Journal of Electron Spectroscopy and Related Phenomena 80 (1996) 445-448

C o m m i s s i o n i n g a n d p e r f o r m a n c e o f t h e P o h a n g L i g h t S o u r c e

T. Lee, S. S. Chang, J. Y. Huang, M. Kwon, S. H. Nam, M. Yoon, and T. N. Lee Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang, 790-784 Korea

Commissioning results and the operational performance of the Pohang Light Source, the first syn- chrotron radiation facility in Korea, are described.

1. O V E R V I E W

The Pohang Light Source (PLS) is the first syn- chrotron radiation source built in Korea. It con- sists of a 2 GeV energy injector linac, a storage ring with 280.56 m circumference, and two initial beamlines. The storage ring is designed to be a low emittance third generation machine with a 12 superperiod TBA lattice. It has a total of 22 beamports from bending magnets and 10 from in- sertion devices. After the installation of the PLS storage ring at the end of August, 1994, the com- missioning was accomplished in two steps [1]. The first commissioning, which began on September 5, 1994 and ended in the early morning of December 24, 1994, was primarily concerned with beam cur- rent storage as high as possible. At this time, the aluminum chambers were not baked out in-situ. The vacuum pressure was at the order of 10 - 9

torr, which is good enough for commissioning pur- pose but not good enough for sufficient lifetime necessary for normal operation. The initial injec- tion energy was chosen to be 1.4 GeV instead of full 2 GeV in order for the kicker magnet to make sufficient bump of 21 mm. In the fourth day of the commissioning the first 500 turns of electron bunches in the storage ring were observed, with- out the power to the RF cavity. The stored cur- rent was increased gradually when the RF power was on along with the adjustment of magnet cur- rents and timimg. On October 26, 1994, an elec- tron beam of more than 100 mA at 2 GeV was achieved. At this point, the commissioning team made global orbit corrections for the first time and the dramatic increase in stored current was observed particularly when the sextupoles were

turned on on November 26. As the injection ef- ficiency improved, the commissioning team could further increase the stored beam current. Thus, in the early morning of December 24, 1994, it reached to 300 mA at 2 GeV. However the life- time of the beam storage was short due to a poor vacuum (lifetime of 47 minutes with 100 mA).

Starting from January of 1995, in-situ bake out of the storage ring vacuum chamber and re- alignment of all the magnets have been made. The vacuum chamber pressure droped to the or- der of 10 - l ° torr. The storage ring has been re-commissioned from April 3 to July 20. The purpose of the second commissioning was to pre- pare the storage ring for the opening of the facility to general users, which will start from September. Therefore main concerns of the sec- ond commissioning could be described as follows: (1) It should be possible to reproduce routinely stored current more than 100 mA in a reason- ably short time. (2) The beam lifetime should be long enough. After the photon beam cleaning of the vacuum chamber amounting to 100 Ampere- hours in the period of the second commissioning, the lifetime of 100 mA beam has been improved to 10 hours approximately. Further improvement is expected from more photon beam cleaning and the increase of RF voltage which increases the Touschek lifetime. (3) The storage ring parame- ters should be tuned to the design values, partic- ularly the tunes, the beta function, and the dis- persion function. These numbers showed some discrepances in the early stage of the commis- sioning, which were probably due to the magnet measurement error. It is important to recover

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the design values of these quantities, in order to maintain the tunability, reproducibility, and sta- bility of the storage ring. (4) Finally instabilities should be overcome. One of the serious difficulties for storing high current was a multi-bunch insta- bility, which gave a multi-bunch current thresh- old. Higher order modes (HOM) of RF cavities are sources of this multi-bunch instability. It was possible to reduce this instability significantly by temperature regulation of the cavities. Details of commissioning results are explained below.

On the other hand, two initial beamlines (VUV and X-ray) have been aligned and commissioned. They are ready for the opening of September.

2. C O M M I S S I O N I N G DETAILS

2.1 In jec t ion efficiency It is necessary to obtain reproducible injection

efficiency for normal operation. During commis- sioning, it was observed that the injection effi- ciency depended on the position and angle of the injected beam besides its intensity. Hence two position monitors were installed in the injection area. The injection efficiency also depends on the amount of orbit bump made by kicker mag- nets. In the PLS commissioning, the kicker mag- net power supply could make only 15 mm orbit bump to 2 GeV beam instead of the design bump of 21 mm. Hence a DC bump of a few mm had to be added by correctors near the injection straight section. With 3 or 4 mm DC bump, The injec- tion rate was normally 1 - 1.5 mA/sec, with the best performance of 3.5 mA/sec which means ap- proximately 25 % storing of the injected current. Since the DC bump distorts the stored beam or- bit unnecessarily, it acts as a limit to the injec- tion efficiency at some level. Hence more powerful kicker power supply system [2] has been devel- oped, which is supposed to supply sufficient orbit bump and better injection efficiency. Of course, the current 1 - 1.5 mA/sec injection rate is good enough for normal operation.

2.2 Closed orbi t correc t ion The closed orbit distortion (COD) of PLS is

measured by 108 beam position monitors (BPM) in the ring. The main correction scheme was MI- CADO and it was possible to correct COD to a RMS value less than 0.4 mm, both horizontally

and vertically. In the third generation machine, which needs strong sextupole fields required for chromaticity correction, COD should be reduced as small as possible, because it generates non- negligible quadrupole effects at the position of sextupole magnets. To improve the orbit correc- tion, beam-based measurements of BPMs will be carried out in the near future.

2.3 Tune and dispersion The fractional tunes can be measured in terms

of spectrum analyzer and the integer tunes can be read from the display of BPM readings. Dur- ing the first commissioning, the operational tunes showed some discrepancies from the design val- ues, v~ = 14.28, z/y = 8.18. On the other hand the dispersion values, which can be measured by ob- serving the change in BPM reading as a function of RF frequency, were also different from the de- sign values. Adjusting quadrupole families, it was possible to recover the design tune values and, at the same time, to obtain dispersion function close to the designed function. But more improvement is expected on the dispersion function, especially in the should-be dispersion free straight sections. Since the dispersion function depends on COD, future improvement on COD correction will give more refinement to the dispersion function. On the other hand, a finite vertical dispersion has been observed with RMS value of 0.93 cm, which is supposed to contribute to the vertical beam emittance.

2.4 B e t a f u n c t i o n The fl function has been measured by observ-

ing the betatron tune variation as a function of quadrupole strength. The resulting values are close to the design values as shown in Figure 1.

2.5 Chromat i c i ty correct ion The PLS storage ring was designed to give

large natural chromaticity values of ~, = -23.4, ~y = -18.2, which are responsible for head-tail instability. By turning on sextupoles, it was pos- sible to set these values to small positive numbers, ~, = 2.9 and ~u = 1.4. Figure 2 shows chromatic- ity measurement.

2.6 Linear coupling The linear coupling between the two betatron

motions is one of the causes for vertical beam

emittance. The linear coupling can be measured by observing the difference between the two be- ta t ron frequencies as the coupling resonance is crossed. The minimum difference was measured to be At, = 0.1, which corresponds to ey/e= = 0.008. If e~ is not far from the design value of 1.21 × 10 - s m, the measured linear coupling con- tributes to the vertical beam emittance by the amount of % = 0.96 × 10 -1° m. Measurement of % will be carried out soon.

03 Z _o t - O z LL LU a z~

,_1 o.. :E ,<

30

20

10

0 0

• Bx (measured) • By (measured)

. . . . . . . Bx m / A .y/m 2

lO 20 BEAM PATH LENGTH

Figure 1. j3-plot, ideal and measured values.

15

~. lO

-10

'= dFx (kHz)

I ! ! "15-4 -2 0 2

RF FREQUENCY VARIATION(kHz)

Figure 2. Chromatici ty measurement.

2 . 7 B e a m l i f e t i m e The beam lifetime has been gradually increased

as the time-integrated current has cumulated. Hence the residual gas pressure may still be the most important factor determining the beam life- time. The other crucial factor is the intra-beam scattering (Touscheck effect), which is manifested by measurement of single-bunch lifetime as a

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function of current (figure 3), and by the observa- tion that increasing the RF voltage increases the beam lifetime. Now with RF voltage of 1.2 MV, the life time of 150 bunches and 100 mA beam is approximately 10 hours. After a sufficient time of beam cleaning of the vacuum chamber, the Tou- schek effect will become more important than the residual gas pressure. In PLS, currently three RF cavities are working, but one more cavity will be added at the end of this year to make a total volt- age of 1.6 MV, which will certainly increase the lifetime [3].

(/) r¢ D 2 o I

U_ _.1

Vrf = 1.2 MV

|

I i i= • •

I I

05 10 15 20 I(mA)

Figure 3. Single-bunch lifetime.

2.8 I m p e d a n c e a n d in s t ab i l i t i e s Impedances due to various parts of the storage

ring are sources of instability. Hence it is desir- able, from the design stage, to try to minimize the total impedance budget. The main contributions to the impedance come from RF cavities. Es- pecially their HOMs are sources of multi-bunch instabilities. Other parts mostly contribute to broad band impedance, which is responsible for single bunch instabilities. The total sum of the broad band impedance of the PLS storage ring has been measured to be ] Z / n I ,,~ 0.8~. This reasonable value includes contributions of various parts such as RF cavities, photon masks, bellows, BPMs, transition pieces, and so on. Although no serious single bunch intability has been ob- served, a multi-bunch instability (mainly longitu- dinal) was found to give current threshold, which has been one of the most serious obstacles for high current storing in PLS. In order to suppress this multi-bunch instability, we used temperature regulation of RF cavities and was able to reduce

4 4 8

the instability significantly. A more systematic s tudy on this mat ter is planned and a method like RF knockout will be used to identify the seed frequencies causing the instabilities.

3. S U M M A R Y A N D F U T U R E P L A N

The PLS commissioning carried out in two steps has been successful, in that the goals de- scribed in the OVERVIEW has been more or less achieved. But note that the PLS storage ring is not perfectly adjusted and thus its full capacity is yet to be reached. Hence a more careful study will be necessary in the future. However it is cer- tain from the results of the two commissionings that the PLS is on the right track and thus its performance is getting bet ter as time goes on

The PLS storage ring will provide a total of 32 beamports, 22 from bending magnets and 10 from insertion devices. Presently two beamlines, one for VUV (12 - 1230 eV) and the other for X- ray (4 - 12 keV), are operational. However, four more beamlines will be ready for use by mid-1996.

The future plans are mainly concerned with providing more varieties to radiation users. For

hard X-ray users, two methods are considered; energy ramping to 2.3 GeV and a 7.5 Tesla super- conducting wiggler [4]. Also an undulator (U7) is currently under construction.

A C K N O W L E D G M E N T

Work supported by Ministry of Science and Technology of Korean Government and Pohang Iron and Steel Company.

R E F E R E N C E S

. M. Yoon et al., To appear in the proceed- ings of Particle Accelerator Conference, Dal- las, Texas, 1995.

2. S. H. Nam el al. In preparation.

. M. Kwon et al., To appear in the proceed- ings of Particle Accelerator Conference, Dal- las, Texas, 1995.

. T. Lee and M. Yoon, To appear in the pro- ceedings of Particle Accelerator Conference, Dallas, Texas, 1995.