Improvement of the PF ring vacuum system

Y. Hori*

High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan

Received 4 August 1999; accepted 3 February 2000

Abstract

For an upgrade of the PF ring, about half of the vacuum ducts were replaced by newly made or improved ones, which were

designed and manufactured to achieve the required beam lifetime. The concept of pumping was maintained. Every vacuum

duct was pre-baked and installed with care so as not to introduce contamination caused by exposure to the atmosphere. The

ring vacuum was successfully conditioned without an in situ bake-out. The operating pressure was speedily reduced by beam

cleaning as the operation time increased. A low operating pressure and resulting long beam lifetime was achieved within a

short operation period. # 2001 Elsevier Science B.V. All rights reserved.

Keywords: Synchrotron radiation; Light source; Vacuum duct; Beam lifetime

1. Outline of the reconstruction

The photon factory storage ring (PF ring) has been

operated as a dedicated synchrotron light source. As

one of recent activities, a new con®guration of the ring

was proposed and the ring was reconstructed [1,2].

The substance of the upgrading was to achieve low

beam emittance by reinforcing the focusing magnets

in the so-called `̀ normal-cell'' sections. In this project,

about half of the beam ducts of the ring had to be

replaced by new or improved ones to be compatible

with the new magnets. The ring was shut down from

December 1996 to September 1997 for the reconstruc-

tion.

Since neither the beam energy nor the bending

radius was changed before or after the reconstruction,

the outgassing rate per stored current per length by

photon stimulated desorption (PSD) must be almost

the same as before. Besides, the same ultimate pres-

sure as before is also enough for the new ring. Thus,

the pump location and pumping speed were main-

tained in order to guarantee a long beam lifetime. The

required pressure was below 4� 10ÿ8 Pa at 400 mA

of the stored beam. One of the key points was how

install bellows, beam position monitors (BPMs) and

pumping ports in a narrow space restricted by the

reinforcement of magnets.

2. Modi®cation of the vacuum duct

The main part of the replacement was carried out in

the normal-cell sections. There are two normal-cell

sections around the ring, and each section consists of

eight unit cells. Sixteen normal-cell ducts were neces-

sary for these sections. Since the bore radii of new

magnets were reduced compared to before, the cross-

sectional form of the beam duct was changed. The

design of the normal-cell duct was ®xed after the

fabrication of a prototype one. A major part of the

duct was made of aluminum alloy, as before. A typical

Applied Surface Science 169±170 (2001) 728±731

* Tel.: �81-298-64-5672; fax: �81-298-64-2801.

E-mail address: hori@mail.kek.jp (Y. Hori).

0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 7 7 6 - 5

view of the duct is shown in Fig. 1. A bending part of

the old duct with an installed distributed ion pump

(DIP) was reused for the new duct because of a

budgetary restriction. This, however, resulted in dif®-

culties in the design, process and schedule. The old

ducts were removed from the normal-cell section, and

their bending parts were then improved and welded

with other prepared new parts during the shutdown. In

order to reduce any broadband impedance, the inner

walls of the ducts were connected as smoothly as

possible. Also, RF contacts were mounted in every

bellows and ¯ange mounted in the new ducts. Non-

circular bellows was adopted so that it could be

installed between the coils of the bending magnet.

Due to the limited space, no bellows could be mounted

between a crotch part and its downstream BPM, both

of which are ®xed points. In order to set these ®xed

points precisely, the normal-cell duct was assembled

within an accuracy of 1 mm. The pumping system was

not changed. The same three pumps (a titanium sub-

limation pump (TSP), an ion sputter pump (SIP) and a

DIP) were reused in every new duct. There is pumping

box between the bending part and its downstream

straight part. There are pumping slits on beam channel

in the box, of which total area is wider than 70 cm2 so

as to maintain an effective pumping speed. Only

extruded aluminum alloy ducts, from which the

straight ducts were made, were subjected to alkaline

etching before machining. The bending part was made

with machining, where ethyl alcohol was used as a

lubricant. No oil or grease was used. The machined

surface was subjected to no treatment by a chemical

attack. Every ®nished duct was ®lled with dry nitrogen

in the factory after it had passed a vacuum leak test,

and was brought to our facility.

The BPM was modi®ed according to the change in

the duct. Four pick-up electrodes were directly welded

on to the extruded straight duct, because the deviation

in the sensitivity caused by the nominal error of the

duct was simulated to be allowably small [3]. This

made the design and manufacturing of the duct simple

and easy. Every BPM was calibrated in order to

determine its electrical center. All of the measured

offsets were <500 mm, and typically 200 mm. This is in

good agreement with a simulation result obtained

from the boundary-element method, assuming a fab-

rication error. The measurement was reproducible

within 30 mm of dispersion. The BPM was mechani-

cally mounted on the quadrupole magnet within an

accuracy of 100 mm.

Some beam ducts were designed and fabricated

with special care. Four kicker ducts were fabricated

for the new kicker magnets. Their ceramic parts were

manufactured by sintering and planing. In order to

maintain a wide vertical aperture, the top and bottom

walls were made to be thin. Another beam duct with a

ceramic break was fabricated for a dc current trans-

former (DCCT) for measuring the stored beam cur-

rent. The existing DCCT had a problem of heat

generation caused by high-frequency noise leaked

from a ceramic break. A thin, wide ceramic plate

was used as a break for the new duct in order to reduce

the leakage. Its width and capacity are 0.5 mm and

3 nF, respectively. The same type coil as before is used

and its observed heat-up was effectively reduced

compared to the old DCCT [4].

3. Installation and evacuation

Every vacuum duct was pre-baked in our facility

and ®lled with dry nitrogen until installation to the

ring. The nitrogen was introduced through a micro-

®lter cooled by liquid nitrogen to eliminate water and

Fig. 1. Schematic drawing of a typical normal-cell duct.

Y. Hori / Applied Surface Science 169±170 (2001) 728±731 729

micro-dust. The normal-cell duct is directly laid on the

bending magnet. First priority of positioning is given

for the BPM. Because there is no bellows between the

®xed points of the SR port and its downstream BPM,

the SR port is set up with the error of manufacturing,

namely <1 mm. This leads to an angle error of less

than �0.7 mrad. A crotch absorber, TSP and BA

gauge were mounted in the duct after being pre-baked

as well as the beam duct. The SIP was kept in a

vacuum after being pre-baked to minimize pre-

adsorption. Dry nitrogen was always ¯owing through

the previously installed ducts during exposure those to

the atmosphere.

The average pressure of the ring decreased to

2� 10ÿ7 Pa just before beam injection on 1 October.

It was mainly determined at the normal-cell sections

where the local pressure was higher by roughly one

order than that at the other sections. No bake-out was

taken place after the installation, except at the section

where the RF cavities were installed, because it was

expected based on a previous experiment that the PSD

could be effectively reduced by SR irradiation, even

without in situ baking [5]. The yield per current was

very large in the beginning of ring operation. But it

became low enough to continue beam cleaning at

500 mA of the initial current after 1 A h of operation.

The TSP was refreshed whenever necessary to recover

the pumping speed.

4. Pressure and beam lifetime

The ring pressure is monitored every second by 48

BA gauges located around the ring. Two parameters

are conveniently used to evaluate the vacuum perfor-

mance independent of the stored current. One is the

product of the lifetime and the stored current (I � t) to

evaluate the lifetime. The other is the pressure normal-

ized to the stored current (p/I) to evaluate the pressure.

The pumping speed and its distribution was almost the

same; thus a required pressure (p/I) below

1� 10ÿ7 Pa/A must be achieved in the new ring.

The practical change in these two parameters is shown

in Fig. 2, where the operation time is represented by

the time-integrated stored current. The irradiating

photon dose is proportional to the operation time.

An operation time of 1 A h is equivalent to an aver-

aged photon dose of 3:89� 1022 photons/m. The

pressure speedily decreased as the operation time

was increased. The beam lifetime increased, being

inversely proportional to the pressure. The ring was

operated using the modulated optics until high-bril-

liant optics could be put into practice in May 1998.

The pressure once deteriorated around the time of

operation exchange, but gradually recovered again

with operation. A beam lifetime of 500 A min or more

was achieved in June, 1998.

The dominant process of beam loss in the ring was

collisions with residual gas molecules and the

Touschek effect in the low-pressure range. Fig. 3

shows the expected beam lifetime (I � t) as a function

of the pressure (p/I) of carbon mono-oxide for two

cases, each using modulated and high-brilliant optics.

The practical change in the lifetime is re-plotted in the

®gure. There is, of course, a difference between the

observed lifetime and the expected lifetime (calcu-

lated above), caused by the gas composition and gauge

location. The correction factor for them was estimated

to be 1.5. In any case, a simple inverse proportional

relation was observed. The beam lifetime became

longer as the pressure became lower. It was, however,

obvious that the lifetime was always shorter during

modulated operation than expected, just as the actual

acceptance was smaller than estimated.

Large outgassing was observed in the beginning

stage of high-current operation with low-emittance

Fig. 2. Beam lifetime and pressure as a function of the operation

time.

730 Y. Hori / Applied Surface Science 169±170 (2001) 728±731

optics. This resulted from a heat-up of the bumped

inner wall by shortened beam bunches. A number of

unshielded ¯ange gaps and bellows remained in the

unchanged sections. The outgassing was, however,

reduced during the tuning period of low-emittance

operation, and a low pressure was maintained during

user-mode operation. Though the pressure once dete-

riorated in the end of the modulated operation due to

consumption of TSPs, the lifetime maintained to some

extent, as shown in Fig. 2. It seemed that the Touschek

effect was also dominant during modulated operation

in the low-pressure range, as in the high-brilliant case.

On the other hand, the lifetime in the new operation

also agreed well with the expectation; thus, why the

observed behavior was contrary to the expectation

between the two operations is again incomprehensi-

ble. The reason is not yet clear.

References

[1] M. Katoh, et al., J. Synchrotron Radiation 5 (1998)

366.

[2] M. Katoh, et al., in: Proceedings of the 8th European Particle

Accelerator Conference, Stockholm, Institute of Physics

Publishing, 1998, p. 590.

[3] K. Haga, T. Honda, T. Kasuga, T. Obina, M. Tadano, in:

Proceedings of the 8th European Particle Accelerator Con-

ference, Stockholm, Institute of Physics Publishing, 1998,

p. 1517.

[4] T. Honda, Y. Hori, M. Tadano, in: Proceedings of the 8th

European Particle Accelerator Conference, Stockholm, Insti-

tute of Physics Publishing, 1998, p. 1526.

[5] Y. Hori, M. Kobayashi, Vacuum 47 (1996) 621.

Fig. 3. Expected beam lifetime as a function of pressure compared

with measured ones.

Y. Hori / Applied Surface Science 169±170 (2001) 728±731 731