A CW Linac scheme for CLIC drive beam acceleration. Hao Zha, Alexej Grudiev 07/06/2016

CLIC drive beam linac design (Pulse mode) Drive beam Linac IP 21.5 Km 540 Klystrons, 22 MW, 140 us 2.37 GeV, 4.2 A IP 3.5 Km Drive beam Linac 540 Klystrons, 22 MW, 35 us …… Feed PETs of positrons linac Time Stored RF power in the structure 140 us 20 ms 10 GW Time Stored RF power in the structure 20 ms 35 us 10 GW 35 us 10 GW Final 3 TeV Scheme : Normal conducting structure operated at pulse mode 375 GeV Scheme: Shorter pulse length but same power): means need same accelerator length and klystron amounts (~ 1.2 Billion $ per linac).

CLIC drive beam linac design (CW mode) Drive beam Linac IP 21.5 Km 200 Klystrons, 0.35 MW, CW 2.37 GeV, 4.2 A IP 3.5 Km Drive beam Linac 50 Klystrons, 0.35 MW, CW …… Feed PETs of positrons linac Time Stored RF power in the structure 35 us 20 ms 17.5 MW Time Stored RF power in the structure 140 us 20 ms 70 MW Release energy (Discharging) Store energy (Charging) Release energy (Discharging) Final 3 TeV Scheme: Superconducting structure run at CW mode (but beam comes at pulse mode) like a battery (Super capacitors). Less amount of power source and no modulator. 375 GeV Scheme: Need less energy storage. Less power source and shorter structure length (possible lower cost). Saved money/ 40000

Big challenge: Beam loading effect! Energy filling time (0.14 ms) >> releasing time (20 ms); Filing power (70 MW) << Beam power (10 GW); Cavity stored energy (voltage) dropped due to strong beam loading during beam time; We need a flat energy distribution (variation < 1%) of drive beam for the acceptance of combiner ring. Cavity stored energy 70 MW 10 GW Time Energy variation

Parameters in beam loading study Cavity stored energy Time : Maximum stored energy in the cavity (Capacity) : Beam (in a pulse) absorbed energy: here 1.4 MJ represents how many cavities do we need (cost). : Stacked beam voltage (Maximum beam loading) : Required accelerating voltage: here 2.37 GeV represents which cavity do we need (geometry). Two parameters are defined:

Parameters for a reasonable cost

1. Regular accelerating structure Time Normalized accelerating voltage Full pulse length (140 us) Not applicable Go back

2. Structure with frequency shift (to beam frequency)

2. Parameters for CLIC-DB- 3TeV Real Imag 2 ** This plot is only for demonstration (here energy variation limitation is 10%) Not applicable Go back

3. Structure with variable frequency shift The phase shift is changing by a given function of time in order to make every bunch seeing the same voltage. Variable phase shift is achieved by tuning the resonant frequency of cavity. Real Imag Time Frequency Energy releasing Energy filling

3. Frequency tuning function Frequency change rapidly

3. Tuner speed limitation Real Imag Stage5 MHz/s400 MHz/s 375 GeV TeV

3. Parameters for CLIC-DB-Linac Newly build structure Upgrade 375 GeV 3 TeV L μLμL old structure Optimum for 3 TeV stage Optimum for 375 GeV stage Stage375 GeV3 TeV Cost0.41 B2.29 B

4. Mixed structures with various frequency shifts Accelerating voltage in the structure with constant frequency shift : sine or cosine function. If phase shift for full pulse is 2n, it provides harmonic voltage profile (Fourier series). Average accelerating voltage is 0, they only correct voltage distribution. We need the fundamental section to provide the DC component to the voltage profile. Voltage profile =? Fundamental section: Provide average voltage Harmonic sections: Provide voltage correction = Flat voltage f=500MHz +7kHz*1 f=500MHz +7kHz*2 f=500MHz +7kHz*3 f=500MHz +7kHz*4

4. Choices of fundamental section Sine series (Odd functions) Cosine series (Even functions) st Harmonic 2 nd Harmonic Fundamental See hereSee here for feature of this fundamental structure See here See here for feature of this fundamental structure 1/2 Harmonic 1 st Harmonic Fundamental + + f=500MHz+7kHz*1/6 f=500MHz+7kHz*1 f=500MHz+7kHz*2 f=500MHz f=500MHz+7kHz*0.5 f=500MHz+7kHz*1

4. Even structure (cosine series)

4. Odd structure (sine series)

4. Upgrade plan mLength m m m m mLength m m m Final stage 3 TeV Phi = 78.5 degree First stage 375 TeV Phi = 52 degree Cost = 2.36 BCHFCost = 0.32 BCHF Applicable

Summary We explore the beam loading compensation of CW drive beam linac scheme: Tuner based scheme : reply on tuner speed, easier to upgrade Mix structures scheme : more stable, sophisticate upgradation.

Thank you

SC Linac design Klystrons 350 kW CW RF Loads Circulator 1 Cryo-modules with 2 couplers, Length : 4.5 m RF couplers 175 kW 1 RF Unit 1 Cryo-modules 2 cavities per module: Length: 4.5 m Frequency500 MHz, TM020 Length1.67 m Diameter1.08 m G-factor900 Store energy capacity 3.5 KJ (3 Ohm/m) Q-ext3.5 * 10^7 4K~5 * 10^10 Half of the cavity Item3 TeV375 GeV RF units540 Cryogenics (4.5K) 34.5 MW RF (67%)250 MW Total284.5 MW Max-H field: 100 mT

Power feeding issue

Decay effect

Stability of variable frequency shift For 100 KHz/ms (1 MHz/ms) tuner speed, to order to achieve < 1% energy flatness, require: -Frequency calibration error < 0.2% (0.07%) -Cavity resonant frequency vibration < 6000 Hz (500 Hz) -Initial gradient change < 0.3% (0.08%) -Initial phase shift < 0.06° (0.04°) For 1 MHz/mm tuner speed For 100 KHz/mm tuner speed

Stability of mix frequency shifts. This system works fine with any Qe (support all power feeding scheme)->Normal conducting for Harmonic structures??? To order to achieve < 1% energy flatness gain, require: -Frequency calibration error < 0.8% -Cavity resonant frequency vibration < 3000 Hz -Initial gradient change < 1.3% -Initial phase shift < 0.2°