the machine determined by fixingsynchrotron radiation loss, and the b *’s for the protonbeam at the interaction point and limiting the beam beamtune shifts, x , for the proton and electron beams, are given---------------------------------------------* e-mail:  norem@hep.anl.govin the following table.  We have assumed that the protonring would be the low field VLHC injector described in[2], the injector chain would be located at Fermilab, andthat the synchrotron radiation power is 50 MW.PARAMETERSCircumference 33962.13 mep Luminosity 2.6 × 1032 cm-2s-1Center of Mass energy 1 TeVElectron energy, Ee ~80 GeVProton energy, Ep 3 TeVSynchrotron radiation Power 50 MWElectron ring dipole field 0.009 - 0.06 TRF frequency in electron ring 352 MHzBunch Spacing 100 nsNumber of bunches 1140Arc Cell length 100 mBend Radius 4451 mb max/ b min in cell 171/29 mb*p,x/ b * p,y, b * e,x/ b * e,y 2/0.5, 0.115/0.115 mx e,x/ x e,y, x p,x/ x p,y 0.011/0.021, 0.0014/0.0068Equilibrium emittance 28 nm-radElectron Beam current 55.3 mAProton beam current 200 mAprotons/electronsperbunch 1.25 × 1011/3.43 × 1010Energy loss per turn 0.814 GeVelectron rf Voltage 1.09 GVelectron energy spread, s E/E 1.03 × 10-3Proton emittance, e RMS,N 3.6 × 10-6 mm-mrExpected luminosity lifetime ~20 hrSokolov-Ternov polarization time 0.9 hrA detector similar to the HERA/ZEUS or H1 would haveadequate range for the majority of experiments planned onthis machine, with smaller detectors for low Q2 events.The electron ring would be located in the tunnelabove the proton ring, and would be installed at the sametime.Figure 1, The collider rings in the tunnel.0-7803-5573-3/99/$10.00@1999 IEEE. 2635Proceedings of the 1999 Particle Accelerator Conference, New York, 19992.1 Lattice DesignOur primary design criteria are a low beam energy spread,a low emittance beam to get a small spot size at the IPand a large bending radius to reduce the synchrotronradiation power. The electron ring contains two 180 degreearcs connected by 1.8 km straight sections.  The arcscontain half cells with 46 m long dipoles, 1.5 m quadsand 2.5 m straights.  A phase advance  of 900 is chosen inthe horizontal plane to get an equilibrium  emittance of 28nm-rad.  The phase advance in the vertical plane will bechosen to minimize the  chromaticity burden  and ensuresufficient dynamic aperture. We have assumed anemittance coupling ratio of 25 % to equalize the beamsizes of the protons and electrons at the IP. The Sokolov-Ternov polarization time for electrons is sufficiently shortto produce polarization, but the large tune spread anddensity of depolarization resonances may require snakes tomake the spin tune independent of energy.A preliminary design of the triplets used to focus theelectrons to the  required spot sizes  at the IP suggests thatsuperconducting quadrupoles will be required for  theinteraction region. The smallest b * achievable isdetermined by the available  gradients, the   distance to thefirst quadrupole and the maximum value of b  in thesequadrupoles.   In order to ensure a good quantum lifetime,the magnet aperture must be greater than 10 s .  With thevalue of b * shown in Table I, the first quadrupole mayhave to be placed as  close as 1m and no further than 3mfrom the IP. The focusing of the  protons will start afterthe two beams are sufficiently separated so that theelectrons will not be  subject to the fields of the protonmagnets. The design could be similar to that of theinteraction region  in HERA.2.2 LifetimeThe electron beam lifetime has been calculated withthe inclusion of the following effects: i) residual gasscattering with an initial pressure of 10-9 Torr  togetherwith synchrotron radiation induced desorption assuming adesorption coefficient  of 2x10-9, ii) e-p bremsstrahlung atthe single IP, iii) quantum lifetime assuming a physicalacceptance of 10 s in the transverse planes and an RFacceptance of 10 s E, iv) Touschek  scattering. These foureffects lead to an electron lifetime of 26 hours.  Othereffects not  included such as ion trapping, effects of orbitdistortion, larger  emittances and energy spread  on thequantum lifetime, scattering off thermal photons etc. willreduce  the lifetime somewhat from this value.3 MAGNET / VACUUM CHAMBER3.1 Vacuum ChamberThe vacuum chamber aperture is determined by requiring10 s  plus 0.2 cm for closed orbit distortion in bothhorizontal and vertical planes.  We have used anantechamber to increase the pumping conductance alongthe length of the chamber. The vacuum chamber is madeof 6063-T5 aluminum extrusions with the profile shownin Fig. 2  There are 2 channels included to heat thechamber during bake-out with pressurized, hot water.  Thelength of the chamber sections between bellows is limitedby the thermal expansion and other effects to about 9 m.With a chamber section anchored at the center of a dipolesection,  both ends of the chamber expand about 2 cmduring bake-out.  The ends of the chamber sections areconnected to 20-cm diameter, stainless steel vacuumflanges with bimetallic transition pieces.  The transitionpieces on the down stream ends also contain simple watercooled absorbers in the aluminum parts to intercept thesynchrotron radiation that would strike the walls andbellows that are located in the sections just beyond thedipole sections.The vacuum pumps for the ring are 30 l/s ion pumpslocated at the ends of each dipole section.  Theconductance of the chamber is sufficient to give pressures<10-9 torr at the centers of the chamber sections includingthe effects of both thermal and photo desorption.Provisions are included in the chamber profile to permitthe use of NEG strips.  This may allow the use of smallerand fewer numbers of ion pumps.3.2 DipolesThe electron ring must operate in close proximity to theproton ring, which produces peak fringe fields that arelarge compared to the field strength in the dipole atinjection (90 G).  The electron ring, therefore, is locatedinside at least a single layer, magnetic shield as shown inFig. 2.  The shield also provides the support structure forthe ring.  The choice of thickness and the number oflayers in the shield depend on the separation between thetwo rings.  A single layer of 6-mm thick low carbon steelhas been calculated to reduce the fringe field strength toless than about 1 G near the center of the electron dipolewhen located at a distance of about 2 m from the center ofthe proton dipole.   The core of the electron dipole isstacked from 1.5-mm thick low carbon steel laminations.Each dipole has a water cooled, copper coil consisting ofone turn per pole.Figure 2.  The magnet, vacuum chamber and support.2636Proceedings of the 1999 Particle Accelerator Conference, New York, 1999Each of the 608 lattice dipoles is made up of 5 sections.The sections have effective lengths of 8.96 m with 30-cmspacings.  Each dipole section has a 3.5-cm gap heightand produces central fields ranging between 0.009 T and0.0616 T.  The conductor size is 1.55 cm by 1.0 cm witha 0.66-cm diameter hole.  The peak current through thedipole is 876 A resulting in peak power losses of 4.3 kWper section for a total of 6.6 MW for all the dipoles.Using a single cooling circuit in a section, a water flow of0.1 l/s is required.  This gives a temperature rise of 10 ° Cin the water and a total flow of 160 l/s for the dipoles inthe ring.3.3 QuadrupolesEach of the 608 lattice quadrupoles has an integratedstrength of 7.545 T, a bore radius of 2.5 cm, and coresstacked from 1.5-mm thick low carbon steel laminations.3.4  Vacuum  Chamber  CoolingMost of the radiated synchrotron power is absorbed onthe outside radius wall (right inside face in Fig. 2) of thechamber with an imbedded water channel used for cooling.The average power distribution of 1.64 kW/m through the180 °  arcs gives 14.7 kW over the length of a chambersection. A water flow of 0.18 l/s gives a 20 ° C watertemperature rise across a cooling channel and a total flowof 552 l/s(8750 gal/min).  The heated water would becollected in pipes which would be located underneath thefloor and these pipes would discharge into the two injectortunnels and then into the cooling ponds.4 RF SYSTEMSuperconducting rf is required to obtain high luminositiesat high energies, since the losses in normal rf cavitieswould require reduction in the electron beam current.  Theavailable luminosity at high energies is shown in Fig. 3.00.20.40.60.8180 85 90 95 100 105 110 115 120Scaled LuminosityEnergy (GeV)Luminosity scaling with energyFixed number of cavitiesFixed peak voltage per cavitySuperconducting RFFigure 3, Luminosity for different rf options.We require 1.1 GV of rf to maintain the electron beam andwe assume this would be at 352 MHz.  The cavitieswould be located in the long straight section opposite thedetector.  Minimizing the cost of SRF cavities is obtainedby optimizing the RF power per coupler, or the numberof cells per cavity. We are looking at: 1) geometrychanges to spoil multipactor resonances, 2) hightemperature in situ bakable vacuum seals on RFcomponents for high temperature bakes, and, 3) in situRF processing utilizing a variable standing-waveapparatus to scan peak electric field over the couplerregion.  These improvements should allow delivery ofbeam power per coupler to levels limited by RF sourcesand vacuum windows, about 1 MW at 352 MHz. The use of the LEP rf would require rebunching theprotons at injection into the 3 TeV ring. We assume theelectrons occupy one bucket in 35 at the LEP frequency(1140 bunches), and proton rf operates at 1/7 of the leptonrf, i.e. ~50.3 MHz.  Proton rebunching should be doneadiabatically, initially at 10 MHz. Rebunching andacceleration would be done at 50.3 MHz, after furtherbunching this frequency.5  INJECTOR CHAINIn addition to the present Fermilab injector, a new e+ / e-linac would be required, which would include anaccumulator ring for positrons.  The present Booster andMain Injector could be used up to energies of 4.5 and 10.5GeV, respectively, with existing rf.  The Booster wouldrequire a lattice correction package to adapt the combinedfunction lattice to electrons. Present positron sourcesproduce ~9 × 1010 e+/sec, and the collider requires 3.26 × 1013e+ , so e+  production would take 6 min.  An electroninjector could look like Figure 4.Main Inj  to 10.5 GeVLinac + accumulator ringBooster to 4.5 GeVFigure 4.  An electron injector chain.6  CONCLUSIONSWe have done a preliminary study of an ep collider thatcould be installed in the low field booster of the VLHC.This machine could be operational before the LEP/LHCand would have a higher luminosity than HERA/TESLA.REFERENCES[1]  Recent information is updated at http://vlhc.org[2]  S. Mishra,VLHC Information Packet, FNAL (98)2637Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

An ep collider with $E_{cm} = 1$ TeV in a VLHC booster tunnel

An ep collider with $E_{cm} = 1$ TeV in a VLHC booster tunnel

Abstract

Similar works

Full text

Available Versions

CERN Document Server

An ep collider with Ecm=1E_{cm} = 1Ecm​=1 TeV in a VLHC booster tunnel

Abstract

Similar works

Full text

Available Versions

CERN Document Server

An ep collider with $E_{cm} = 1$ TeV in a VLHC booster tunnel