QUICK RECOVERY OF THE KEK E-/E+ INJECTOR LINACFROM THE GREAT EAST JAPAN EARTHQUAKE
Atsushi Enomoto, -KEK, Tsukuba, JapanAbstract
The KEK 8-GeV e-/e+ injector linac is currently underoperation for the 2.5-3 GeV KEK Photon Factory (PF)storage ring and 6-GeV Photon Factory - Advanced Ring
(PF-AR). In addition the linac had just started the upgrade
for the SuperKEKB project since the summer in 2010. On
March 11, 2011, the linac suffered great damage from theGreat East Japan Earthquake. Due to an extraordinarystrong vibration, many bellows of vacuum pipes were
violently torn and the entire linac vacuum was exposed tothe atmosphere. Without electricity, highly humid air
entered the inside of accelerator structures. Some people
supposed the linac would not be recovered within a year.
However, it resumed operation after only two months. We
record the memorable disaster and the valuable
experience how we recovered the linac so quickly.

INTRODUCTION
The KEK linac was initially constructed as a 2.5-GeV
electron injector for PF (1978–1981), adding a positronsource for TRISTAN (1982–1984), and further upgradedto provide 8 GeV electrons (e-) and 3.5 GeV positrons(e+) for KEKB (1994–1998). It has been operated for
5000-7000 hours annually since 1982. In FY2010 it again
entered into a construction phase for SuperKEKB, while
continuing the operation for PF and PF-AR. The PF andPF-AR injection is conducted by using the downstream
part of the linac, which is isolated from the upstream by athick radiation-shield concrete wall and can accelerate
electrons up to ~3 GeV. In FY2011 the most severe andtough work was the recovery of the linac from the EastJapan earthquake happened on March 11, 2011. The PF
and PF-AR injections were successfully resumed aftertwo months by quickly repairing the downstream linac,
though the upstream part is still under restoration because
of severe damages along the entire length (~600 m) of thelinacs.

(Fig.1), inflow water coming from the tunnel joints andspilled over the floor, etc.

Figure 1: The linac damage in accelerator tunnel.

Figure 2: Inflow water into the tunnel.

QUAKE AND FIRST ACTION FORRECOVERYThe peak earthquake strength was 6-lower in theJapanese scale which has been never experienced so far,and strong oscillation lasted nearly five minutes. The
acceleration of the ground motion was recorded 0.3 – 0.4g
in Tsukuba. Just after the quake went away a visual
investigation was initiated in the darkness without
electricity. And it was immediately turned out the linac
was caused heavy damages: In the accelerator tunnel
there were many vacuum bellows torn hardly, broken
girders, a quadrupole magnet dropped down to the floor

Figure 3: The linac damage in klystron gallery.

The damages were found more as the investigation
went on; and, the mostly feared thing was that ~240
pieces of accelerator sections in the tunnel were supposed
to be damaged because they were purged to a very humid
atmosphere under the non-ventilated condition withinflow water. The high-gradient performance ofaccelerator guides might be degraded if inner surface
becomes eroded; in a worst-case situation, they might beunusable if metallic dusts flying from torn bellows enter
inside. Therefore, it was an issue of vital importance thatthe atmosphere inside the accelerator guides wasevacuated as soon as possible. Taking account of thisconcern, the following procedures were introduced:(1) The first action was, for all of the accelerator guides,
to make “nitrogen purge” (to purge wet atmosphere insideaccelerator by dry nitrogen).
(2) Recovery of the downstream part was prioritized inorder to quickly resume the PF and PF-AR injection.
(3) The remaining part of the linac (the upstream 5sectors) is then recovered for the SuperKEKB injector by
the end of FY 2014.

TOWARDS OPERATIONThe next step is to resume the linac operation. Thevacuum should be recovered to an operation level (< 104) and the entire beamline must be established. To repair
beam position monitors (BPMs) was one of the criticalworks. The broken bellows were removed using a lathe
and new ones were welded. To save time it was carried at
KEK workshop. After a performance check the BPMs
were incorporated into the beamline. Iterations of vacuum

Figure 4: A cut-view of 2-m accelerator section.

leak search and its repair were continued. Due to anextended damage of vacuum components, it took one
month to fix the downstream 3 sectors. In addition to thevacuum recovery, the accelerator girders were reinforced.
The standard accelerator girder mounts four 2-m
accelerator sections with service equipment such as
waveguides, a vacuum manifold, and a cooling-water
header. The net load is ~2 tons. It adopted a quakeabsorbing structure with damper. After the quake, it wasfound it has a resonance frequency at around 3.5 Hz in
longitudinal direction. Since the resonance is supposed toenhance the damage the immediate improvement wasrequired. The girder oscillation is restricted with amassive stopper at the mid-bottom instead of the damper.
By the end of April infrastructure necessary for the linac
operation was recovered. The inflow water wassuppressed by grout injection into the tunnel joints. Thenthe linac was conditioned with high-power RF up to an
operational strength. The PF and PF-AR injections wereresumed on May 16 and July 1, respectively..
“Nitrogen replacement” takes the process as follows:
After the broken components are replaced, the accelerator
is roughly evacuated. When the vacuum pressure canreach ~1 Pa, then it is purged with dry nitrogen (~0.1 MPa= ~1 atom). Though the linac recovery was prioritizedover the other facilities at KEK, it was initiated under thelimited condition of infrastructures, such as electricity,compressed air, and cooling water. Nevertheless, it wasdone within two weeks, since this task was addressed by asupport from outside of injector vacuum groups.

Figure 5: The linac vacuum purge with dry nitrogen.

Figure 6: Accumulation current and rate of the PF storage ring.

THE ENTIRE LINAC RECOVERYThe upstream recovery necessary for the SuperKEKB
upgrade was started on July 2011 and is still continuing
while operating PF injections. For this purpose, it is really
helpful that the upstream part had been separated by thickconcrete wall in FY 2010. In the upstream part, many
accelerator girders are put on square logs because theirsupport legs are left broken. The beamline among them
are not repaired. Thefore, the vacuum recovery of theupstream 40 accelerator units were made by isolatingthem from the beamline. Thus the accelerator vacuum
was finally recovered by the end of November. A highpower test was finished by mid December and it turned
out that the accelerator did not have a severe damage. It isplanned the entire beamline will be restored before
FY2014 when the linac commissioning for SuperKEKBstart.

photo-cathode material. The beam-test is conducted using
the linac machine-study time. The beam with an intensityof about 0.5 nC/bunch could be successfully accelerated
to the end of the linac. Increasing and stabilizing the beam
intensity is under investigation. For the positron source, aflux concentrator will be used because more intense
magnetic field is necessary to increase positrons. For thispurpose a conventional system which is already used inother laboratories will first be used and a new type to
produce more intense field is under R&D. Together withstrengthening the focusing field, the aperture of thepositron capture sections will be expanded. An L-band
accelerator section was fabricated and is presently readyfor the high-power test in the following fiscal year

Figure 9: RF-gun studies underway downstream of linac.

ACKNOWLEDGMENT
The author thanks to the injector linac people who
committed to the hard recovery work as well as thepeople who helped us, especially for the linac vacuum
recovery and alignment.
Figure 7: Broken girders are put on square logs.

REFERENCES[1] T. Higo, “Linac upgrade in intensity and emittance forSuperKEKB”, IPAC2012 (this conference), TUPPR005.

Figure 8: Beamlines are not yet connected (upstream).

UPGRADING TO SUPERKEKB LINAC
TheThe R&D for upgrading the linac for the SuperKEKB
was resumed in parallel. During the two-months summer
shutdown in 2011, an RF gun system was installed justafter the thermionic electron gun for the PF injections. It
is a new type employing a disk-and-washer (DAW)
structure for the accelerating cavity and LaB6 for the