A new target design and capture strategy for high-yield positron production in electron linear colliders

A new target design and capture strategy for high-yield positron production in electron linear colliders

Nuclear Instruments and Methods in Physics Research A307 (1991) 207-212 North-Holland 207 A new target design and capture strategy for high-yield po...

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Nuclear Instruments and Methods in Physics Research A307 (1991) 207-212 North-Holland

207

A new target design and capture strategy for high-yield 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 high-energy electron beam incident on thin wire targets. A 1 nun diameter, 10 radiation length long tungsten-alloy wire was chosen as an optimal target. The new target, combined with an appropriate capture strategy, could triple the yield of the high-energy positron production systems currently in use.

I. I n t r o d u c t i o n

Positron beams at high-energy accelerators are produced in an electromagnetic cascade shower initiated by a high-energy electron beam incident on high-Z 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 low-energy positrons are produced as the shower progresses. Most of these low-energy 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. semi-infinite 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 low-energy 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 DEAC03-76SF00515. ] Present address: Reed College, Dept. of Physics, Portland, OR 97202, USA.

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10-2 TARGET RADIUS

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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. semi-infinite tungsten target).

0168-9002/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

208

M. James et al. / High-yield positron production in electron linear colliders

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positrons can be captured, accelerated, a n d successfully injected into the SLC.

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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 high-current, short-time 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 phase-space transf o r m a t i o n is achieved by immersing the emerging positrons in a quasi-adiabatically 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, high-gradient (30 M e V / m ) accelerator section which accelerates the positrons to 45 MeV. This high-gradient 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, low-gradient 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 (5-20 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. semi-infinite 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 low-energy 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. Positron-dedicated linac and initial transport system.

209

M. James et aL / High-yield 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

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_

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(~ 6 0 o E)')

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-100 PHASE

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100

200

(degrees)

Fig. 4. Longitudinal phase-space 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.

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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 phase-space 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!

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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

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100

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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 / High-yield 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. 5a-5e 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 S-band - 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 ray-tracing 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 low-gradient 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 low-gradient 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. / High-yieldpositron 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 broad-band 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. Low-energy 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 high-gradient accelerator section, to minimize any further phase spread we chose to inject the beam into a back-based, low-gradient 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 low-gradient 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 low-gradient ( E = 12 M e V / m ) accelerator (fig. 6c), followed by 6 m of high-gradient ( 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 low-gradient 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.

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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 / High-yieM positron production in electron linear colliders I 100

,-

x

=

~.~

10

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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] low-energy 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 phase-space 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, SLAC-265 (1985). [3] This energy range is consistent with the current SLC target-collector system. [4] EGS4 was modified to include time as a particle property. [5] J.E. Clendenin, IEEE Part. Acc. Conf., Chicago, IL, 1989; SLAC-PUB-4743 (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).