A Very Large Lepton Collider in a VLHC tunnel

Transcription

1 A Very Large Lepton Collider in a VLHC tunnel Tanaji Sen Fermilab, Batavia, IL Design strategy Intensity Limitations RF and Optics parameters: Arc, IR Lifetime Scaling the beam-beam parameter Luminosity, Energy reach Parameters of a 233 km ring, L = cm 2 sec 1, E = 185 GeV Low energy design at Z0 mass E cm = 90GeV Accelerator Physics Challenges e + e Workshop March 9, 2001

2 Design Strategy Use the maximum RF power available Operate at the beam-beam limit Synchrotron radiation power lost by both beams, P T = 2C γ E 4 I eρ C γ = (4π/3)(r e /(m e c 2 ) 3 ) = [m/gev 3 ]. Luminosity L = f rev M b Nb 2 4π σx σ y Vertical beam-beam tune shift (1) (2) ξ y = r e 2π N b β y γσ x σ y Replacing one power of bunch intensity, or Interpretations Lγ 3 =, σ y σ x (3) L = 1 ξ y 2er e βy γi (4) 3 ξ y P T 16πre 2(m ρ (5) ec 2 ) βy At fixed L, P T and ξ y E ρ 1/3 This determines the maximum allowable energy at these parameters. At fixed bend radius or circumference C, P T and ξ y L γ 3 At constant luminosity, the maximum energy E increases if C, P T, ξ y increase and β y decrease....

3 Intensity Limitations Bunch intensity limitations At top energy, the limit is set by the beam-beam interactions. Limits from the desired collisions are included in the design, there may be additional limits from parasitic collisions. At injection energy, the transverse mode coupling instability sets the limit. At the threshold the m = 0 and m = 1 modes of the betatron modes ω β + mω s become degenerate. Threshold bunch current I T MCI b 8f revν s E e i β i k i (σ s ) k i is a bunch length dependent transverse mode loss factor. At LEP TMCI limits the bunch current to below 1mA. I assume that similar bunch intensities as in LEP will be stable in the large ring but this may be optimistic... (6) Beam intensity limitations This is primarily determined by the available RF power. Cryogenic cooling power. The dynamic heat load on the cavities includes a contribution from the beam HOM power in cavities. P beam dynamic = 2R m(σ s )I b I e (7)

4 Basic Parameters Choose β x, β y (limitations determined by IR optics, bunch length) Determine the maximum energy E from L, β y, P T and ξ y (choice of ξ y has to be self-consistent with the energy) for a given circumference. Bunch intensity N b is set by TMCI limitations Choose a coupling ratio (determined by β y /β x ) Equilibrium emittance is found from ɛ x = N b γξ y κ = ɛ y ɛ x (8) r e 2π β y κβx where factors within () are assumed to stay constant. (9) Choose a phase advance per cell µ C (upper limit usually determined by chromaticity sextupoles). The cell length L C is determined by the equilibrium emittance ɛ x C q = 55 hc/(32 3(m e c 2 )) = [m] Filling factors f 1 and f 2 ( ) Cq R J x ρ [L c γ ] 3 2 (10) µ c R 3 R = f 1 C 2π, and ρ = f 2R, f 1, f 2 < 1 (11) R is the arc radius, ρ is the bend radius. Typically 1 < R/ρ Maximum number of bunches is determined by the beam power P T M max b = ( P T ρ ) 2C γ f rev N b E 4 (12)

5 RF parameters Requirements The RF must replenish the energy lost per turn. The RF must provide an acceptable quantum lifetime. Energy Gain E 4 ev RF sin φ s = U = C γ ρ The longitudinal quantum lifetime is determined by the energy headroom N QL = E RF /σ E as where 1 τ quant;s = τ s N 2 QL πhη slip ev RF EG(φ s ) = N QL (13) exp[ 1 2 N 2 QL ] (14) C q E 2 J s ρ m e c 2 (15) G(φ s ) = 2 cos φ s (π 2φ s ) sin φ s (16) Typically N QL 10. The two requirements determine the equation for the synchronous phase φ s and the RF voltage V RF. RF frequency The RF acceptance ( E/E) accep 1/ h so lower RF frequencies increase the acceptance. However high power klystrons are cheaper above frequencies of 300MHz. LEP operates with 352MHz. For this design we chose an RF frequency of 400MHz.

6 Arc parameters: phase advance and cell length Equilibrium emittance The emittance decreases as the phase advance increases, reaching a minimum at 135. In a lattice with FODO cells, but ɛ x (µ C x ) = 4C qγ 2 J x θ sin2 (µ C x /2) sin4 (µ C x /2) sin 2 (µ C x /2) sin µc x. (17) Stronger focusing increases the chromaticity and the strength of the chromaticity sextupoles which can limit the dynamic aperture. Typically 60 µ c < 120 For example, LEP has operated with (60, 60 ) at 45GeV, and since then (90, 60 ), (90, 90 ) and (102, 90 ) at higher energies. TMCI threshold Ithresh T MCI ν s β 1 cos( µ c L c 2 ) (18) The TMCI threshold increases if the cell length L C and phase advance per cell µ C decrease. Emittance control by changing the RF frequency. dj x dδ = dj s dδ = 4L D L Q sin2 µ C /2 sin 2 µ C /2 (19) L D : length of dipoles in a half cell, L Q : length of a quadrupole. Required RF frequency shift is related to the momentum deviation δ by f RF f RF = R R = α Cδ (20) Important to keep R small to minimize loss in physical aperture and transverse quantum lifetime, i.e. design J x / R to be large. This requires lower µ C and L Q /L D to be small i.e. weaker focusing. Example: C = 228km, L D = 103.7m, L Q = 4.1m, µ C = 90, α C = , J x R This is large enough to be useful. = 0.54 /[mm]

7 Beam-beam limit Beam-beam parameter at different coupling ratios Beam-beam limit ξ y = ξ x ξ y κ < β y β x ξ y > ξ x κ > β β ξ < y y x ξ x κ = β y β x N opt Bunch Intensity

8 IR Parameters Lower limits on β Set by the tolerable beam size in the IR quadrupoles and the chromaticity of these quadrupoles. With βy β x, aperture and chromaticity limitations will first arise in the vertical plane. βy σ s to prevent the luminosity loss due to the hourglass effect. β, coupling and the beam-beam limit Beyond the beam-beam limit, ɛ x I, ξ x, ξ y const. and L I. ξ x = κ βy ξ y, L ξ y (21) /β x If κ > β y /β x, then ξ x > ξ y, the beam-beam limit is reached first in the horizontal plane. ξ y never reaches its maximum value and since L ξ y the maximum luminosity is not obtained. So require κ β y /β x or ξ y ξ x. Optimal coupling: κ = β y /β x and ξ x = ξ y. If κ < β y/β x, the limit is reached at intensity N limit = 2πγɛ x r e N opt = 2πγɛ x r e ξ y (22) κ ξ βy/β x y < N opt (23)

9 Beam Lifetime Radiative Bhabha scattering process e + e e + e γ. τ L = 1 = N IP M b N b Lσ e+ e 2r e N IP β y ξ y 1 1 γξ y σ e+ e 1 γf rev (24) The cross-section σ e+ e depends on the energy acceptance and has a weak logarithmic dependence on energy. In the energy range from GeV per beam, σ e+ e 0.36 mbarns assuming an RF acceptance of 1%. Beam-gas scattering. Compton scattering off thermal photons. Total lifetime Example: LEP 1 τ = i 1 τ i (25) Process τ[hrs] Radiative Bhabha scattering 5.8 Compton scattering 60 Beam-gas scattering (pressure=0.6 ntorr) 80 Total 5.0

10 0.12 Scaling of the beam-beam parameter 0.11 Vertical beam-beam parameter VLLC34 LEP: 97.8GeV LEP: 65GeV VLLC LEP: 45.6GeV Damping decrement

11 450 Energy vs Circumference: synch. rad. power = 100MW Energy per beam [GeV] Luminosity=10 32 Luminosity= Luminosity= Circumference [Km] 100 RF Voltage vs Circumference: synch. rad. power = 100MW Luminosity=10 32 RF Voltage [GV] 10 1 Luminosity=10 33 Luminosity= Circumference [Km]

12 Bremmstrahlung Lifetime [hrs] Luminosity lifetime vs Circumference: synch. rad. power = 100MW Luminosity=10 34 Luminosity= Luminosity= Circumference [Km]

13 e + e Collider Parameters Energy [GeV] Luminosity Synch. radiation power(both beams) [MW] σx, σy [microns] , Number of bunches 126 Bunch spacing [km] Particles per bunch Bunch current [ma] Emittances [nano-m] 6.009, Beam-beam parameter Damping decrement Single beam current [ma] Brho [Tesla-m] Arc tune Phase advance per cell [deg] Dipole field [T] Focal length of cell [m] Quad gradient [T/m] Quad field at 1σx max /dipole field Cell: β max, β min [m] , Cell: σx max, σx min [mm] 1.524, Cell: σy max, σy min [mm] 0.341, Max apertures required [cm] 2.524, Max and min disp. [m] 1.117, Momentum compaction E-04 Energy loss per turn [GeV] Damping time [turns] 46 RF Voltage [GV] Synchronous phase [deg] Relative energy spread E-03 RF acceptance E-02 Synchrotron tune Bunch length [mm] Longitudinal emittance [ev-sec] Bremm cros-section [barns] Bremm lifetime [hrs] 23.6 Polarization time [hrs] Critical energy [kev] Critical wavelength [A] Number of photons/m/sec 0.115E+17 Gas load [torr-l/m-sec] 0.104E-07 Linear Power load(both beams) [kw/m] specif press. rise [Torr/mA] 0.138E-12 specific current [ma] spec. current/beam current

14 Comments on Parameters C(V LLC33)/C(LEP ) 8.5, P T (V LLC)/P T (LEP ) 7 L(V LLC33)/L(LEP ) 10 at almost double the energy. The e + e bremmstrahlung lifetime in VLLC33 is significantly longer at 23 hours. The beam sizes in the two machines are comparable. Hence vacuum chamber dimensions in VLLC33 can be similar to those in LEP if a two-ring machine (long-range interactions in a single ring machinemay require a larger aperture). B dip (V LLC)/B dip (LEP ) 1/5. Warm iron magnets will suffice. But good shielding from stray magnetic fields will be more important. The quadrupole gradient is determined by requiring that synchrotron radiation in quadrupoles be small. Require B quad (r = 1σ)/B dip = 1 (E. Keil). The critical energy is smaller in VLLC33 so shielding against synchotron radiation as in LEP should be adequate for VLLC33. The photon flux per unit length is almost the same in the two machines. The RF voltage required for VLLC33 is higher at 4.7GV compared to 3.1GV for LEP. f 1 = f 2 = 0.84 was chosen to have the same ratio ρ : C/(2π) as in LEP. A more aggressive choice of 2πρ/C = 0.81 yields e.g. maximum energy E max = 193GeV, RF voltage V RF = 4883MV. We chose optimum coupling, i.e. ɛ y /ɛ x = βy /β x 0.05 which ξ x = ξ y. If we reduce it to ɛ y /ɛ x = 0.025, then ξ x = 0.071, ξ y = 0.1. Optics and beam size parameters change, e.g. ɛ x = 11.8nm, cell length=278m, β max = 475m, Dx max = 1.72m, σx max = 2.4mm, ν s = 0.156, σ l = 8.1mm. The RF voltage increases to 4780MV, most other parameters are relatively unaffected. We chose N QL = 10 to ensure sufficient quantum lifetime. At LEP N QL = 6.6. If we assume this value for the 228km ring, the RF voltage is lowered from 4.66GV to 4.43GV.

15 Comparison of parameters Parameter Bunch current I b = 0.1mA Bunch current I b = 0.05mA κ = βy/β x κ = 0.5βy/β x κ = βy/β x κ = βy/β x κ = βy/β x ξ y = 0.1 ξ y = 0.1 ξ y = 0.08 ξ y = 0.1 ξ y = 0.08 Energy [GeV] Emittances ɛ x, ɛ y [nm] 6.1, , , , , 0.2 Number of bunches τ bremm [hrs] Arc tune Cell Length [m] Arc σx max, σx min [mm] 1.53, , , , , 0.5 Synchrotron tune ν s The luminosity is cm 2 sec 1 in each case. Ring circumference is 233 km and the synchrotron radiation power is fixed at 100 MW.

16 Low Energy Operation At low energies, the ring is not limited by available power only constrained by the beam-beam tune shift L = π r 2 e M B f rev [ σ xσ y (β y )2]γ2 ξ 2 y = π r 2 e M B f rev [ κβ x (β y) 3]1/2 γ 2 ξ 2 y ɛ x In this regime, Luminosity increases with the emittance L ɛ x This requires filling the aperture at these energies. Sufficient physical aperture Aperture 10 [σ 2 x + (D x δ 2 p)] 1/2 + 1cm(c.o.d) (26) The bunch intensity is low in order to limit the beam-beam tune shift. Number of bunches have to be increased to increase the luminosity. What is the minimum bunch spacing at these intensities? Assume (without justification) a minimum spacing of 5m.

17 Low Energy Design Strategy Increase the emittance by lowering the phase advance per cell µ C. The emittance Find the smallest phase advance so that ɛ x = ( C q J x R ρ )[ L C Rµ C ] 3 γ 2 10 [σx 2 + (Dmax x δ p ) 2 ] 1/2 + 1cm Aperture Find the bunch intensity from the beam-beam tune shift Check that N b N T MCI b N b = ( 2π r e κ )γɛ βy x ξ y /β x Find the number of bunches M B from the minimum bunch spacing M B f rev = c S B Luminosity L = π r 2 e M B f rev [ κβ x (β y) 3]1/2 γ 2 ξ 2 y ɛ x Alternative strategy of increasing the emittance: Double the cell length by turning off half the quadrupoles.

18 Aperture= 10*[σ x 2 + (Dmax δp) 2 ] 1/ [cm] Circumference=233km, Energy = 45 GeV Bunch spacing =5 m Cell Length = m Phase Advance [degrees] Aperture Emittance Emittance [nm]

19 Luminosity [times ] cm -2 sec Circumference=233km, Energy = 45 GeV Phase Advance [degrees] Luminosity Power Bunch spacing =5 m Cell Length = m Beam Power [MW]

20 Circumference=233km, Energy = 45 GeV 4 1 Quad Gradient [T/m] Gradient RSBC parameter Bunch spacing =5 m Cell Length = m RSBC parameter Phase Advance [degrees]

21 Luminosity [times ] cm -2 sec Circumference=233km, Energy = 45 GeV L C = m L C = m Bunch spacing =5 m Phase Advance [degrees]

22 Energy [GeV] Luminosity Synch. radiation power(both beams) [MW] σx, σ y [microns] , Number of bunches Bunch spacing [km] Particles per bunch Bunch current [ma] Emittances [nano-m] , Beam-beam parameter Damping decrement Single beam current [ma] Brho [Tesla-m] Arc tune Phase advance per cell [deg] Dipole field [T] Focal length of cell [m] Quad gradient [T/m] Quad field at 1σx max /dipole field Cell: β max, β min [m] , Cell: σx max, σx min [mm] 4.011, Cell: σy max, σy min [mm] 0.897, Max apertures required [cm] 5.011, Max and min disp. [m] , Momentum compaction E-03 Energy loss per turn [GeV] Damping time [turns] 3216 RF Voltage [GV] Synchronous phase [deg] Relative energy spread E-03 RF acceptance E-02 Synchrotron tune Bunch length [mm] Longitudinal emittance [ev-sec] Bremm cros-section [barns] Bremm lifetime [hrs] Polarization time [hrs] Critical energy [kev] Critical wavelength [A] Number of photons/m/sec 0.430E+18 Gas load [torr-l/m-sec] 0.387E-06 Linear Power load(both beams) [kw/m] specif press. rise [Torr/mA] 0.336E-13 specific current [ma] spec. current/beam current 15.41

23 Luminosity[1.E33/(cm 2 -sec)] Circumference=233km, synch. rad. power = 100MW ξ y = Luminosity RF Voltage ξ y = Energy [GeV] RF Voltage [GV]

24 Stored Energy, Site Power Stored energy per beam LEP (E=98GeV, I=2.88mA, frev=11.25khz) Stored energy 25kJ VLLC (E=185GeV, I=12.4mA, frev=1.315kh) Stored energy 1.74MJ VLHC (E=7TeV, I=0.53A, frev=11.25khz) Stored energy 334MJ Stored energy is not large in comparison to hadron colliders. Site Power requirements Beam power 100MW. Assuming a klystron efficiency of 50%, this requires a wall plug power of 200MW. Cryogenic cooling power 100 MW (?). 100 MW of heat has to be extracted by cooling water. Some fraction of this power will be required to pump the water. Power requirements are significant.

25 Accelerator Physics Challenges Combating TMCI The transverse impedance of the beam pipe alone is close to the threshold impedance. Other major contributions to the impedance from cavities, bellows,... will likely increase the impedance to beyond threshold. Possible solutions Coalescing bunches at top energy Feedback system Raising the injection energy Increasing the bunch length and the synchrotron tune.... If a single ring machine, then long-range beam-beam interactions with many bunches will limit the beam stability. Will likely affect the required physical aperture. With many bunches, multi-bunch instabilities may be an issue. Avoiding synchro-betatron resonances e.g. those driven by dispersion in the cavities. The beam-beam limit may not increase with damping decrement as hoped for. In that case, achievable luminosities will be lower. At low energy (45 GeV), the beam current is high ( 2 A) and the number of bunches is very large (about 47000). Operation in a single ring machine at these parameters will be extremely difficult if not impossible and will be challenging even in a two ring machine of this circumference....