RF and feedback for bunch shortening CARE HHH-APD WS CERN, 8-11 Nov 2004 J. Tückmantel, CERN Abstr.: The motivation for, possibilities and limitations of bunch shortening by an additional superconducting RF system at 1.2 GHz supplying 43 MV in LHC are examined. Possible technical realizations are examined and critical issues shown

Contents Why shorter bunches, a heuristic sketch Bunches in Buckets: which RF system System parameters Hardware considerations Summary: what is ready to use, needs engineering or strong R&D effort J. Tückmantel, 8 Nov 04

bunch shortening : 1/2 w. resp. Desired: bunch shortening : 1/2 w. resp. 2.5 eVs, 16 MV 400 MHz -> 1.08 ns J. Tückmantel, 8 Nov 04

Invest money elsewhere: • Keep ‘good’ intra beam scattering lifetime: -> blow up bunches to 2.5 eVs -> 1.08 ns (‘party-line’) • Beam captured at 1 eVs (confident estimate, wait for 2007) • If we understand LHC/SPS better, maybe 0.6-0.7 eVs ( wait 200.) 1.0 eVs -> 0.66 ns = 60% of 1.08 ns 0.7 eVs -> 0.55 ns = 50% of 1.08 ns -> goal reached ( but at 0.7 eVs) Invest money elsewhere: To be studied Is beam still stable ??? Intra beam scattering ??? J. Tückmantel, 8 Nov 04

How to shorten bunches ? Increase RF voltage slope Increase voltage of (existing) 400 MHz (brute force method) 4th-power ‘law’ (*) : for 1/2 b.l. 8 cavities ->16 * 8 = 128 cavities Impedance increase: factor 16 Cost factor 16; no space in ring (& power converters) anymore (technology copy/paste) • Add system at higher frequency The wavelength has to be longer than bunch length The cut-off tube diameter has to be ≥ 5 cm (standard LHC pipe) (in RF section as planned pipe diameter is 10 cm !!!) RF manipulations -> multiple of basic RF frequency: n*400 MHz --> 1200 MHz ( -> voltage: 43 MV ) (*) short bunches J. Tückmantel, 8 Nov 04

2.5 eVs: 16 MV @ 400 MHz (standard for IBS) J. Tückmantel, 8 Nov 04

Brute force: 250 MV @ 400 MHz, 2.5 eVs J. Tückmantel, 8 Nov 04

(only 43 MV @ 1.2 GHz: bucket too small !!) ±2440 MeV 2.5 eVs: 16 MV @ 400 MHz + 43 MV @ 1.2 GHz (only 43 MV @ 1.2 GHz: bucket too small !!) J. Tückmantel, 8 Nov 04

Reduced emittance: 1.75 eVs ??? IBS ??? J. Tückmantel, 8 Nov 04

Brute force: 120 MV @ 400 MHz, 1.75 eVs J. Tückmantel, 8 Nov 04

Reduced emittance : 1.75 eVs, double RF system: tight ±2120 MeV Reduced emittance : 1.75 eVs, double RF system: tight J. Tückmantel, 8 Nov 04

How to get to 7 TeV/c Bunches of 1.75 eVs (2.5) with 400 MHz system only too long ≈0.9 ns (1.1) to be captured by 1.2 GHz system (‘wave length’ 0.833 ns) -> Transfer to 1.2 GHz has to be done before reaching the flat-top Before start acceleration, in parallel: - Ramp 400 MHz from 8 MV (capture) to 16 MV - Ramp 1.2 GHz form 0 MV (enforced) to 5 MV (‘abuse’ as Higher Harmonic System) • Accelerate to ≈4.5 TeV/c, bunches shorten for higher Eb • In parallel to continued acceleration to 7 TeV/c: - Ramp 1.2 GHz to 43 MV - (smooth) blow up of bunches -> 1.75 (2.5) eVs --> Movie J. Tückmantel, 8 Nov 04

<- Bunch shortening Open parenthesis Bunch lengthening (for different Machine condition + crossing angle) J. Tückmantel, 8 Nov 04

Bunch 'lengthening' (Piwinski): Bunch 4 eVs: 3 MV @ 200 MHz, -1MV @ 400MHz (HH) -> 2.7 ns Ramp to 7 TeV/c as usual, then transfer + 2.5 -> 4 eVs J. Tückmantel, 8 Nov 04

Bunch shortening -> Close parenthesis Bunch shortening -> J. Tückmantel, 8 Nov 04

Which cavity to choose ? • 2 independent RF systems, 1 per beam (as 400 MHz) Superconducting: - high gradient -> less additional impedance - as for 400 MHz system: beam gaps & reactive beam loading compensation -> low R/Q, high V best The 400 MHz sc. cavity was designed for high currents: HOM modes (TM011) ‘tuned’, good HOM propagation into beam tubes, HOM coupling scheme can be scaled --> scale 1:3 (& adapt) 400 MHz cavity J. Tückmantel, 8 Nov 04

Scaling of 400 MHz cavity • Assume 25 MV/m (*) -> 3.1 MV/cav -> (14 ->) 16 cavities at 2.7 MV do the job • R/Qfundam. (identical) = 45 W • long. HOMS (400 MHz calc. and f-extrapolated 3:1) (assume Qext,damp as measured for 400 MHz): 2350 MHz 1.5 W*270 -> 0.4 kW/cav 2700 MHz 9 W*270 -> 2.4 kW/cav 3200 MHz 4.5 W*1000 -> 4.5 kW/cav 3300 MHz 2 W*400 -> 0.8 kW/cav -> 8 kW/cav + + + + (some more) (problem: cell to cell coupling …) (*) 400 MHz: 5.5 MV/m -> 2 MV/cav J. Tückmantel, 8 Nov 04

Fundamental mode impedance: Qext,max = 200000, R/Q = 45W -> bare Z = 9 MW/cavity (16*9 MW = 144 MW) Fast RF vector feedback: delay in loop T=600 ns (tunnel layout) gain limited to half of ginst= Qext/(2 fRF T) -> ZFB= 4 (R/Q) fRF T = 130 kW / cav (16*130 kW = 2.08 MW) The 1-turn-delay F.B. may give another factor 3 … 5 (comb profile) : use coming experience of nominal LHC with 400 MHz J. Tückmantel, 8 Nov 04

TESLA Technology at 1.3 GHz 25 MV/m reliably in ‘machine’ (more was done: LHC needs ‘absolute’ reliability !!) Bulk Nb cavities, run at 2 K (not 4.5 K) Q0 ≥ 1010 TESLA is pulsed: 5 (or 10) Hz with 1.4 ms RF-on time per pulse -> average LHe cons. limited LHC in CW : ≈ 250W dynamic load @ 2K -> cryostat through-put !! TESLA uses 9-cell cavities -> LHC needs 1-cell cavities (fast RF feedback !!) J. Tückmantel, 8 Nov 04

RF power : variable coupler Qext To make cavity ‘invisible’ (e.g. injection), lowest Qext -> Qext,min = 10,000 (else antenna ‘scratches’ beam) Detune (far away) cavity, possib. without power ? -> Impedance is still somewhere -> CB instabilities -> At one instant cavity has to be brought to tune • 400 MHz system Qext-stroke = factor 20 Qext,max = 200,000 J. Tückmantel, 8 Nov 04

Beam current options Full B.L. = 4*3.8 cm = 15.2 cm At 1.2 GHz RF current reduced (cos2) by 0.78 with respect to point bunches • (n)ominal: 0.56 A (DC) (u)ltimate: 0.85 A (DC) (s)uper: 1.41 A (DC) Keep cavity ‘invisible’ on tune (force V=0): -> n: 43 kW; u: 100 kW; s: 275 kW per cavity J. Tückmantel, 8 Nov 04

Necessary RF power (for V=2.7 MV) current Qext,opt (half-detuning) <P> (*) (full control) P (adapting FB) (+) Q=Qmax n 135000 300 kW … -> 100 kW u 90000 450 kW … -> 100 kW s 55000 750 kW … ->100 kW (*) Power spikes: about 50% more installed power !! (+) OK in theory and ‘honest’ simulation program (with 400 MHz system) Will be very delicate to handle in reality ( … beam loss …) -> No guarantee, has to be tried out in truly existing LHC J. Tückmantel, 8 Nov 04

RF Power converters Info Erk Jensen (no exhaustive research) > TESLA MBK's typically 150 kW CW (1.3 GHz) Raytheon's "Amplitron": 425 kW CW (3 GHz) 76 % efficiency (for ‘s’ also the 400 MHz needs an upgrade above 300 kW !!) J. Tückmantel, 8 Nov 04

The Power coupler Problem: sc. cavities may reflect full power (e.g. when fast vector feedback wants to lower field) On coupler (and transmission lines) fields may double i.e. local power may be 4 times nominal one [ average power - thermal - OK, but arcing ] 400 MHz system: Pnom = 300 kW, peaks 1.2 MW: Tests show it should work, but at limit, long processing 1.2 GHz: already for ‘nominal’ we need for full control more than 300 kW at 1.2 GHz J. Tückmantel, 8 Nov 04

LHC 400 MHz single cell cavity (Nb/Cu technology) J. Tückmantel, 8 Nov 04

High field: bulk Nb, TESLA technology (1.3 GHz) J. Tückmantel, 8 Nov 04

J. Tückmantel, 8 Nov 04

J. Tückmantel, 8 Nov 04

400 MHz sc. cavity module with 4 cavities Main coupler HOM coupler power outlet Main RF inlet (door knob) accelerated beam ‘guest’ beam 400 MHz sc. cavity module with 4 cavities J. Tückmantel, 8 Nov 04

LHC 400 MHz 4-cavity module (mirrored..) J. Tückmantel, 8 Nov 04

Challange: HOM outlet lines, 'density' of wave-guides and couplers Each cavity has 1 MC (>300 kW) 4 HOM couplers, 1 tuner, … Challange: HOM outlet lines, 'density' of wave-guides and couplers J. Tückmantel, 8 Nov 04

Challange: Cryo insulation (+ see above) Each cavity has 1 MC (>300 kW) 4 HOM couplers, 1 tuner, … Challange: Cryo insulation (+ see above) J. Tückmantel, 8 Nov 04

Technology Summary: Ready: Cavity design: scale 400 MHz cavity - HOM coupling scheme: scale (HOM power ?? ) 1-cell sc. cavities at 1.2 GHz, bulk Nb : TESLA technology 25 MV/m: reliable Cryo supply at 2 K (TESLA, CEBAF, … LHC) Tunnel length needed : 15-20 m (16 cav parall.) J. Tückmantel, 8 Nov 04

• No fundamental problems (but to be engineered/checked): Cryostat with a multitude of ‘outlets’ (‘hedgehog’) cryo-losses ? mounting a ‘crowded’ RF supply system ? Allocation of power converters, wave guide passage into tunnel (radiation !!) –––––––– Check beam-stability issues -Check intra beam scattering issues (2.5 -> 1.75 eVs) J. Tückmantel, 8 Nov 04

(possible show-stoppers): • R&D challenges (possible show-stoppers): RF power converter 300 - 750 kW CW at 1.2 GHz (combining smaller ones ugly and expensive…) Main coupler 300 - 750 kW CW at 1.2 GHz (superimposed: spikes of up to 4 times this value) Diameter 1/3 of known 400 MHz/300 kW system: 3 * field-strength, 9*√3 ≈ 15.5 power density and finally …. Run LHC with double RF system with fast vector FB without losing coasts too often (integrated luminosity counts !!) J. Tückmantel, 8 Nov 04