First commissioning of the SuperKEKB vacuum system

The first (Phase-1) commissioning of SuperKEKB, an asymmetric-energy electron-positron collider at KEK, began in February 2016, after more than five years of upgradation work on KEKB and successfully ended in June 2016. A major task of the Phase-1 commissioning was the vacuum scrubbing of new beam pipes in anticipation of a sufficiently long beam lifetime and low background noise in the next commissioning, prior to which a new particle detector will be installed. The pressure rise per unit beam current decreased steadily with increasing beam dose, as expected. Another important task was to check the stabilities of various new vacuum components at high beam currents of approximately 1 A. The temperature increases of the bellows chambers, gate valves, connection flanges, and so on were less than several degrees at 1 A, and no serious problems were found. The effectiveness of the antechambers and TiN coating in suppressing the electron-cloud effect (ECE) in the positron ring was also confirmed. However, the ECE in the Al-alloy bellows chambers was observed where TiN had not been coated. The use of permanent magnets to create an axial magnetic field of approximately 100 G successfully suppressed this effect. Pressure bursts accompanying beam losses were also frequently observed in the positron ring. This phenomenon is still under investigation, but it is likely caused by collisions between the circulating beams and dust particles, especially in the dipole magnet beam pipes.

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Figure 1

Layout of SuperKEKB at the KEK Tsukuba campus.

Figure 2

Present view of the SuperKEKB tunnel.

Figure 3

Histories of the average pressure in the whole ring (red circles), average pressure in the arc sections (blue squares), and stored beam currents for (a) the LER and (b) the HER. Arrows indicate the NEG conditioning times; here, “T” and “W” mean that the NEG was conditioned only at the “Tsukuba straight section” and “Wiggler sections,” respectively.

Figure 4

$dP/dI$ ($\mathrm{Pa}{\mathrm{A}}^{-1}$) and $\eta$ (molecules ${\mathrm{photon}}^{-1}$) in the arc sections as functions of the beam dose (Ah) and $D$ (photons ${\mathrm{m}}^{-1}$) in the arc sections for (a) the LER and (b) the HER.

Figure 5

(a) Ion currents for typical gases normalized by beam current (A ${\mathrm{mA}}^{-1}$) measured by a residual gas analyzer in an arc section of LER as functions of the beam dose (Ah). (b) Mass spectra before and after the activation of NEG at a beam dose of approximately 180 Ah and a beam current of 510 mA.

Figure 6

Typical temperature changes (°C) of (a) five MO-type flanges and (b) five bellows chambers near the beam collimators versus the beam current (A), where the bunch fill pattern was $1/1576/6\mathrm{n}$.

Figure 7

(a) Conceptual structure and (b) test model of the new beam collimator installed in the LER arc sections.

Figure 8

Electron monitors attached to a test chamber with antechambers in an arc section of the LER. Here, [Al_Ante] and [$\mathrm{Al}\_\text{Ante}+\mathrm{TiN}$] denote the Al region without and with TiN coating, respectively.

Figure 9

Measured electron currents for four types of beam pipes for bunch fill patterns consisting of one train of 1500–1576 bunches with 6 ns bunch spacing. Here, [$\mathrm{Cu}\_\text{Cir}+\mathrm{TiN}$] and [$\mathrm{Cu}\_\text{Cir}$] present the data obtained from Cu beam pipes having circular cross sections with and without TiN coating, respectively, which were measured in KEKB. [$\mathrm{Al}\_\text{Ante}+\mathrm{TiN}$] and [Al_Ante] denote the data from Al-alloy beam pipes having antechambers with and without TiN coating, respectively. Note that the integrated beam doses achieved when the data of [$\mathrm{Cu}\_\text{Cir}+\mathrm{TiN}$], [Cu_Cir], [$\mathrm{Al}\_\text{Ante}+\mathrm{TiN}$] and [$\mathrm{Al}\_\text{Ante}$] were measured were approximately 3200, 3900, 760, and 760 Ah, respectively.

Figure 10

Behaviors of (a) average pressure in an arc section and (b) vertical beam size versus beam current without and with permanent magnets on Al-alloy bellows chambers for the $1/1576/6\mathrm{n}$ bunch fill pattern.

Figure 11

(a) Al-alloy bellows chambers in the LER and (b) permanent magnets and yokes attached to the chambers.

Figure 12

Measured vertical (y) and axial (z) magnetic fields inside a yoke on the upper inner wall of the beam channel (point U in the drawing).

Figure 13

Typical example of a pressure burst accompanying beam loss, followed by beam abortion.

Figure 14

Number of pressure bursts per 50 h (red bars), beam current when each burst occurred (blue circles), and maximum beam current (black line) as functions of the duration of operation with a beam current greater than 50 mA.