Electron cloud and ion effects in SuperKEKB and FCCee K. Ohmi (KEK) Beam dynamics meets Vacuum Mar 8-10, 2017, Karlsruhe Thanks to H. Fukuma, Y. Funakoshi, Y. Suetsugu, M. Tobiyama

Contents SuperKEKB FCCee Beam size blow-up due to Electron cloud Coupled bunch instability due to electron cloud Tune shift along bunch train in LER Coupled bunch instability in HER Tune shift along bunch train in HER FCCee Electron cloud Ion

Instability simulation at SuperKEKB design stage Using code PEHTS re=4.2x1011 at 4000-th turn Simulation, reth=3.8x1011 m-3. Design target for vacuum system: re<1011 m-3 in average of whole ring

Beam size blow-up in LER Beam-size blowup observed in KEKB has been seen in early stage of SuperKEKB commissioning Threshold I~300mA in Apr 19 (Y. Funakoshi) Electron cloud has been monitored at AL chamber w and w/o TiN coating (Y. Suetsugu). Beast study threshold I~600mA, Nbunch=1576 in May 17 (Nakayama et al) Aluminum bellows, which were not coated by TiN, were suspected as an electron source. Permanent magnets were installed at the aluminum bellows.(Y. Suetsugu et al.) The blow up was suppressed. Systematic studies in 8 July ( H. Fukuma et al.) sy Before perm. mag installation June 1, 2016 4 train x150 bunches, Nbunch=600 Threshold beam current 160, 200, 260,500 mA for 2, 3, 4, 6 bucket spacing H. Fukuma et al.,

Simulation studies using beam study condition Threshold of the electron density ex=2nm, ey=15pm, sz =6mm, ns=0.019 Np=1.6x1010 Ith=160mA, 4ns spacing Np=2.7x1010 Ith=260mA, 8ns spacing Np=3.65x1010, Ith=350mA, 6ns Np=6.25x1010, I=600mA, 8ns Np=2.1x1010 Ith=200mA, 6ns spacing Np=5.2x1010 Ith=500mA, 8ns spacing

Simulated threshold electron density (before/after permanent magnet installation) Nb=600, ex=2nm, ey=15pm, sz =6mm, ns=0.019 Np,th (1010) we/2p (GHz) wesz/c reth (Q=10) (1011m-3) reth (Q=6) reth (Simu) spacing Ip,th (mA) 1.6 61 7.7 1.91 2.45 3.2 2 (4ns) 160 2.1 71 8.9 1.65 3.4 3 (6ns) 200 2.7 80 10.1 1.47 3.6 4 (8ns) 260 5.2 111 14.0 4.0 6 (12ns) 500 3.65 91 11.5 3.8 350 6.25 122 15.3 4.2 >600 6

Electron cloud measurement Retarding bias, V=500V or 0V, select electrons with E>500eV or all, respectively. Electron E=500eV, v=1.3x107m/s. R/v=3.8ns, R=5cm, density can be estimated considering volume with energy gain 500eV from a bunch. 500eV may be critical for 2 (4ns) spacing. For V=0V, electron rate production including secondary is detected.

Measured electron current Y. Suetsugu 8x10-7 　　Vr = -500 V 　　Vr = -500 V 1x10-8 Al部分 Al wo coating Al w TiN coating 4x10-8 　　Vr = 0 V 1x10-6 　　Vr = 0 V Al wo coating Al w TiN coating For V=0V, 1mA, electron production rate is 0.3x109 m-1/bunch

Electron density at the blow-up threshold Simulated electron density at the threshold current Measured threshold current and density Only Al part r Simple formula Q=6

Installation of permanent magnets produce solenoid like field Before 2016/6/1 sy n After : 2016/6/8 Beam size blow-up is suppressed, but remains especially in narrow bunch spacing 4/150/2 4/150/3 sy 4/150/4 r Pressure rise is also suppressed, but still remains.

After installation of permanent solenoid at Al bellows Simulated electron density at the threshold current sy After : 2016/6/8 Ith=330 mA Ith=200 mA 4/150/2 4/150/3 4/150/4 r Ith>600 mA Ith/600bunches 160mA to 200mA by 2 bucket 200mA to 330mA by 3 260mA to >600mA by 4 SuperKEKB nbunch=1500-2500 Simple formula Q=6

Electron cloud build up simulation Electron distribution Electric potential electron cloud build up 10cm Detail studies can be done-> Terui

Simulated electron cloud build up in Drift Np=2.5x1010, I=240 mA for Nb=150x4=600 Np=4x1010, I=380 mA for Nb=600 1011m-3 1011m-3 re(r=0)~lex1000 Np=9.4x1010, I=900 mA for Nb=600 Beam current: Slightly higher than measured threshold 1011m-3 Ith=330 mA 2016/6/8 4/150/3 Ith=200 mA 4/150/4 4/150/2 Ith>600 mA Y1=Photon numberx0.1x0.01

Coupled bunch instability caused by electron cloud Electron cloud motion reflects coupled bunch instability mode. Electrons in drift, short range wake ~10ns Electrons in solenoid, slow rotation around chamber surface. KEKB 400 f0 1000 f0

Electron distribution and Wake field drift space Weak Solenoid positive negative Slow freq. corresponds to electron rotation along chamber. Higher Q~>10 Electrons oscillate 1 or 2 period, low Q=1

Unstable mode (before solenoid installation) M. Tobiyama Typical signal of coupled bunch instability caused by drift electrons 1000 f0 400 f0 Fast, strong 400 f0

Mode spectrum (after solenoid installation) positive modes for W(z~0)<0 negative modes W(z~0)>0 By 2 ver 300mA 400mA By 4 ver 350mA 600mA Solenoid origin Drift origin Electrons in drift space dominate for narrow bunch spacing and high current Solenoid origin Solenoid origin

Unstable mode (after solenoid installation) Typical mode caused by electrons in solenoid is seen. Mode seems to change to those of drift origin at high current. Solenoid modes 400 f0 540 f0 700 f0 Fast, strong

Tune shift measurement along bunch train Tune shift along bunch train in LER （Ohmi,Tobiyama,Fukuma） ∆ 𝜈 𝑥 =∆ 𝜈 𝑦 Δ𝜈 𝑦 = 𝜌 𝑒 𝑟 𝑒 𝛽 𝑦 2𝛾 𝐶 Δ𝜈 𝑦 =0.005 より 𝜌 𝑒 =8× 10 11 m-3 ∆ 𝜈 𝑥 =0 ∆ 𝜈 𝑦 = 𝜌 𝑒 𝑟 𝑒 𝛽 𝑦 𝛾 𝐶 𝜌 𝑒 =4× 10 11 𝑚 −3 ∆ 𝜈 𝑦 =0.005 Agree with density measurement

Ion effects in SuperKEKB

Coupled bunch instability in electron ring (HER) M. Tobiyama Ion? Fast, strong 𝑀𝑜𝑑𝑒=5120− 𝜔 𝑖,𝑥𝑦 𝜔 0 Resistive wall 5120-1=5119 CO fi/f0(measure)=30(x)-60(y) fi/f0=220(y)

Ion production Produced ion for a bunch(population：Ne) pass through 1m dm: molecular density, sm: ion cross-section 𝑛 𝑖 =𝑑 𝑚 𝜎 𝑚 𝑁 𝑒 𝑑 𝑚 ( m −3 )=2.42× 10 20 𝑃 𝑚 (Pa) 𝜎 𝐶𝑂 7𝐺𝑒𝑉 =190× 10 −24 𝑚 2 𝜎 𝐻2 7𝐺𝑒𝑉 =32× 10 −24 𝑚 2 𝑛 𝐶𝑂 ( 𝑚 −1 )=0.045 𝑁 𝑒 𝑃 𝑚 (Pa) 𝑛 𝐻2 ( 𝑚 −1 )=0.0077 𝑁 𝑒 𝑃 𝑚 (Pa) For 𝑁 𝑒 =6.3× 10 10 1mA bunch 𝑃 𝑚 = 10 −7 Pa 𝑛 𝐶𝑂 =283 m-1 𝑛 𝐻2 =48.5 m-1 𝑁 𝑒 =2× 10 10 𝑛 𝐶𝑂 =90 m-1

Tune shift measurement along bunch train in HER Ohmi,Tobiyama,Fukuma Dnx>Dny pilot bunch 23 bucket after the train end 𝑁 𝑒 =2× 10 10 What we know from these facts. 𝛽 𝑥,𝑦 =12m ∆ 𝜈 𝑥 +∆ 𝜈 𝑦 = 𝜌 𝑖 𝑟 𝑒 𝛽 𝑥,𝑦 𝛾 𝐶 𝜌 𝑖 =2.8× 10 11 𝑚 −3 2𝜋 𝜎 𝑥 𝜎 𝑦 𝜌 𝑖 =5.3× 10 3 𝑚 −3 Number of ions (CO) at the train end Beam size 𝑁 𝑖 =90×1576=1.4× 10 5 𝑚 −1 𝜎 𝑒,𝑥 =0.25𝑚𝑚 Ion cloud size 𝜎 𝑖,𝑥 =0.28𝑚𝑚 𝜎 𝑖,𝑦 =0.53𝑚𝑚 𝜎 𝑒,𝑦 =0.012𝑚𝑚

Ion distribution in simulation Feedback OFF at 1000 turn fi/f0=5120/(3x8)=210 fi/f0(measure)=30(x)-60(y) 78-th bunch 780-th bunch 1576-th bunch Ion cloud increases, but unstable frequency is kept fast.

Comments on Ion instability Modeling of ion instability is quite insufficient. Mode spectrum is consistent, if ion cloud with 𝜎 𝑖,𝑦 =40 𝜎 𝑒,𝑦 moves correctively. How ions are cleared at the abort gap NSLSII E=3GeV, C=792m, h=1320, ex~1nm, ey<8pm. Hor. FB ON, V FB gated OFF fx,y/f0=15,47 at 100mA Weixing Cheng (BNL) fy/f0(measure)=15 Dny=0.0026 𝜌 𝑖 =6.8× 10 11 𝑚 −3 𝑁 𝑖 =1× 10 5 𝑚 −1 2𝜋 𝜎 𝑥 𝜎 𝑦 𝜌 𝑖 =4.3× 10 3 𝑚 −3 I=100mA

FCCee works

Electron cloud effects in FCC-ee Photons are emitted by positron/electron beam. Number of photon per revolution Critical energy Electrons are produced at the chamber wall due to photo-emission. Quantum efficiency Y1=0.1-0.2. Electron production by a bunch per m passage. ne,pri= Ng Y1 Np (m-1) Secondary electron 𝑁 𝛾 = 5𝜋 3 𝛼𝛾 𝑢 𝑐 = 3ℏ𝑐 2 𝛾 3 𝜌 𝑏𝑒𝑛𝑑

Synchrotron radiation Number of photon per meter, proton rbend=11.3km(TLEP), 6km(CEPC) Critical energy Quantum efficiency of electron production, Y1=0.1-0.2 Electron production by a bunch per m passage. Chamber cross section 0.005 m-2 (tentative) Electron density exceeds re,th=7.8x109 m-3 even in a bunch passage. Antechamber and other cures are necessary. ne,pri= Ng Y Np (m-1)

Simulation of electron cloud build up in TLEP FCCee-Z FCCee-W FCCee-H FCCee-t

Electron cloud effects in FCC-ee FCCee-Z Nb=90300, Lb-b=1.1m Np=3.3x1010, <b>=100m, sx=95mm, sy=10mm we=2px127GHz, wesx/c=13 re,th=7.8x109 m-3. Threshold is very weak density. KEKB 5x1011, SuperKEKB 1x1011

Parameters related to EC instabilitiy CEPC TLEP-Z TLEP-W TLEP-H TLEP-t Circumf C (km) 54 100 Energy E (GeV) 120 45.5 80 175 No. bunches Nb 49 90300 5162 770 78 Bunch pop Np (1011) 3.8 0.33 0.6 0.8 1.7 Beam l-density l (1010m-1) 0.74 3.0 0.3 0.06 0.0013 Beam size (av.) sx/sy (mm) 583/32 95/10 164/10 247/11 360/16 Bunch length sz,tot(mm) 2.6 5.0 2.4 2.5 Synch. tune ns 0.18 0.015 0.037 0.056 0.075 g prod. rate Ng (/m/e+) 0.28 0.059 0.10 0.16 0.23 e- prod. rate ne,prod (1010m-1) 1.1 0.020 0.062 0.12 0.39 Electron freq. we/2p (GHz) 137 127 171 174 Electron osci. wesz,tot/c 7.5 13 11 8.7 9.0 Thr. density re,th (1010m-3) 104 0.78 3.4 7.7 15 Tune shift Dn at re,th 0.034 0.0025 0.0061 0.0092 0.012

Simulation for FCCee-Z Growth of emittance Coherent motion in y+(z), y-(z),sy+(z) Threshold is re,th=0.8x1010 m-3 , agree with the analytic formula 0.78x1010.

Ion instability in FCCee -Z Nb=90300, Lsp=0.75m(2.5 ns) Ne=3.3x1010,<b>=50m, sx=67mm, sy=7mm Dispersion increases sx , assume 21/2 times controlled by ex in simulation. ex =0.09nm, wi=2px103MHz, weLsp/c=1.61<- critical for trap in the bunch train, ex =0.18nm, wi=2px88MHz, weLsp/c=1.38 Ion production rate for CO, ni =0.045 Ne P(Pa), ni (10-8 Pa)= 14.9 (/(m.e-)).

Simulation results for P=10-8 Pa Ions are trapped partially for ex=0.09nm Growth does not depend on train length. Bunch-by-bunch Feedback G=0.1(10 turn) is necessary.

Summary Commissioning of SuperKEKB (Phase I) had started at Feb 2016 without collision point. Beam-size blow-up caused by fast head-tail instability due to Electron cloud had been observed. Aluminum bellows without TiN coating are suspected as electron source. Installation of permanent magnets on the bellows worked to suppress the instability. It is good for Phase II (1A 3-4 spacing). Furether cures are necessary for Phase-III (>2A). Ion instability and tune shift were observed. The instability is less serious and ion density near the beam is weak. Horizontal tune shift is bigger. Ion seems to distributes si,x<si,y.