Nuclear Instruments and Methods in Physics Research A307 (1991) 207212 NorthHolland
207
A new target design and capture strategy for highyield positron production in electron linear colliders * Mary James 1, Rick Donahue, Roger Miller and W.R. Nelson Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309, USA Received 17 May 1990 and in revised form 22 April 1991
Monte Carlo calculations were performed using EGS4 to simulate the production of positrons by a highenergy electron beam incident on thin wire targets. A 1 nun diameter, 10 radiation length long tungstenalloy wire was chosen as an optimal target. The new target, combined with an appropriate capture strategy, could triple the yield of the highenergy positron production systems currently in use.
I. I n t r o d u c t i o n
Positron beams at highenergy accelerators are produced in an electromagnetic cascade shower initiated by a highenergy electron beam incident on highZ target material. To maximize positron yield, the thickness of the target is shown to be equal to shower m a x i m u m in the target material. Simulations Of the electromagnetic shower reveal that large numbers of lowenergy positrons are produced as the shower progresses. Most of these lowenergy positrons are reabsorbed before reaching the downstream edge of the target. Present targets have transverse dimensions on the order of 1 cm. In this article we examine the possibility of using a small diameter " w i r e " target in order to allow positrons to emerge from the sides, thereby avoiding reabsorption. A different capture strategy [1] designed to maximize the capture of positrons emitted from the proposed wire target is also presented.
energy range of 5 to 20 MeV [3] for tungsten (and uranium) wires with radii ranging from 0.001 to 1 cm surrounded by vacuum. The yields are normalized to the calculated yield from a 6 r.1. semiinfinite slab of tungsten, which reasonably approximates the present SLC positron target. YieM is defined as any positron that ultimately passes through the plane of the downstream end of the target, regardless of its distance from the central axis, its transverse momentum, or its arrival time at the plane [4].
At very small wire radii, h i g h  e n J g y charged particles and photons escape from the sides of the wire, precluding the development of a substantial electromagnetic cascade. At large wire radii the shower reaches maximum development, but many of the lowenergy positrons are reabsorbed before reaching the down
3 2. P o s i t r o n y i e l d s f r o m a w i r e t a r g e t
The EGS4 Code System [2] was used to simulate positron production in tungsten wires of varying diameters. The model assumed a 33 GeV electron pencil beam incident on a 6 radiation length (r.1.) wire. A Gaussian smearing was performed on the output rays to account for finite beam spots. Fig. 1 shows positron yields in the
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* Work supported by Department of Energy contract DEAC0376SF00515. ] Present address: Reed College, Dept. of Physics, Portland, OR 97202, USA.
: i t
9
102 TARGET RADIUS
10 q
10 0
(crn)
Fig. 1. Yield of 5 to 20 MeV positrons from 6 r.1. wires as a function of radius (normalized to the yield from a 6 r.l. semiinfinite tungsten target).
01689002/91/$03.50 © 1991  Elsevier Science Publishers B.V. All rights reserved
208
M. James et al. / Highyield positron production in electron linear colliders
t
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positrons can be captured, accelerated, a n d successfully injected into the SLC.
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~ Beclad~
3. Positron capture and acceleration The present SLC p o s i t r o n source is s h o w n schematically in fig. 3. A b e a m of 33 G e V electrons is incident from the left u p o n a 0.6 cm radius, 2 c m long tungstenalloy cylinder. T h e positrons emerge with a time structure (or b u n c h length) similar to that of the incident electron b u n c h (  6 ps, F W H M ) . In order to preserve this highcurrent, shorttime d u r a t i o n pulse, it would be best to accelerate the b e a m to relativistic velocities as quickly as possible; however, the positrons also emerge from the target with a small radial extent a n d large transverse m o m e n t a . T h e p o s i t r o n b e a m e m i t t a n c e must b e m a t c h e d to the acceptance of the current accelerator capture section, which has a relatively large radial extent b u t small transverse m o m e n t u m acceptance. The required phasespace transf o r m a t i o n is achieved by immersing the emerging positrons in a quasiadiabatically decreasing longitudinal magnetic field, which t r a n s f o r m s most of their transverse m o m e n t u m into longitudinal m o m e n t u m . T h e decreasing magnetic field is p r o d u c e d b y a " f l u x conc e n t r a t o r " m a g n e t which [6] extends for 10 c m from the d o w n s t r e a m edge of the target. In the current SLC source, the flux c o n c e n t r a t o r is followed b y a 1.5 m long, highgradient (30 M e V / m ) accelerator section which accelerates the positrons to 45 MeV. This highgradient section is followed by a 0.5 m i n s t r u m e n t drift space, a n d then one 3 m, lowgradient accelerator section. Next comes a 3 m i n s t r u m e n t a t i o n section in which a magnetic chicane separates the orbit of the positron b e a m from the electron beam. Beyond this point, profile m o n i t o r s a n d current m o n i t o r s give u n a m b i g u o u s information. This i n s t r u m e n t section is followed by two more 3 m accelerator sections which
>
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~ o 4
6 e+/inc.) I I I 12 6 8 10 TARGETLength (tungstenr.I.)
Fig. 2. Positron yield vs target length for 1 mm diameter tungsten wires clad with various materials (520 MeV).
stream edge of the target. Fig. 1 suggests that the m a x i m u m positron yield will be o b t a i n e d with a 1 m m d i a m e t e r wire. This yield is f o u n d to b e 2.5 times greater than t h a t achieved by similar E G S 4 calculations for the present SLC p o s i t r o n source [5], the increase due to particles emerging from the sides of the target. After fixing the d i a m e t e r of the tungsten wire to 1 mm, we simulated p o s i t r o n p r o d u c t i o n in wires of varying lengths s u r r o u n d e d by vacuum, beryllium a n d carbon. Fig. 2 shows the positron yield for wire targets r a n g i n g from 5 to 10 r.1. T h e curves have again b e e n normalized to the yield o b t a i n e d with a 6 r.1. semiinfinite slab of tungsten chosen to a p p r o x i m a t e the geometry of the current SLC target. As fig. 2 shows, a yield of almost 100 positrons per electron can be achieved using a 10 r.1. wire suspended in v a c u u m  a factor of 4.3 increase over the 6 r.1. infinite slab target. F u r t h e r simulation showed t h a t 1 m m was still the o p t i m u m d i a m e t e r for a 10 r.1. wire. Since most of the increased yield is due to lowenergy positrons emerging from the sides of the wire, we now turn our a t t e n t i o n to the question of whether these
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Fig. 3. Positrondedicated linac and initial transport system.
209
M. James et aL / Highyield positron production in electron linear colliders
boost the mean particle energy to 200 MeV  the design energy of the 180 ° bend magnets that steer the beam into the positron return line. To understand and optimize positron capture downstream of the target, the dynamics of positrons undergoing acceleration in a sinusoidal longitudinal electric field must be examined. The change in the particle's energy as a function of position along the central axis is given by
d7
eE sin/9 mo c2 '
_
dz
80
(~ 6 0 o E)')

40
2~r( 1 3` flw
1) flz '
dz
2~(1 
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7
flw
2,1/2 (3`2 _ 1  P t )
2nr mo c2 [
X
~
i¸

i 200
100 PHASE
i
i
0
100
200
(degrees)
Fig. 4. Longitudinal phasespace trajectories of positrons with transverse momentum (0 < Pt < 2moc). The horizontal axis is phase with respect to the accelerating field phase null. The vertical axis is longitudinal momentum in units of moc.
8°i 60t (a) 80
o_
4O
40
2O
2O 0
0
) .
~3`_(3'2_1_pt)
(3)
2,tl/2~ ),
I,
200
100
 100
400
2O0 (c)
300 II
I
2OO 100
oov/
0
100
ii(d)
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I
200rl
II1 lOO I
200
J 2o0
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(4)
which gives the orbits in longitudinal phase space, 3', O. Particles whose positions in phase space are on the right side of fig. 4 are decelerated by the E field until they lag in phase enough to drop behind the field null (O = 0) and are accelerated. They eventually reach a velocity nearly synchronous with the wave velocity, after which their phase with respect to the wave is constant• The shaded area represents the phasespace trajectories of particles with transverse m o m e n t a between 0 and 2moc and with an asymptic phase of  9 0 o. The dashed line represents particles with transverse m o m e n t u m equal to 2moc. Given their higher total m o m e n t u m for the same longitudinal momentum, they are accelerated
(b)
60
"F
For a velocity of light capture section, flw = 1. If the section is immersed in a uniform axial magnetic field, the transverse momentum, Pt can be considered constant, since the radial rf forces are small. For this case we can integrate eqs. (1) and (3) yielding cosO==cosO
111
(2)
where 3' = wavelength of the accelerating field, flw = velocity of the accelerating wave in units of c, and flz = axial positron velocity in units of c. Expressing flz in terms of 3' and the transverse m o m e n t u m Pt in units of moc, eq. (2) becomes dO
if!
LJ
0
dO dz
I!i II
uJ o9 < " r 2O I:L
(1)
where mo c2 = rest mass energy of the positron, 7 = particle energy in units of m0 c2, e = charge of the positron, E = electric field strength, and z = distance along the central axis. The change in the particle's phase with respect to this accelerating field, 8, is given by
ii ii
O
100
100
uJ
200
 2 0 0 100 0 100 PHASE (degrees)
I
200
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2500
(e)

1500
v 13_
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ILII
ooilii i, 0
200
III ~"
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'
I
'
100 PHASE
100 (degrees)
200
Fig. 5. Longitudinal emittance of positron bunch at five positions along the current SLC capture system (see text). The vertical smear of points in (a) is a direct result of time not being included in the original EGS calculations.
210
M. James et aL / Highyield positron production in electron linear colliders
less by the E field and therefore follow a trajectory with higher m i n i m u m longitudinal momenta. Figs. 5a5e represent the calculated longitudinal emittance of the positron bunch at five positions along the current SLC capture system. Fig. 5a represents some 63 e + / i n c i d e n t e  emerging from the target with a phase of  3 ps (1 o at Sband  1 ps). Fig. 5b represents the longitudinal emittance of the bunch as it emerges from the flux concentrator. The particle trajectories in the capture system were calculated using the E T R A N S raytracing code [7]. The lower longitudinalm o m e n t u m particles lag in phase because of their smaller longitudinal velocities and their longer (spiral) pathlengths through the flux concentrator. By the end of the flux concentrator, the bunch has a phase extent of  80 ps. Only 8 e + / e  arrive at the entrance to the highgradient accelerator section, where the mean energy is increased to  45 MeV. At the end of the 9 m lowgradient accelerator the bunch has a mean energy of  190 MeV (see fig. 5c). A total of 5.3 e + / e  reach this point, 90% of which are within the transverse acceptance of the positron return line. About 70% of the remaining positrons, or 3.5 e + / e , are within the 20 MeV acceptance of the 180 o return line bends (see fig. 5d). The length of the positron bunch determines its final energy spread at the entrance to the SLC damping rings. The 30 ps bunch length results in a 4% energy spread by the time the positrons are accelerated to 1 GeV at the entrance to the damping ring. As the damping ring acceptance is _+1%, a loss of 25% occurs (see fig. 5e). Thus in the current positron source,  2.5 e + / e  arrive at the injection point of the damping ring.
4. W i r e t a r g e t a n d n e w c a p t u r e s t r a t e g y
Our proposed design combines the higher yield "wire" target with a lowgradient continuous acceleration capture scheme proposed in ref. [1]. To understand how to optimize the capture in longitudinal phase space, we look at eqs. (3) and (4). To the second order in Pt/"/ and 1/"/, eq. (3) can be approximated by
dO 2,~ dz  ~ 
1 fiww
(
1 1 +  2"/2 +
1 1 ]1/2
2y2
/ (5) In a drift region "/ is constant, and if we let flw = 1, this equation represents the phase lag of a particle with energy "/and normalized transverse m o m e n t u m Pt relative to a velocity of light particle moving parallel to the
axis. In an adiabatically tapered magnetic field the transverse m o m e n t u m varies as 
[ B(z)
~1/2
Pt(z) Pt0~}
•
(6)
In drifting a distance D between the e + target and the accelerator the phase slip relative to a velocity of light particle is A 0 = ~,,
+ 2~00f
° Be dz
.
(7)
This can also be written as A 0 =  ~,,
( 0 + 2021p, ) K
(s)
where 1
D
K = ff( fo Bz dz;
(9)
B 1 = the uniform magnetic field in the capture accelerator section, and Ph = the transverse m o m e n t u m in the capture section. Using eqs. (4) and (6) we can find the condition for two positrons which were created in the target at the same time to have the asymptotic phase. First, consider two positrons with Pt = 0 and different energies, "/1 and "/2. They will have the same asymptotic phase if
eE2sln~o12= 1 [ ]'2"/1 1
moc
D l ~
]'
(10)
where ~ is the mean sin 0 for the two particles as they enter the accelerator sections. Clearly, sin 0 must be positive which is the region where the particles are decelerated. We can also make the asymptotic phase O~ independent of the transverse m o m e n t u m for some energy "/3 chosen to be between "/1 and "/2. Using eq. (6) to find the phase with which the particles enter the accelerator, and eq. (4) to determine their asymptotic phase, we find the condition to be
e E ~ ~"' = T rv03   1
mocz
"y32
K ("/2 1) 1/2'
(11)
where sm~ff~o 3 is the mean value of sin 0 for the two particles entering the structure with differant transverse momenta. Again we find we want to decelerate. If "/2 >> "/1 (in our case, they are 40 and 2, respectively), "/2 >> 1, and ~ = sm~ff~0 > we can simplify further and combine eqs. (10) and (11) to yield K "/3 D "/1'
(12) 1
eE2sln~m2 9"/1,
mo£
(13)
M. James et al. / Highyieldpositron production in electron linear colliders where K / D is the ratio of the average magnetic field in the flux concentrator (or tapered field solenoid) drift region to the constant magnetic field in the accelerator section. Since we would like to cancel the transverse m o m e n t u m effect on asymptotic phase near the minim u m energy, where it has the largest impact, we want K / D to be as near to unity as possible. This means we must taper down the magnetic field in the flux concentrator as fast as possible, consistent with the large broadband acceptance in transverse phase space. Secondly, since the left hand side of eq. (13) is roughly the rate of acceleration in the capture region, we would like D to be rather small. We propose to replace the first 1.5 m accelerator section and the following 0.5 m instrumentation section of the SLC positron source with a 2 m section closely coupled to the following 3 m section. This 5 m capture region is to have an accelerating gradient of 12 M e V / m . Figs. 6 a  6 e represent the calculated longitudinal emittance of the positron bunch at five positions along the proposed capture system. Fig. 6a represents 195 e ÷ / e  emerging from the 1 mm diameter, 10 r.l. wire target. Lowenergy positrons emerging from the sides reach the plane of the downstream edge of the target as
211
much as 100 ps behind the first positrons. Fig. 6b shows the longitudinal phase space occupied by the positrons which emerge from the flux concentrator Some 16.5 e + / e  occupy a phase extent of  80 ps. Their distribution in phase space is quite similar to that of fig. 5b. Rather than inject this beam into a highgradient accelerator section, to minimize any further phase spread we chose to inject the beam into a backbased, lowgradient accelerating field. We chose the phase and gradient of the rf such that the phase occupied by the beam lay along the " c o n t o u r lines" of the particle orbits as shown in fig. 6b. The beam's subsequent trajectory in phase space results in the smallest possible asymptotic phase spread. In order for this strategy to work the positrons must be uniformly accelerated with no drift sections until all the positrons have energies > 10 MeV. Since some positrons are decelerated from 20 MeV down to low energy and then are reaccelerated, this takes about 30 MeV of acceleration, or about 3 m of the 5 m lowgradient accelerator. Thus, for this strategy, we eliminate the 0.5 m instrumentation section that serves little purpose since both electrons and positrons are in the beam at this point. Figs. 6 c  6 e illustrate the motion of the beam as it traverses the 5 m lowgradient ( E = 12 M e V / m ) accelerator (fig. 6c), followed by 6 m of highgradient ( E = 29 M e V / m ) accelerator (figs. 6d and 6e). We propose to replace the quadrupole focusing on the final 6 m accelerator section with solenoidal focusing, which eliminates losses due to the represent transverse acceptance at the entrance to this structure. We find that 9.5 e + / e  arrive at the end of the 6 m lowgradient structure with a phase extent of  15 ps. Note that most of the positrons are initially decelerated, yet the final phase spread of the beam is significantly less than that of the present source. Because of the reduced phase extent of the beam at the 190 MeV point, only 2% are lost in the 180 ° bend leading to the positron return line (fig. 6f). The 15 ps bunch length is small enough to accelerate all particles to 1 GeV within the 1% energy acceptance of the damping ring. Our simulations predict that 9.3 e + / e  can be produced within a suitable phase space for damping ring acceptance  more than tripling the present yield.
looi.:i, loo_ :t///////l/ll/l//I 60
0
0
200
100
0
100
0
0
200
100
5OO
100
200 200
200
100
200
:J
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IJ II II (e) 500
400
(f)
400 300
T300 v
5. Summary of results
2O0
2OO
100
IO0 0 200
0
100
100 PHASE
100 (degrees)
C 200 200
100 PHASE
100 (degrees)
I 200
Fig. 6. Longitudinal emittance of positron bunch at six positions along the proposed capture system (see text).
Since it is possible to upgrade the target and capture system separately, we divided the simulations as follows: 1) The production of positrons by the wire target (WT), but injected into the current capture system (CC). 2) The production of positrons by the SLAB target
212
M. James et aL / HighyieM positron production in electron linear colliders I 100
,
x
=
~.~
10
I
I.
I
~
o
=~
"" . . . . . . .
9.3
:'c~2.~.. . . . '6.8 .................
a.
]
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43
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Fig. 7. Positron yields at various stages of capture for the four combinations discussed in the text.
m i g h t be i m p r o v e d with existing hardware, possibly tripling the current yield. F u r t h e r work includes additional simulations with E G S 4 to explore shaping the target to o b t a i n the highest possible p h o t o n track length, a n d including recent [8] lowenergy e n h a n c e m e n t s to EGS4. F u r t h e r study of the capture system is needed to explore possible imp r o v e m e n t s in the transverse phasespace capture. In particular, we wish to explore the possibility of immersing the d o w n s t r e a m e n d of the target in the field of the flux c o n c e n t r a t o r to reduce loss of positrons emitted from the sides of the target. A d d i t i o n a l study is also required to u n d e r s t a n d a n d realize suitable mechanical properties of the wire target including strength, heat transfer, durability, a n d the effects of finite b e a m sizes.
(ST), but injected into the proposed capture system
(PC).
References
T h e results for all four c o m b i n a t i o n s are s u m m a r i z e d in fig. 7 a n d table 1.
6. Conclusions
W e presented a M o n t e Carlo simulation of p o s i t r o n yields from wire targets d e m o n s t r a t i n g that the current p o s i t r o n p r o d u c t i o n a n d capture system of the SLC
Table 1 Summary of target yields (e÷/e  )
Current capture Proposed capture
Slab target
Wire target
2.7 4.3
6.8 9.3
[1] B. Aune and R.H. Miller, Proc. 1979 Linear Acc. Conf., Montauk, NY (1979). [2] W.R. Nelson, H. Hirayama and D.W.O. Rogers, SLAC265 (1985). [3] This energy range is consistent with the current SLC targetcollector system. [4] EGS4 was modified to include time as a particle property. [5] J.E. Clendenin, IEEE Part. Acc. Conf., Chicago, IL, 1989; SLACPUB4743 (1989). [6] Y.B. Kim and E.D. Platner, Rev. Sci. Instr. 30 (1959) 524; and M.N. Wilson and K.D. Srivasta, Rev. Sci. Instr. 36 (1965) 1096. [7] H.L. Lynch, Documentation of ETRANS Program, SLAC internal memo to the SLC Positron Source Group (1988). [8] T.M. Jenkins, W.R. Nelson and A. Rindi, Monte Carlo Transport of Electrons and Photons (Plenum, 1988).