Proceedings of the 20th Linear Accelerator Meeting in Japan (September 6—8,1995, Osaka，Japan)

B a d lE

Status of 2-GeV Linac at Pohang Accelerator Laboratory^5In Soo Ko, Moo-Hyun Cho, Heung Soo Lee, J o o S h i k B a k , J i n h y u c k C h oi , a n d W o n N a m k u n g
Pohang Accelerator Laboratory, Pohang 790-784, Korea

ABSTRACT
Pohang Accelerator Laboratory (PAL) has constructed the 2-GeV Pohang Light Source (PLS) on August, 1994.
The 2-GeV electron linear accelerator is used as a full energy injector to the storage ring. There are 42 accelerator
structures,11 klystrons o f 80-MW maximum which are driven by 200-MW modulators, and 10 rfpulse compressors in
the 150 meter long linac, Ifs construction and commissioning was completed on June 30,1994. We present themachine scheme and parameters，the commissioning results and the present status of the 2-GeV linac.

I. INTRODUCTION
The 2-GeV linac is a full energy injector for thePohang Light Source (PLS) which is a low-emittancesynchrotron radiation source. The construction andcommissioning of the PLS linac was completed by theend of June, 1994. Since then, it has been serving
2-GeV electron beams to the storage ring (SR) without
any significant failure. The PLS linac consists of 42
accelerating sections,11 klystron-modulator modules，
10 SLAC-type pulse compressors (SLED), 6 quadrupole
triplet magnets, a prebuncher, a buncher, and three beam
analyzing stations (2 installed currently) [1].
The storage ring and the linac are connected b y 9 6 - mlong beam transport line (BTL). It consists o f 5 b e n d i n gmagnets, 24 quadrupoles, 5 vertical correctors, a n d 8
horizontal correctors. The 2-GeV beam leaving thelinac is bent to 20 degrees horizontally b y t w ohorizontal bending magnets toward the injection area ofthe storage ring. After about 65-m behind the end of thelinac, the beam is bent upward to the beam plane of thestorage ring which is 6-m higher than that of the linac.
There are three vertical bending magnets and o n eLambertson-type DC septum magnet in the verticalsection. Each vertical bending magnet and the septum
magnet bend the 2-GeV beam by 8 degrees.
The performances of the linac and the BTL areheavily depended on the reliability of individualcomponents and the best beam optics including magnet
settings and RF phasing. The first-year experience o f
the 2-GeV linac and the BTL operations is reported in
the following sections.

II. G E N E R A L DESCRIPTION
A. Linac
The norminal beam energy of the PLS linac is 2-GeV
with the operating frequency of 2,856 MHz. Thereare 42 SLAC type 2k/3 mode accelerating sections,11klystron-modulator modules including those for thepreinjector. The total length of the linac is 150-m. The* Work supported by Pohang Iron & Steel Co. andMinistry of Science and Technology, Korea.

r eq u i r e d accelerating gradie nt o f the m a i n linac is atleast 15.8 MV/m; If w e consider one or t w o klystrons
as stand-by, it requires a n accelerating gradient o f 17.8
a n d 19.8 MV/m, respectively. In o r d er to a c h i e v e thisaccelerating gradient, w e a d o p t e d h i g h - p o w e r klystronso f 8 0 - M W a n d S L A C - t y p e p o i s e c o m p r e s s o r s e Inaddition, w e required the R F p u l s e length at least 4 |isfor a hig h e r e n e r g y gai n factor f r o m S L E D cavities [2]sThe energy spread with energy 100-MeV at BAS # 1 isless than 0.4 % a n d w i t h e n e r g y 2 - G e ¥ at B A S # 3 isless tha n 0.3 % . T h e e - g u n current in the linac is 1 〜 2A with 1 0 - H z repetition rate. M a j o r p a r a m e t e r s o f theP L S linac are s u m m a r i z e d in T a b l e I.
T a b l e 1 : M a j o r p a r a m e t e r s o f the P L S linac
Beam Energy
2 GeV+/- 0.3 % or less
Energy Spread
2,856 MHzRF Frequency
320 7i.nm.rad at 2 GeVEmittance (Measured)
Current Delivery Ratio> 60 %
10 Hz (60 Hz Max.)
Repetition Rate
15.5 MV/m (min.)Accelerating Gradient> 1 A/ 1.2. or 40 ns
E-gun
50 MW (80 MW Max.)
Klystron Output Power150 mMachine Length
No. of Klystrons
11(=1+10)10
No. of Pulse Compressors
No. of Accelerating Sections 42 (= 2+40)
No. of Quadrupole triplets 6
100 M eV ,1 GeV, 2 GeV
Beam Exit

B_ Beam Transport Line (BTL)The 2-GeV electron beam from the linac is injectedto the storage ring through the beam transport line.
Total length of the BTL is 96-m. There are two 18-m
long branches for the beam analyzing station and for thebeam dump, respectively. There are 5 bending magnets
to form the beam path and 24 quadrupole magnets tofocus the beam in the BTL. The maximum design value
for px is 57-m, and for Py is 56-m. The maximum valuefor T]x is 0.6-m between 2 horizontal bending magnets,

and for i^y is 1.2-m in the last vertical section. The
inner radius of the BTL chamber is 21-mm. The
maximum beam radius calculated from the operation
data taken in last April is 21.9-mm in the verticalsection. The beam delivery rate measured by using 3
beam current monitors in the BTL is more than 90 %.
The major parameters of the BTL are summarized inTable 2.
Table 2. Major parameters of the BTL2 GeV
Beam Energy+/•
0.3 % or less
Energy Spread
320
7C.nm.rad at 2 GeVEmittance (Measured)>90%
Current Delivery Ratio
600 - 1,000 mA (Pulsed)
Injection Current to SR
Machine Length
96 m
5 (2 horizontal,3 Vertical)
No. of Bending Magnets
No. of Quadrupole Magnets 24
Maximum Px (Designed) 57 m
Maximum Py (Designed) 56 m
Maximum T]x (Designed) 0.6 m
Maximum T]y (Designed) 1.2 m
Injection Beam Size
< 3.0 mmHorizontal< 3.0 mm
VerticalIII. O P E R A T I O N R E S U L T S
A. RF Power, Phase
The most important parameter in the linac operationis the beam energy. The RF power is a primary factor togain the beam energy. In the PLS linac, eleven
klystron-modulator modules including one preinjector
module are used to supply high RF power. Each
module except the preinjector supplies the RF power of48 MW to each of four accelerating sections. Without
SL AC-type pulse compressor (SLED), we can get
1.5-GeV energy with that RF power. Using 10 SLEDs,
we can achieve 2-GeV beam or higher. The highest
energy we achieved so far is 2.34-GeV beam on
November 7 ,1 9 9 4 . In the normal operation, the SLED

efficiency to improve enegy is 150 %. During two
weeks on last April, one module was stand-by, so only
10 modules were used to get 2-GeV energy. In thiscase, we improved the SLED efficiency to 163 % byadjusting RF phase.
The RF phase is as important as the RF power toachieve the beam energy. It influences the beam
qualities such as the energy spread, the beam emittance,
and the current delivery ratio. So the RF phase control
is another main factor in the linac operation. As amatter of fact, in the early period of the linac operation,
we sometimes deccelerated the beam because of
mismatched RF phase. Adjusting the RF phase is also
related to the drive power supplied by the klystron
modulator modules. In the PLS linac, the isolator-phase

phase angle
shifter-attenuator (IPA) system controls the phase andthe amplitude of the drive power for each klystron [3].
Fig.l shows the relation of RF phase angle, modulator
high voltage, and the beam energy achieved by theklystron #11. The energy is measured at the beam
analyzing station #3.
The prebuncher and the buncher are another parts
related to the RF power and phase. The electron beamis firstly bunched in the prebuncher and the buncher.
They influence the beam delivery ratio in thepreinjector. Table 3 shows the improvement of thedelivery ratio in the preinjector by adjusting the RFpower and the phase at one particular case.
Table 3. Delivery ratio in the preinjector

Prebuncher Input Power
Prebuncher Phase
Buncher Input Power
Buncher Phase
Delivery RateBefore
AdjustAfter
Adjust31.1 kW334°
0.54 MW284°

6.1 kW154°
1.1 MW
296°
>80%

<50%

B. Beam Analyzing
There are three beam anlayzing stations in the linac.
One is located at the end of the preinjector for 100-MeVenergy (BAS #1). Another is located behind theswitching magnet in the BTL horizontal line for 2-GeVenergy (BAS #3). The third is not installed but ispreserved with its location for 1-GeV energy (BAS #2).

3530

T nif tr .tion

25Stored Current'• Injection Efuciency
o f Operations : 38 Daysノt : 1 Min. ______ ^wfncieincy{%)
11505200-40「 0<s )ISJJnuv(l20TO—781213141718192024252627288910111415161718212223252829301415162021222324
OctoberNovember

December

DateFig. 4 Beam current and the injection efficiency during
the period of October-December in 1994
Emls
lasos
101
5's sa)vzls

20-

The schematic diagram of the beam analyzing station isshown in Fig. 2. The beam energy is measured by thecontrolling of the bending magnet field. The bending
angle a is fixed to 24° at BAS # 1 ,and to 10° at BAS#3, so we control the bending magnet field to locate thebeam to the center of the target. From the magnet field,
we can calculate the beam energy. The energy spread isobtained by the measurement of FWHM of the beam
image on the target by the equation 8p/p=5x/2x (p is themomentum of the beam, 8x is the FWHM of the beam,and x is the distance from the straight beam path to thecenter of the target.) [4]. In the early operation of thelinac, the energy spread of 2-GeV beam was above 0.6%. We tried to improve the beam qualities by adjusting
beam optics. Now we have injected 2-GeV beam to SR
with the energy spread of 0.3 % or less.
Beam emittances are measured by varying
quadrupole magnet fields located in front of the bending
magnet. It can be calculated from the relation of thequadrupole fields and the beam size on the target.
Measured beam emittance is 320 Ti.nm.rad or less inboth horizontal and vertical plane. In the future, we will
expect to obtain more accurate values by using beam
position monitors.
C. Beam Diagnosis
There are 13 beam-current-monitors (BCM) and 12
beam profile monitors (BPRM) for the diagnosis of thebeam in the PLS linac and the BTL including two beam
analyzing stations. The delivery rate of the beam current
is affected mainly by the beam optics. In the early
operation, the delivery rate is less than 20 % in the linac
and less than 60 % in the BTL. We have improved theoptics and now we obtained more than 60 % in the linac
and 90 % in the BTL. By the TUTLE CODE simulation
we expect above 96 % of the delivery rate in the BTL.
The operation results show a good approach of thesimulation results. The electron gun current also affects
the delivey rate. Fig.3 shows the delivey rate in thelinac and the BTL by in terms of e-gun current.

Fig. 5 Beam envelopes in the BTL
efficiency were also increased drastically. We measured
the beam size from the image on the target of the BPRM
in the BTL. It is compared with the simulation resultsby TRANSPORT CODE and is presented in Fig.5,
which shows a good agreement.

IV. S U M M A R Y
The PLS 2-GeV linac has been successfullycommissioned in June 1994 as scheduled, and its
performance exceeds the design values. We obtain
2-GeV energy, high current, low emittance and low
energy spread beam routinely. We injected the beam toSR without any significant failure. During the second
maintenance period in 1995, we will install beam
position monitors and beam loss monitors in the BTL,
so more accurate operations will be expected.

V. R E F E R E N C E S

Fig. 3 Beam delivery rate by the e-gun current
The history of the linac operations are summarized
in Fig. 4 during October-December period in 1994. It
shows clearly that the beam current from the linac wasincreased as we accumulated more experiences for thelinac operation. The stored beam and the injection[1]Designe Report of Pohang Light Source (reviseded.), Pohang Accelerator Laboratory, 1992
[2] W. Namkung et al., "Operation of 2-GeV PLS
Linac,” Proc. 1995 Particle Accelerator Conference,Dallas, Texas, U.S.A. (To be published)[3] H. S. Lee et al., "Microwave System of PLS 2-GeV
Linac,” Proc. 1995 Particle Accelerator Conference,
Dallas, Texas, U.S.A. (To be published)[4] Y. X. Luo et al, "Beam Emittance Measurement
Technique for PLS-IM-T Linac," IEEE *91 Particle
Accelerator Conference, San Fransisco, U.S.A., May
6-9,1991.