DR Vacuum
The 22nd KEKB Accelerator Review Committee
March 14, 2018
Kyo Shibata(on behalf of KEKB Vacuum Group)
Inter-University Research Institute Corporation High Energy Accelerator Research Organization (KEK)
大学共同利用機関法人 高エネルギー加速器研究機構 (KEK)Contents• Overview• Construction& NEG activation• Dynamic pressure& Vacuum Scrubbing• Concerns• Summary– “Updates of MR Vacuum system” will be presented by Suetsugu-san (3/14)– “IR assembly” will be presented by Kanazawa-san (3/15)
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2Overview• Vacuum system is divided into 7 sections
by GVs.
– East arc sect. : SR monitor, FB monitor & kicker,
DCCT, RGA
– West arc sect. : Inj. kicker, Ext. kicker– South straight sect. : Ext. septum, Beam stopperfor safety system– North straight sect. : Inj. septum– RF sect. : RF cavities– LTR– RTL

Beam energy

1.1

GeV

Bunch number
Circumference
Maximum stored beam
current

4135.5m

70.8mA

Maximum bunch charge

8nC

Bunch length
Bending radius
Critical energy
Total SR power

6.532632/2967
0.93/0.82
7.2mmmm
keVkW

Large-emittancebeam from Linac• Target pressure : < 1×10-5 Pa– Required beam lifetime due toresidual gas scattering : > 1000 sec.
– Vacuum pumps : NEG & ion pumps
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Low-emittancebeam
To SuperKEKB LER
via Linac & BT3

Overview (Arc sect.)• Beam pipes for arc sections (Al alloy)– Two types of beam pipes corresponding to B1&B2 magnets• Each beam pipe covers one dipole, one quadrupole and one sextupole magnet.

– Designed to fit narrow spaces between the closely-located magnets– Having BPM blocks, pumping ports, RF-shielded bellows, CF flange with RF contact– Having antechambers on both sides of a beam channel to deal with SR which irradiates
both side of the beam pipes and reduce beam impedance.
– TiN coating & grooved surfacing are adopted as countermeasures against ECE.
– Having water cooling channels on both sides of beam pipes• Total SR power with design beam current (70.8 mA) : 7.2 kWSRSRSR
Layout of the beam ducts in one cell of the reverse-bend FODO lattice.
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Overview (Arc sect.)
Aluminum beam pipes for arc sections :RF shielded bellows

Type ISpace-saving connection flange∼1.5 mNEG

Pumping port

TMP
RF contact onConflat flange

IPType II∼1.2 m
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Groove structure

BPM block

Cross-section

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TiN coating5Overview (Arc sect.)
Pump location :•••••Two NEG pumps are installed in Type-I.
One NEG pump or ion pump is installed in
Type-II alternately.
Distance between pumping ports : ∼0.7 m
Seff_NEG : ∼0.01 m3/s (10 L/s) for CO
Seff_IP : ∼0.008 m3/s (8 L/s) for CO

Type-I (∼1.5 m)
NEG

Ion pump,rough pumpNEG
Type-II (∼1.2 m)

Type-I (∼1.5 m)

Pumping port (NEG & IP)

NEG
NEG

NEG
Type-II (∼1.2 m)

Ion pump, rough pumpType-II (∼1.2 m)
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Overview (straight sect.)• Beam pipes for straight section. (Al alloy)– Much simpler than beam pipes in the arc sections.
••••

No antechamber
No groove surfacing
No water cooling channel
Isolated bellows chambers instead of bellows part welded tothe beam pipe

Example of beam pipe for thestraight section– Having different cross-sections to fit special
components installed in the straight sections.
• Cross-sections of beam pipes change as smoothly as possible.
Cross-sections of vacuum components inthe straight sections.
RF section
Feedback kicker
Feedback monitor
Inj. and Ext. kicker
Inj. and Ext. septum
Adjacent area of theeast arc section, etc.

Circular
Octagonal
Octagonal
Rectangular
Circular

φ150 mm62.8 mm (face-to-face)
46 mm (face-to-face)
42 mm × 37 mm
φ41.2 mm

Octagonal

46 mm (face-to-face)2018/3/14

Isolated bellows chamber

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Overview (Control system)• Pressures are monitored by ion pumps.
– Number of IPs :• East sect.; 11, West sect.; 11, North sect.; 4 + 4(septum), South sect.; 3• Installation interval of IP : ∼ 6 m– When the pressures exceed 1×10-4 Pa at several points at the same time in one section,
the section is isolated by gate valves and beam injection is inhibited.
– One CCG is also installed in each section. (4 CCGs in DR, 1 CCG in LTR, 1 CCG in RTL)
Thermometer– One RGA is installed in East arc section.

•Temperatures of beam pipes are monitored.
– Number of monitoring points :• East sect.; 13, West sect.; 12, North sect.; 4, South sect.; 1– When the temperature exceeds 80°C, an alarm is raised.
• Temperature rise due to beam operation has not been observed so far. Flow switch• Flow rates of cooling water are continuously monitored.

wrapped inPermalloy– Water flows rate of every blanch is monitored by water flow switch. (10 blanches)– When the flow rate falls below the threshold value, an alarm is raised.
• Since two flow switches occasionally malfunctioned due to the magnetic field fromthe magnet, they were wrapped in high permeability magnetic shield (Permalloy).
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The 22nd KEKB Accelerator Review Committee8Construction 1• Pre-installation works :– Almost all beam pipes (105) were coated with TiN film (t200 nm)
and baked (150°C, 24 h) at the coating & baking facility in KEK.
• Not including ceramics pipes for Inj. & Ext. kickers, Ext. septum, special
beam pipes for RF sections, FB monitor & kicker, DCCT, Inj. & Ext. septums.
• NEG pumps were activated during the baking.
• Not baked in-situ after installation.

Coating & baking facility in KEK
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TiN coated beam pipe for arc sections
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Magnetron discharge sputtering
9Construction 2• Installation works :– The installation of the beam pipes commenced in May
2016 and finished in January 2018.

Inserting beam pipe into
B magnet• Evacuation from atmospheric pressure :– Each section was evacuated by rough pumps (TMP +scroll pump) & ion pumps before and during NEGactivation.
• When the pressure decreased to ∼10-4 Pa, ion pumps
were operated.
• When the pressure decreased to ∼10-5 Pa, NEG
activation was started.
• It takes about several days before NEG activation.

– After NEG activation, rough pumps were isolated byangle valves and removed.

Connecting beam pipes
L type valve
TMP

IP
Scroll pump

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NEG activation• Heating power for NEG activation was controlled so that the pressure did notexceed 1×10-4 Pa.
– The pressure was monitored by ion pumps and vacuum gauges at manifold between TMP and IPport.
– Since the conductance of beam pipe and the conductance between pumping port and beam duct
are small, the pressure in NEG port during activation is higher than measured value.
– It takes about 3 weeks from the start of evacuation to the end of NEG activation.
Pressure during NEG activation (East arc section)

Start of rough pumping
Angle valves closed
Pressure (vacuum gauge)
Pressure (IP)

Vacuum gaugeAngle valve

TMP

Current for
NEG activation
IP on
NEG activation (13 days)

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IP11

Pressure estimation• Pressure in the beam pipes was estimated by Molflow+ (Monte-Carlo Simulator
developed at CERN)––––

Model : 1 Type-I beam pipe + 1 Type-II beam pipe
Pumps : 2 NEG pumps (0.05 m3/s) in Type-I, 1 ion pump (0.04 m3/s) in Type-II.
Mass : 28, Temp. : 20 °C, Thermal gas desorption rate : 1×10-9 Pam3/s/m2 = 1×10-11 mbarL/s/cm2
PSD rate : η=0 molec./photon (No stored beam)

CCG port
IP portAverage in beam channel

CCG port

Result is similar to themeasured pressure
before beam operation.

IP port
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12Dynamic pressure 1• Beam operation started on Feb. 8th.
– Storage of beam started on Feb. 9th.
– The pressure exceeded 1×10-5 Pa, when the beam was accumulated for the first time.

1 bunch, 1 Hz, 1.5 mA

Current [mA]First storage day!!

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Dynamic pressure 2• Beam current gradually increased and pressure gradually decreased.
– Maximum storage current reached 10 mA (4 bunch, 25 Hz) on Feb. 23rd.

Current [mA]

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Vacuum scrubbing• Vacuum scrubbing has been progressing smoothly so far.

West arc

East arc

Beam current

Beam dose : 1 Ah =Photon dose 5.2×1022 photon/m
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Beam lifetime• Beam lifetime was larger than 1000 s when beam current was 10 mA.
– Operation mode : 4 bunch, storage mode
– Beam dose : 0.7 Ah (Photon dose : 3.6×1022 photon/m)

Lifetime

Lifetime

Pressure
current

Time [hh:mm]

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Time [hh:mm]

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PSD rate estimation• Pressure in the beam pipes was estimated by Molflow+ (Monte-Carlo Simulator
developed at CERN) to estimate PSD rate.
– Pumps : 2 NEG pumps (0.05 m3/s) in Type-I, 1 ion pump (0.04 m3/s) in Type-II.
– Mass : 28, Temp. : 20 °C, Thermal gas desorption rate : 1×10-9 Pam3/s/m2 = 1×10-11 mbarL/s/cm2– PSD : η=1×10-4 molec./photon, stored beam current = 8.85 mA• SR irradiates both sides of antechambers evenly.
NEG port
CCG port
IP portAverage in beam channel
The calculated pressure is close to themeasured pressure.
(Pmeasured_CCG : 1∼2×10-8 mbar)η ≈ 1×10-4 molec./photon (beam dose : 0.7Ah, photon dose : 3.6×1022 photon/m)
Almost as expected!!

η ≈ 1×10-5 molec./photon is required to obtain enough lifetime (1000 s) with design beam current (70.8 mA).
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Residual gas components• RGA data of the dynamic pressure was taken at the east arc section.
• Main gas components at beam dose = 0.7 Ah– H2 (beam current = 0 mA, PIP = 4.3×10-7)– H2, CO, CH4, CO2 (beam current = 9.8 mA , PIP = 1.4×10-6)• They are typical residual gas components in beam pipes irradiated by SR and evacuated by NEG.

H2COCH4

CO2
H2ORGA
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IP18

Concerns 1• Pressure rise during beam operation at a repetition rate of 25 Hz.
– Pressure at a repetition rate of 25 Hz is larger than that of 1 Hz.
• Vacuum scrubbing was performed by storage mode or at a repetition rate of 1 Hz, and time ofbeam operation at a repetition rate of 25 Hz is very short.
• It may be caused by the increase in the time stored with large-size beam.
• It seems there are places where only large-size beam can scrub.

– It is expected that the vacuum scrubbing of such places will progress during Phase-IIoperation.
1 Hz

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25 Hz

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Concerns 2• Saturation of NEG pump
– Re-activation (conditioning) of NEG pumps is required when pumping speed of NEGpump becomes small.
– It takes several days to re-activate NEG pumps.
• DR can not be operated during re-activation of NEG pumps.

– There is no indication that the pumping speed of NEG decreases up to now.
– Re-activation will be performed during long shutdown. (this summer?)• Electron cloud issue– Antechamber, TiN coating and Grooved surfacing are adopted as countermeasuresagainst ECE.
– It is expected that electron cloud density integrated over the ring is 0.13×1014 m-2 byusing TiN coating and antechamber (not including grooved surface) when stored beam
current is 70.8 mA.
• Assumption : δmax of TiN coating = 1, photons irradiating beam channel/whole photon = 0.1• Threshold of electron cloud instability : 1.4×1014 m-2– Electron cloud instability will not occur in DR.
• It is necessary to continuously observe whether the ECE occurs or not as the stored beam
current increases.
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20Summary• Beam operation of DR started on Feb. 8th, and DR vacuum system have noproblems so far.
• Vacuum scrubbing has been progressing smoothly so far.
– The pressure in the beam channel is estimated to be less than 1×10-5 Pa with a stored
beam current of 10 mA at present. (beam dose = 0.7 Ah)– Currently, beam lifetime is larger than 1000 s when the stored beam current is 10 mA.
– It seems that vacuum scrubbing by beam operation at a repetition rate of 25 Hz isinsufficient.
– ∼10 times the beam dose is required to achieve enough lifetime (>1000 s) with design
beam current (70.8 mA).

• There is no indication of decrease in the NEG pumping speed.
– Re-activation will not be required until this summer shutdown.

• As countermeasures against ECE, antechamber, TiN coating and Grooved surface
are adopted.
– It is necessary to continuously observe whether the ECE occurs or not as the stored
beam current increases.
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Fin.
Thank you for your attention.

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Back-up

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Average pressure estimation• Rough estimation of average pressure in arc sections– Required averaged pressure is lower than 1×10-5 Pa to obtain enough beam lifetime.
– Beam pipes have three pumping ports in one cell with a length of about 2 m. (d ≈ 0.7 m)– Two of them are for the NEG pumps and one is for ion pump.
• Average effective pumping speed will be 0.01 m3s-1 (just after activation) .

– Photon stimulated desorption by SR is major dynamic gas load during beam operation.
• After the sufficient scrubbing, η will drop to below less than 1×10-5 molec./photon.

– 1×10-5 Pa with a stored beam current of 70 mA is achievable goal with sufficient scrubbing.

Stored beam current : 70 mA
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Average pressure estimation• Rough estimation of average pressure in arc sections– Required averaged pressure is lower than 1×10-5 Pa to obtain enough beam lifetime.
– Beam pipes have three pumping ports in one cell with a length of about 2 m. (d ≈ 0.7 m)– Two of them are for the NEG pumps and one is for ion pump.
• Average effective pumping speed will be 0.01 m3s-1 (just after activation) .

– Photon stimulated desorption by SR is major dynamic gas load during beam operation.
• After the sufficient scrubbing, η will drop to below less than 1×10-5 molec./photon.

– 1×10-5 Pa with a stored beam current of 70 mA is achievable goal with sufficient scrubbing.

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