Beam Loading experiment at KEK ATF Multitrain acceleration

Beam Loading experiment at KEK ATF (Multi-train acceleration at KEK-ATF Injector) KEK Masafumi Fukuda and Junji Urakawa LCWS 2014/10/05 2 train acceleration at KEK-ATF 1

Introduction • Drive electron positron beams are accelerated with the form of triplet multi -bunch mini-train. • The beam-loading effect should be compensated to be accepted into the ILC -DR. – Beam loading compensation in a mini-train – Energy compensation between multi mini-trains Control of input RF power by phase shifters Time structure of beam 0<t<264. 45 ns, i=0. 532 A 264. 45<t<362. 85 ns, 0 A 362. 85 ns<t<627. 3 ns, 0. 532 A 627. 3 ns<t<725. 7 ns, 0 A 725. 7 ns<t<990. 15 ns, 0. 532 A is beam loading current. T. Omori et al. , Nucl. Instrum. Meth. A 672 (2012) 52 -56. 2014/10/05 2 train acceleration at KEK-ATF 2

3 x 1010 positron/bunch 300 Hz triplet beam Less than +/- 0. 3% Beam loading current 0. 78 A. 300 Hz Power Supply 80 MW Klystron We need the precise control of the phase shifters. Low Level RF Phase Shifter and Amp. 3 d. B High Power RF Combiner 50 W High Power Terminator 3 m long constant gradient travelling wave structure Also, I assume 10% margin as wave guide loss and so on because of the experience at ATF Linac. So, klystron output power 80 MW and 2 ms pulse width are necessary.

Beam loading compensation experiment at ATF Extraction line Damping Ring Photo-cathode RF gun (electron source) S-band Linac

Two train acceleration at KEK-ATF injector 166. 4 ns 53. 2 ns ~60. 0 ns In this experiment at KEK-ATF injector, 2 mini-train beams are accelerated. Multi-bunch beam: 2 x 1010 with 6. 15 ns bunch spacing corresponds to 0. 9 x 1010 in the case of 2. 8 ns bunch spacing as same beam loading in multi-bunch trains. Two mini-train beam : 3 x 1010 with 100 nsec train gap and 6. 15 ns bunch spacing corresponds to 1. 4 x 1010 in the case of 2. 8 ns bunch spacing as same beam loading in multi-bunch trains. 3. 6 cell RF Gun 2014/10/05 3 m long 2 train acceleration at KEK-ATF Bending magnet 5

Experimental Setup KLY#0 KLY#1 40 MW, 1. 5 ms 12. 5 Hz Power Supply Amplitude Modulation Low Level RF Phase Shifter and Amp. KLY#2 80 MW, 3 ms 12. 5 Hz Power Supply 60 MW Klystron 3 d. B High Power 50 W High Power Terminator RF Combiner Speed of phase control is about 100 degrees/10 ns. 45 degrees bending magnet to measure the energy of multi-bunch 3. 6 cell RF Gun Chicane (Standing wave) 2014/10/05 3 m accelerating tube (Traveling wave) 2 train acceleration at KEK-ATF 6

Waveguide circuit KLY#0 KLY#1 KLY#2 SLED 3. 6 cell RF Gun SW 3 m long TW KLY#0 Before SLED 3 m long TW KLY#1 KLY#2 After 3 d. B High Power RF Combiner SLED 3. 6 cell RF Gun SW 2014/10/05 Chicane 3 m long TW 50 W High Power Terminator SLED 3 m long TW 2 train acceleration at KEK-ATF 45 degrees bending magnet 7

We completed the change of the waveguide system by 21 st August. Vacuum level around both klystrons is 5 -6 x 10 -6 Pa. New waveguide system for beam loading compensation experiment To L 0 KLY#2 KLY#1 To Gun KLY#0 KLY#2 KLY#1 3 d. B Directional Coupler Ion Pump 75 L/s Dummy Load

Generation of two mini-train per pulse L 0 3. 6 cell RF Gun SW 3 m long TW Chicane KLY#1 45 degrees bending magnet to measure the energy of multi-bunch 3 m long TW L 1, L 2 in KLY#0 L 0 in KLY#0 Gun in

Several modules and function generators were prepared for the control of RF phase and amplitude which are necessary for the beam loading compensation experiment. Then, we started the RF aging and test new instrumentation from end of August. Test of I-Q modulation & detection was done by using LLRF at LUCX and the enough performance was confirmed. Function Generator (AFG 3000 C)

2856 MHz Oscilloscope RF Signal 2856 MHz All modules were checked. The performance was confirmed by measurements of setting accuracy and linearity.

Experimental Setup UV 60 ns 20 bunches LUCXでのテスト結果 357 MHz mode-locked laser PC 2 ・・ ・・ PC 1 Pockels cell 1: 60 pulses are clipped out from pulse train with 357 MHz (2. 8 ns pulse spacing). Pockels cell 2: Train gap with 60 ns is made by this pockels cell. 2014/10/05 2 train acceleration at KEK-ATF 12

12. 5 Hz Power Supply 40 MW, 1. 5 ms Amplitude Modulation 12. 5 Hz Power Supply 80 MW, 2 ms Low Level RF Phase Shifter and Amp. 60 MW Klystron 3 d. B High Power RF Combiner 50 W High Power Terminator Generation of three mini-train per pulse 3. 6 cell RF Gun SW 3 m long TW Chicane 150 bunches/pulse with 2. 8 ns bunch spacing fast phase control 45 degrees bending magnet to measure the energy of multi-bunch 3 m long TW Energy spread 0. 1% in rms 2 n. C/bunch

20 bunch – 60 ns – 20 bunch at 1 n. C/bunch Fast amplitude modulator off Fast amplitude modulator on Checked amplitude control accuracy.

Present laser system, Two Pockels cells, 1. 6 cell RF Gun, Chicane and BPM were used to generate doublet pulse train and to measure energy of each bunch. 0. 78 x 1010 electrons per bunch were checked at LUCX.

Beam loading compensation A train with 1. 5 x 1010 e-/bunch and 60 bunches is accelerated. Beam loading is compensated by using Dt method. 5 n. C/bunch Before compensation Big beam loading Energy gain 90 Input RF power: 60 MW 80 Only 8 bunches are observable in this case. 70 60 50 40 After compensation 30 20 10 0 0 1 2014/10/05 2 3 2 train acceleration at KEK-ATF 16 Input RF power: 80 MW

2 train acceleration Two trains with 1. 5 x 1010 e-/bunch and 20 bunches are accelerated. Energy difference at the train gap is compensated by amplitude modulation by phase control. Compensation is not enough. 1. 5 x 1010 e-/bunch 20 bunches/train 60 ns train gap 2. 8 ns bunch spacing After compensation, the energy differnce is within 0. 5 Me. V (pk-pk) which correspond 1. 2% (Peak to Peak). This is less than +/-0. 3%. 2014/10/05 After compensation 2 train acceleration at KEK-ATF 17

RF pulse Phase of KLY#1 and KLY#2 was modulated at the timing of train gap. Pickup(3) Pickup(2) Pickup(4) 2014/10/05 2 train acceleration at KEK-ATF KLY#1 Linac (L 0) KLY#2 18

Summary 3 m long TW 3. 6 cell RF Gun 3 m long TW 45 degrees bending magnet to measure the energy of multi-bunch Beam Transport 3 x 1010 with 6. 15 nsec bunch spacing corresponds to 1. 4 x 1010 in the case of 2. 8 nsec bunch spacing with same beam loading in multi-bunch trains. Amplitude modulation technique for beam loading compensation was confirmed by test at LUCX successfully. Dt and phase amplitude modulation technique for beam loading compensation was confirmed by test at KEK-ATF. Thank you for your attention.

Backup 2014/10/05 2 train acceleration at KEK-ATF 20

Energy measurement 3. 6 cell RF Gun 3 m long Position = S Calibration result (Chicane out) 2014/10/05 Calibration result (Bending magnet out) 2 train acceleration at KEK-ATF 21

RF pulse shapes during 1 train acceleration (60 bunches) KLY#0 KLY#1 Dummy Load L 0 in 2014/10/05 2 train acceleration at KEK-ATF KLY#2 L 0 out 22

Beam loading compensation with step modulation RF pulse with stepped shape was injected to the accelerating tube. 2014/10/05 2 train acceleration at KEK-ATF 23

Beam loading compensation with step modulation Energy in a train could not be compensated because the response time of klystron was slow. 2014/10/05 2 train acceleration at KEK-ATF 24