JAERI-Conf 95-021

PB30
Basic Design of an Asymmetric Double Slow Extraction System for the KEK-PS
Yoshihiko SHOJI, Katsumi MARUTSUKA, Takcshi TOYAMA and Hikaru SATO
National Laboratory for High Energy Physics
1-1 Oho, Tsukuba-shi, lbaraki-kcn, 305, Japan

Abstract
Simultaneous extraction to two beam lines of theKEK 12GeV Proton Synchrotron (KEK-PS) has beentheoretically calculated. Unlike double extractionsat other facilities, the two extraction lines are notequal in resonant phase. The test operation is also
reported.
INTRODUCTION
Resonant slow extraction is widely used inalmost all existing synchrotrons. Some of them
have two extraction lines, and extract beam to boththat of lines simultaneously in order to save on theoperation time. ZGS at ANL, reported by E. A.
Crosbie and Y. Cho et al.[l], was initially designed
for double extraction. Its two extraction lines wereset just at opposite sides of the ring, and otherelements were also symmetrically positioned. SPS
at CERN, reported by M. Gyr et al.[2), had two lines
which were not diametrically symmetric. In orderto realize equality of the two lines in resonant
phase, they divided the set of F-quadrupoles into 2
halves and excited them with slightly different
currents. At the KEK-PS the new slow extraction
line (EP1)[3] for the newly built North CounterHall was constructed in 1991, besides the EP2 for
the East Counter Hall. The present lay-out of theKEK-PS is shown in Figure I. Each extraction line
has its own independent extraction systemdesigned for single extraction; they are notsymmetrically positioned. However, strong request
for a longer machine time makes double extraction
an urgent theme. We report on the possibility of
double extraction with minimum change to thepresent system.
BASIC DESIGN OF DOUBLE EXTRACTION
The extraction system of the KEK-PS wasdesigned based. on theoretical studies by K. Endoand C. Steinbach [4], utilizing the half-integer
resonance (2v h=l5). One perturbing quadrupole
magnet (EQ) is inserted into the lattice in order toproduce a half-integer stop-band. One octupole
magnet (OCT) is excited in order to separate phasespace into stable and unstable regions. The tune isthen approached slowly to the half-integer, thestable region becomes ever smaller, and particles
are ejected from the machine. The shaved beam at
the first septum, ESS (electro-static septum}, isdeflected by 5 magnetic septa (named SeptumA, B,C, D and E)(Figure 2) and guided to the extraction
channel.

Figure 1
Lay-out of the KEK-PS
A resonant phase at the ESS is 2jJ j, where JJ j isthe betatron phase advance from the ESS to EQ. It
determines
theperturbationtothetwiss
parameters (a , p and y ) and the angle of theoutgoing separatrix. In normalized phase space,t:. YI t:..XIx=o= tanJJ j= sin(2JJj)/[l+cos(2JJj}] .

Here, X and Y are coordinates defined
following well-known equations:

Screen monitor.--u==---lll'i:~~~~-------- ~ ~C----..-~~~=-u=----

Figure 2

Septum array of a slow extraction line (EPl).

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X=Y =xh(Pv'J3 [ x' +(j3/o.)x],(2)

are the unperturbed twiss
where o. and J3parameters. An unperturbed lattice means that EQis zero, and thus has no half-integer stop-band.
The 2).l j is (3/8)11 at single extraction, as shown in
Figure 3. In order to guide the extracted beamthrough the septum array 2].l j must satisfy(3)

0 < 2).l j < 11 .

When 2).l j = 11 (the perturbed beta function issmallest) the outgoing separatrix arm would hardly
reach to the ESS. On the other hand, when 2].l j isJess than 0 (the perturbed beta function ismaximum at 2).l i=O) the circulating and theextracted beam would not have a sufficient turnseparation at the down stream septum. For double
extraction, both EPl and EP2 should satisfy
equation (3). A solution could exist because theresonant-phase differnce between EPI and EP2 is.D.(2).lj)=-11/2.

y0. 01

~=

ll'\•

Ll.l:

0.005

~~--~~~~~~~~~~x0.01

-0.01

-0.01

Figure 3 Separatrix in normalized phase space.
The next step

wasto performa single-particle-----'t a)i-60~0)( ('T~JTI-i--·.

II.I

tracking simulation. The main lattice magnets (56quadrupoles and 48 dipooles) wereused in thecalculation as thick lens matrices. Their higher
order components were approximated with two thin
mul tipoles set at both ends of each magnet.
Sextupole magnets for chromaticity control andperturbations (EQ and OCT) were approximated asthin-lens components. The strengths of EQ, OCT
and the sextupoles were set at the values used for asingle extraction (their strengths were 0.03 m- 1
and 4.0 m- 3, respectively [5] and the horizontal
chromaticity was -6). We calculated the outgoing
separatrix lines in the x-x' plane of two particles.
One has a Courant-Snyder invariant of zero and a
momentum displacement of -0.08%; the other has a
Courant-Snyder invariant of 311 mm mrad and a+0.08% momentum displacement. Figure 4 (b)shows the separatrices of single extraction. Here,ESS was set so that the step-size was about lOmm.
Figure 4 (a) shows the separatrices at EPl when EQ
and OCT of EP2 were used. Figure 4 (c) shows
separatrices at the EP2 when EQ and OCT of EPI
were used. The resonant phase (2).l j) is -11 I 8,(3/8)11 and (7/8)11 for (a), (b) and (c), respectively.
Table I shows the performance of extraction inthree cases. Case (b) was the best. Although case(c) was acceptable, case (a) was not. Therefore, thesetting of EQ and OCT for EPl is one possible
solution.
For double extraction it would be important toenlarge the step-size because the same step-size
would be shared with two extraction channels. We
calculated two ways to enlarge the step-size. Oneway is to also excite OCT for EP2 besides OCT forEPl. Another is to set the ESS far from theequilibrium orbit (identical to reducing the bumpheight). As listed in Table II, the first way isbetter because the step-sizes at EPl and EP2 werewell balanced.

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.

ESS·,-ESS·r ·-.- •.

>tJ.

tb)
-(_ ()

I.

406om-41J-20(121]mm )Figure 4 Separatrix at the ESS edge(a) Separatrix at EPl when EQ and OCT for EP2 are excited.
(b) Separatrix at EP2 when EQ and OCT for EP2 are excited or at EPI when EQ and OCT for EPI is excited.
(c) Separatrix at EP2 when EQ and OCT for EPI are excited.

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JAERI-Conf 95-021

Extraction parameters at the entrance ofTable I
the ESSs of EP1 and EP2.
perturbation for EP2
EP1
perturbation for EP1(a)Picture in Figure 3
resonant phase 2}1 j (rad) -11 /8
Extraction Bump (mm)13(mrad) 1.1(mm)step-size
7.5
angle divergence (mrad) 1.23
Expected beam loss (%) 22%(mm mrad) 1.611~miuan~.;eEP2
EP2
EP1(b)(c)(3/8)11 (7/8)1115
29-1.2 -3.9
960.65 0.63
4%
5%

2.211

The parasitic use of an internal target (tripleextraction) was also successful.
Under long-term operation, double extraction
was less stable to same external disturbance thansingle extraction. External disturbances, such asdrifting of the magnetic field or emittance change
due to the condition of the injectors, easily change
the intensity ratio of EP1 to EP2, because they areasymmetric. To stabilize double extraction fordaily operation, we need another kind of feed-backsystem.

1.211
EP1
(2.2X!0 11 ppp)
EP2
(1.3Xl0 12 ppp)

Table II Extension of the step-size by (1) exciting
OCT of EP2 or by (2) lowering the extraction bump.
method· extraction line
Bump height (mm)step-size (mm)

(2)

(1)

EPl15
9.5

EP228
8.5

EP11011

EP2258MR(1.6X10 12 ppp)

EXPERIMENTAL RESULTS
For double extraction we started from single
extraction for EPI, and then gradually raised thelocal bump orbit for EP2 to share the step-size ofthe extracted beam. Here, the servo-spill control
system [6] for EP1 was used. The extracted beam
intensity for EPI and for EP2 changed with thebump height at EP2, as shown in Figure 5. The
required bump height of EP2 was higher by 16mm
and steeper by 4mrad than that of EPI, as had been
predicted. However, the step-size at EP2 appeared
to have a long tail, which was not expected.

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30

When a very small amount of the beam wasextracted to EP2 (less than a twentieth of EP1),
double extraction was stable because of the longtail of the step-size at EP2. However this wasthough to be scattered protons mainly at themagnetic septa of EP1, which therefore would when
the beam loss at EPI is improved.
CONCLUSION
We succeeded to extract beams to two lines
within a short period (less than a day). For its
daily use, an improvement of the servo control
system or. of more stabilization of the entire
machine is desired.
Although this solution is not the best, it ispossibly a solution involving a minimum change ofthe present system. If we can afford otherresonances, v h=20/3 or v h= 16/3 there might bebetter solutions, where the resonant phases (3}1 j)of EPI and EP2 are identical. However, we should
redesign most of the system for the third-integer
resonant extraction.

\IEP2

Figure 6 Spill structures of doubly extracted
beam. The spill was controlled only for EPl. Theirlengths were different because the heat toleranceswere different at EP1 and at EP2 [1].

o40Extraction Bump height of EP2 (mm)Figure 5 Bump height of EP2 and the extracted
beam ratio while the bump of EPI is kept constant.

REFERENCES

After a delicate adjustment of the extraction
elements the beam losses at each of the extraction
channels were about 1.5-times those of single
extraction (totally 3 times). The losses were lessdepended on the extraction ratio. Figure 6 shows
the spill structure of a doubly extracted beam toEP1 and to EP2 with an intensity ratio of I to 4.

[1] E.A.Crosbie et ai.,Proc. of 1973 IEEE PAC, p434
Y. Cho et al., Proc. of HEACC 1974, p.451,[2] M. Gyr et al., Proc. of 1985 IEEE PAC, p2997.
[3] Y. Shoji et al, Proc. of the 8-th Symp. on Ace.
Sci. & Tech., 1991, p.281.
[4] K. Endo and C. Steinbach, KEK-77-13. 1973.
[5] Y. Shoji et al., KEK-Report 93-10, 1993.
[6] H. Sato et al, Prpc. of 1992 HEACC, p.495.

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