The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

A Design of the Slow Extraction System in the JHF 50-GeV Proton Synchrotron
Noboru TOKUDA*, Masahito TOMIZAWA, Yoshitsugu ARAKAKI, Shinji MACHIDA,Yosiharu MORI, Tamaki WATANABE
Accelerator Laboratory, High Energy Acclerator Research Organization
3-2-1 Midori-cho, Tanashi, Tokyo 188, JAPAN*1-1 Oho, Tsukuba, Ibaraki 305, JAPANAbstract

1 Introduction
The JHF 50-GeV main ring has four long straight sections (LSS's), each of which is 60 m long, in the presentlattice version [1]. An ordinary layout of an electrostatic
septum (ESS) and magnetic septa would be such that allof them are installed in one LSS. This is, however, infeasible in our case. The beam energy is so high that thedeflection angle in the ESS is small, and hence, at theentrance of the magnetic septum that follows the ESS,
the separation between the extracted beam and the circulating one is not large enough. We therefore install
the ESS in the LSS located in the opposite side of theLSS for the magnetic septa: the LSS's are apart fromeach other by 1/2 circumference. 1 In the present design,
the length and field of the ESS (we call it preseptum)
are respectively 1.5 m and 6.8 MV /m, yielding a deflection angle of 0.2 mrad. Despite of the small deflection,
the beam-beai:n sepation at the first magnetic septum islarge enough to insert a 1-mm-thick septum. The beam
passes through another four magnetic septa, and at theexit of the final septum, it is apart from the equilibrium
orbit by 44 em.
This paper describes the layout of the extractoin devices and beam simulation results for an extraction
scheme using the third-order resonance.

0QDX

0I0QFT

GFS

A slow extraction systefi.l using the third-integer resonance technique has been designed for the 50-Ge V main
ring of the Japan Hadron Facility (JHF). The septa arean electrostatic septum (ESS) and five magnetic ones.
The former and the latter are installed in different longstraight sections, which are apart from each other by 1/2
circumference. Beam simulations show that the beamloss at the ESS wires is "'1 %.

0QDS

8 ...,

0 I B~m_oQFT

QFT2u•• -•ua...,p A-t-"2A-1-.t._Bump
A 2 ~-2-2

PreeeptumGOTGDS0QFS0QDXFigure 1: Layout of the preseptum and the bump magnets (A group).
the septum wire, and the partiCles in the two arms areextracted. For a remedy, we have installed two focusing
quadrupole magnets (QFT2's) additionally only in thisLSS. During the extraction period, this LSS is operated
in a special manner (slow extraction mode): the QFT2magnets are excited; and the field gradients of the latticequadrupole magnets are different from those in the otherLSS's. In the slow extraction mode, the LSS is matchedto the adjacent arcs: the beam-optics functions are continuous at the LSS ends; the phase advance over theLSS is the same as that in the other LSS's (l:l.J.t:r: = 2 1r,
l:l.J.ty = 1r). At the entrance of the preseptum the beamoptics functions are .Bx = 25 m, ax
0.5, TJx
0.35m. 2 The preseptum wires are positioned so that (x, x')= ( -43 mm, -0.25 mrad) at the entrance end; the extracted beam is kicked inward. Four bump magnets (Agroup) produce an orbit with ( -15 mm, 0 mrad) at theentrance of the preseptum.

==

Figure 2: Layout of the magnetic septa (MS-1 throughMS-5) and the bump magnets (B group).

The layout of the LSS for the preseptum is sketchedin Fig. 1. The preseptum is placed at the exit of a focusing quadrupole magnet, because the ,B-function (.Bx)
is large there and the step size is large accordingly; thisis advantageous to reduce the beam loss at the septum
wires. The a-function (ax) is also large. Owing to this,two of the three arms going out from the separatrix cross

The septa other than the preseptum are five magnetic
ones (MS-1 "'MS-5); their layout is shown in Fig.2 alongwith bump magnets (B group). The phase advance between the preseptum and the first magnetic septum MS1 is 3662°; the extracted particles are kicked inward in
MS-1 as well as in the preseptum. The particles receive
further inward kick in MS-2 and outward kick in MS-3,4, and -5. The bump orbit is such that (x, x') = ( -19.75mm, -2.11 mrad) at the entrance of the first magnetic1 The LSS between the above LSS's is used for the rfcavities.

m.

2 Layout of Devices and Orbits

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1n the normal operation, flx =28m, ax = 2.6, 1/x = Q.29

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

3 Beam Simulation
120 :100 :80 i
60 i)(s(m)

Figure 3: Beam envelopes in the long straight-section:1) 3-GeV injected beam (dotted lines), 2) 50-GeV circulating beam (dashed lines), and 3) 50-GeV extracted
beam (solid lines). The coils of the magnetic septa are
indicated by squares, and the edges of the quadrupole
magnets by vertical lines.
septum MS-1.
Figure 3 shows beam envelopes in the long straightsection for MS-1 through MS-5. The envelopes plottedhere are 1) 3-GeV injected beam (c: = 54 1r mm · mrad,Apfp = ±0.42%, COD= ±1 mm), 2) 50-GeV circulating beam (c: = 6.1 1r mm · mrad), 3) 50-GeV extracted
beam. The parameters of the septa are listed in Tables
1 and 2. At the exit of the final septum MS-5 the beam
is separated so far from the equilibrium orbit that itwill not hit the yoke of the quadrupole magnet: the half
width of the yoke is 410 mm, and the particles distributein the range of x = 445 ,. . ., 472 mm.
Table 1
Parameters of the preseptum.
location
deflection angle
electric field
gap (wire-anode)
voltage

LST3.2-0.2 mrad
6.79 MV/m25mm
0.170 MV

Table 2
Parameters of magnetic septa.
septum manget MS-1 MS-2 MS-3 MS-4 MS-5
LSTl LST2 LST3.2 LST3.2 LST2.2
location
64.0
3.0
9.0-1.8 -1.4
okick (mrad)
0.133 0.068 0.665 0.222 1.673
B (tesla)
6.5
2.3
2.3
3.5
2.3
length (m)4040404040gap (mm)
53.2
7.05
21.2
0.423 2.16
NI (kA-turns)
30
101021tseplum ( mm)244182753106)
lseptum (A/mm

For the design of the 50 GeV MR, a computer code
has been developed. This code exec~tes multi-particle
tracking in a x-x'-y-y'-Apfp phase space using transfer
matrices of the lowest order. A thin lens approximation
is used for the sextupole and higher order fields. The initial beam distributions are assumed to be a uniform one
in a four-dimensional ellipse ( x, x', y, y') and a parabolic
one for Apfp.
The position of the preseptum wires is set to -43mm. The horizontal and vertical emittances are 6.17r
mm·mrad, and the momentum spread is ±0:23%. The
horizontal betatron tune is approached to the resonance 65/3 by ramping the lattice quadrupole magnets(QFNs). The horizontal and vertical chromaticity is set
to zero by the sextupole magnets for the chromaticity.
.
correction.
~4a:s2s'r;loSot······~ ..

"C~-2"C-4-60-40-20020x{mm)Figure 4: Phase space plot of a single particle at theentrance of the preseptum.
Figure 4 shows a phase space plot of a single particle
by the simulation. In this case, the sepratrix is rotatedby 18° in the normalized phase space. As a result, theextracted particles have the negative x' for the bump orbit at the entrance of the preseptum, which is necessary
to deliver the particles inward for the MS-1. The horizontal betatron tune is approached to the resonance of21.666 from a higher value. The tune shift per one turnis 5.5 x 10- 6 . The position and the angle of the bumporbit are -15 mm and 0 mrad at the preseptum. Figure 5 shows the phase space distribution and the profileof the extracted beam at the preseptum entrance. Theloss at the wires depends on the beam density and theangular spread around the wires. The angular spreadnear the wires is about 0.7 mrad. When the preseptum
length is 1.5 m and a thickness of the wires is 0.1 mm,
the obtained minimum beam loss is 1.3%.
The beam loss is also caused by particles not extracted
from the ring in the extraction process. These particles
are remained in the ring and lost in the following deceleration process. This loss rate was investigated from thepoint of view of a tune ramping speed. Figure 6 shows
the rate of remained particles as a function of the tune
ramping speed. The remained rate becomes almost zerofor slower speed than 1 x 10- 6 per turn, which corresponds to the spill time of about 50 ms.

-295-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

T4':ti'Ill

20T

20

42...

!. 0__'tl~ -210l~-~0!.fj 0-10

~ -2-4-70 -60 -50 -40 -30

IFigure 7: Phase space distribution and profile at thepreseptum entrance. The bump orbit is varied during
the extraction.
used in our design. The important issue is to minimize
the beam loss. Simulations show that the beam loss atthe preseptum wires is 1%. We will try to reduce the loss
further, and improve the simulation for more precise extimation (the ripples of the magnetic fields have not beentaken into account). From the point of view of clearing
the beam from the ring, the half-interger resonance with
non-zero stopband width is more advantageous. Studies on the beam loss using half-integer extraction will
be done. The decision which resonance to adopt will bemade after comparing the performances.
From the point of view of hardware, the bump andthe septum magnets will be ordinary ones: their magnetic fields and current densities are not far defferent
from those used now in the KEK 12-Ge V proton sychrotron [2]. The preseptum needs R&D before fabrication. In the present design the requied field strength is6.8 MV /m. The voltage is 170 k V for a wire-anode gapof 25 mm. In order to attain such a high voltage, we areplanning to construct an electrostatic septum (ESS) fortests. The main issues are the structure for the feed ofhigh voltages (the goal is 200 k V) and the material of thewires and the anode. The length of the R&D ESS will
be around 0.7 m, and the transverse cross section will beclose to a real ESS. The start of the tests is scheduled
for the next spring.
Acknowledgment

1.5

1i

1i

Tune shift per turn=5.5/N X 10E..fl:z(mm)2ic-20-70 -60 -50 -40 -30:z(mm)There are two kinds of particles' hit to the preseptum wires. One is the head-on hit: the entrance-end
wire is hit. This loss depends on a particle density near
the wires. To reduce it, the step size should be chosen
to be as large as possible within the gap length of thepreseptum. The other hit is such that particles hit theside of the wires. In this case, the loss depends on theangular spread of the particles near the wires as well as
the particle density. In the present design, the preseptum is placed at the position with the small dispersion(0.35 m), and the chromaticity is set to zero. In thiscase, the angular spread is mainly caused by the emittance of the circulating beam, because the separatrix
shrinks with the tune shift. This angular spread can bedecreased by varying the angle of the bump orbit during the extraction. In the simulation, the angle of thebump orbit is varied from -0.8 mrad to 0.65 mrad tokeep the position at -15 mm. Figure 7 shows the phasespace distribution and the profile of the extracted beamat the preseptum entrance. The angular spread has been
reduced by varying the bump orbit ( cf. Fig. 5). As a result, the beam loss at the preseptum wires decreased to0.9%. The emittance of the extracted beam integrated
over the extraction period- has decreased by a factor ofabout 3.

•';;-10-70 -60 -50 -40 -30:z(mm)Figure 5: Phase space distribution and profile of theextracted beam at the preseptum entrance.

~ 2.5]o-4-20-70 -60 -50 -40 -30:z(mm)102a: 0.5

The authors thank S.Ohnuma for valuable suggestions
and discussions.

••
2References
46810Figure 6: Rate of the remained particles as a function ofthe tune ramping speed.

[1] Beam Optics and Dynamics Working Group,"Design of JHP Synchrotrons -Beam Optics andBeam Dynamics (in Japanese)" , INS, JHP-30, July,
1996.

4 Concluding Remarks[2] Y. Shoji et al.,"Slow Extraction System of the KEKPS", KEK Report 93-10, 1993.

Tune Ramping Factor N

We have found a scheme of the extraction from the 50Ge V ring. The third-order resonance technique has been-296-