Transcript A GRANULAR CONVERSION TARGET FOR THE HYBRID …

A GRANULAR CONVERSION TARGET FOR
THE HYBRID POSITRON SOURCE FOR ILC
P.Sievers (CERN), X.Artru, R.Chehab, M.Chevallier
(IPNL), O.Dadoun, A.Variola (LAL), C.Xu (IHEP and
LAL), V.M.Strakhovenko (BINP)
Thanks to L.Rinolfi, A.Vivoli (CERN), T.Kamitani, T.Omori, T.Suwada,
J.Urakawa (KEK),T.Takahashi (Hiroshima-U)
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P.Sievers/POSIPOL2011/Beijing
A GRANULAR CONVERSION TARGET FOR
THE HYBRID POSITRON SOURCE
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PLAN
* 1- Introduction
* 2- The hybrid positron source: a recall
* 3- The granular amorphous converter
* 4- The simulation results
* 5- Technical Solutions
* 6- Summary and discussion
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1-INTRODUCTION
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The hybrid positron source uses an oriented crystal where channeling
radiation and also coherent bremsstrahlung provide a large amount of
soft photons and an amorphous target, where these photons materialize
into e+e- pairs [ theoretical works from BKS* and others and Experiments
at CERN and KEK] . The hybrid source allows important decrease of the
PEDD (Peak Energy Deposition Density) provided the charged particles
emerging from the crystal are swept off. However, the average heating of
the target has still to be lowered. For this, rotating target wheels have
been considered. High rotation speeds lead to technical difficulties (Eddy
currents brought by pulsed magnetic lenses, rotating vacuum seals,..).
One is therefore led to limit the wheel speed, hence the tolerable amount
of thermal power deposited in each target of the wheel: that implies also
an appropriate time structure of the beam. An interesting solution using
granular targets, developed for Neutrino factories projects and in
Spallation sources [See P.Sievers’s references] can be also interesting
for positron sources. We present here an hybrid positron source using
an amorphous granular substance as converter. This principal will be
applied to the ILC project.
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*BKS: Baier-Katkov-Strakhovenko
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A GRANULAR CONVERSION TARGET FOR
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2-THE HYBRID POSITRON SOURCE: A RECALL
R.Chehab, V.M.Strakhovenko, A.Variola
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A High energy electron beam (5 to 10 GeV) is directed along the crystal
axis . Photons, electrons and positrons are generated. A sweeping
magnet takes off the charged particles and only the photons impinge on
the amorphous converter, placed about 2 m further downstream. For the
ILC we consider an e-beam at 10 GeV with a transverse rms-width of
σ=2.5 mm.
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3-THE GRANULAR AMORPHOUS CONVERTER
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As already pointed out (see P.Pugnat, P.Sievers) [J.Phys.G.Nucl.Part.Phys.29
(2003)1797-1800]
a granular converter made of small
spheres of ~ mm radius offers the
advantages of presenting a relatively
high [surface/volume] ratio which is
interesting for the power dissipation.
Staggered rows of spheres have been
considered, which leads to an effective
density of about 72 % of pure W.
The densest possible packing would be
85 % with special arrangement.
A comparison has been carried out between
the granular and the compact targets
concerning the yield and deposited energy.
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4-THE SIMULATION RESULTS
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Simulations have been made with two dedicated programs:
- the simulation program of V.M.Strakhovenko (VMS), taking into
account the crystal effects and embedded into GEANT4.
- the program G4FOT using the simulation program FOT of X.Artru,
and embedded into GEANT4.
Both dedicated programs are in good agreement. We shall present the results
without specifying the origin of the program.
Results are presented for the total and the accepted yield, the positron energy,
the transverse size distribution and emittance, the time duration of the bunch,…
A table giving the compared features of granular and compact targets is given
below.
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P.Sievers/POSIPOL2011/Beijing
A GRANULAR CONVERSION TARGET FOR
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COMPARISON: GRANULAR & COMPACT
The number of sphere layers is counted along the longitudinal axis
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thickness
yield
PEDD
(ΔE)dep
N-layers
spheres
number
Effective
density
unity
mm
e+/e-
GeV/cm3/
e-
MeV/e-
g.cm-3
compact
8
13.3
2.18
523
19.3
Granular
r=1mm
10.16
12.18
1.88
446
3
864
13.9
Granular
r= 0.5mm
11.60
13.45
2.33
613
7
8064
13.9
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Comments on the table:
* The granular target is thicker than the compact for the same
yield, as the effective density is lower
* The energy deposited increases with the thickness
* The PEDD values are depending on the elementary volume; for
volumes from 1 to 4 mm3 the differences are within 20 %.
* The effective density of the two granular targets (r=1 mm and r= 0.5 mm)
is the same as every granular target is divided in a number of
mini-cubes, containing, each, 8 spheres.
* An additional table gives information on the positron
characteristics ( mean energy, transverse dimension and
momentum, time duration,..)
* The choice of the granular targets have been made to approach
the values of the positron yield of the compact, 8 mm target.
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A GRANULAR CONVERSION TARGET FOR
THE HYBRID POSITRON SOURCE
VMS-ILC
Granular r=1 mm
l = 10.165 mm
Layers = 3
Granular r=0.5 mm
L =11.6 mm
Layers=7
position
Target end
Capture end
Target
rms
x/mm
3,41676
8,5781
Px/MeV
6,44369
3,07627
Energy/MeV
24,2406
19,0081
Time/ps
11,3146
40,5014
mean Energy/MeV
28,143
19,4781
yield
e+/e12,1884
4,2972
pedd/(GeV/cm^3/
PEDD e-)
1,87858
Compact l = 8 mm
Captur
end
Capture end Target end e end
3,41845
8,57933
3,14229 8,545
6,36228
2,83137
6,46721 3,125
23,9412
18,6325
24,0696 18,77
11,2453
42,5559
10,7834 43,33
27,3703
18,7457
27,9593 18,74
13,4544
4,6432
13,26
4,53
2,33801
2,18018
COMPARISON BETWEEN GRANULAR AND COMPACT TARGETS
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We can notice that the PEDD values are different for the 3 cases. A probable
explanation is due to the difference of elementary volumes where it is determined:
Granular (r=0.5 mm): volume=0.5 mm3 and PEDD= 2.33
Compact volume= 1 mm3 and PEDD= 2.18
Granular (r=1mm): volume =4 mm3 and PEDD = 1.88
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A GRANULAR CONVERSION TARGET FOR
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COMPARISON GRANULAR/COMPACT TARGET
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The yield and total deposited
energy/incident electron have
been simulated for a compact
and a granular target
(w.r.t. the number of layers ).
We can see, here, that a granular
target with 3 layers has slightly
lower yield than the compact
one (8 mm thick) and the energy
deposited is also lower. The 3-layer
granularity seems convenient.
.
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A GRANULAR CONVERSION TARGET…
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SIMULATION RESULTS FOR THE PHOTON BEAM ON THE GRANULAR TARGET
The transverse (x-y) dimensions and the marginal photon distribution in x-plane are
represented below: the γ have been created by an electron beam of 10 GeV (having an rms
radius of 2.5 mm ) impinging on a 1mm thick tungsten crystal oriented on its <111> axis.
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X-Y distribution of γ beam on granular target
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Marginal photon distribution on X-plane
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A GRANULAR CONVERSION TARGET FOR
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SIMULATION RESULTS FOR THE POSITRON BEAM: the ILC case
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The positrons created in the granular target (3 layers) are captured by an AMD.
The AMD has the following characteristics:6T-0.5Tand L=50cm. Energy
spectrum, transverse dimensions and momentum are given.
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At the target exit
After capture and 1 meter acceleration
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POSITRON EMITTANCE WITH THE GRANULAR TARGET
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The target is made of 3 layers of spheres (Φ= 2 mm); the case is for ILC
P.Sievers/POSIPOL2011/Beijing
A GRANULAR CONVERSION TARGET FOR
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SIMULATION RESULTS: ENERGY DEPOSITED AND PEDD (ILC)
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The precise configuration is
described below for the case of ILC.
In order to lower the amount of energy
deposited per pulse and per second
- we divide the original pulse into
many minipulses
- we choose a multi-target system
on a rotating wheel
The pulse configuration is that of
Omori/KEK known as the 300 Hz
solution. The new ILC data is taken
with ~ 1300 bunches shared into 13
Mini trains with a separation of 3.3 ms.
Each mini train contains 100 bunches.
The duration of the macro pulse is 40 ms,
leaving 160 ms for damping in DR.
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P.Sievers/POSIPOL2011/Beijing
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THE SIMULATION RESULTS: ENERGY DEPOSITED AND PEDD (ILC)
For the chosen system (300 Hz)
We have determined the energy
deposition density in the (x,z)
plane; where z is the propagation
axis. The spheres have 1 mm
radius and correspond to a volume
of 4mm3. The largest value (PEDD)
is 1.8 GeV/cm3/e-; it is slightly lower
than that of a compact target.
The total energy deposited in the
target is ~ 446 MeV/e- for the
3 layers target.
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The case of small spheres (r=0.5 mm)
We report the energy deposition density
The maximum PEDD is
About 2.33 GeV/cm3/eThe PEDD is higher than
in the case r=1 mm (1.88).
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5- The heating and cooling of a granular target (for ILC)
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In granular targets, consisting of densely packed spheres with sizes smaller than the rmstransverse beam profile of 3.5 mm, only small and essentially linear temperature gradients
will be created across each individual sphere. Since this will be able to expand relatively
freely it will be submitted to negligible thermal stresses.
Thermally induced shocks can be neglected when: R/2<<to.c; R: radius of the sphere of 1
mm, to: Pulse duration of a micro pulse of 0.6 micro seconds; c: velocity of sound in
Tungsten of c=4.103 m/s. For ILC this is satisfied in good approximation.
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Another important feature of the granular target is that the deposited heat can
be evacuated rapidly and at the location of heat deposition by the cooling fluid
passing between the spheres.
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A GRANULAR CONVERSION TARGET FOR
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A GRANULAR TARGET DISTRIBUTED ON A ROTATING WHEEL
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A stationary target would receive a too large amount of deposited energy,
leading to serious heating problems. -To reduce the adiabatic temperature rises
induced by one macro pulse, the beam energy can be diluted by sweeping, rotating
the target.
Since the deposited beam energy is concentrated within a diameter of about 1 cm
( see previous plot), with a linear velocity of the rim of the rotating wheel of about 3
m/s, the rim is displaced by 1 cm over 3.3 ms, so that the energy of each micro pulse
is separated from the adjacent, following micro pulse. Only very little pile up occurs.
The calculated maximum temperature rise in the target rim, resulting from the PEDD
per micro pulse, is not exceeding 222 K. Details on the wheel and on the target
container (Be and Ti) are presented below.
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A GRANULAR CONVERSION TARGET FOR
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THE GRANULAR WHEEL TARGET
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A GRANULAR CONVERSION TARGET FOR
THE HYBRID POSITRON SOURCE
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The W spheres are filling a ring container made of a light element as Titanium, Beryllium in order to
lower the energy deposition and the multiple scattering.
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The entrance and exit windows (1 mm thick in beam direction) are Be whereas the upper and lower
container structures are made of Ti. The data obtained for the hybrid source in ILC conditions [E=10 GeV, L(crystal) = 1 mm, Granular target with 3 layers] is:
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Energy deposited
PEDD
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MeV/eGeV/cm3/eBe/up
0.12
0.0016
Be/down
10.6
0.17
Ti/up
0.14
0.0021
Ti/down
0.14
0.0021
The beam parameters are for ILC: E-=10 GeV; L(crystal)=1mm; Distance Rad-Conv =2 m
The PEDD in the downstream window has been calculated as 30.6 J/g per micro pulse which leads
to an adiabatic temperature rise of 16.3 K/micro pulse. This low temperature is clearly due to the
low ionization rate dE/dx in Be.
Also the thermally induced stress per micro pulse are about 30 MPa, 13% of its yield strength.
Material fatigue with about 3 Mio thermal cycles per 100 days of continuous operation and
radiation damage may however limit their lifetime. Similarly, fatigue of the W-spheres, although
considered as stress free, may occur due to the temperature pulses of 222 K/micro pulses, again
at a rate of 3 Mio/100 days.
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The Granular Target Container
A GRANULAR CONVERSION TARGET FOR
THE HYBRID POSITRON SOURCE
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COOLING OF THE ROTATING WHEEL
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-The minimum periphery of the rotating wheel would be 13 cm, i.e. a diameter of 4 cm, to distribute
uniformly a macro pulse over its rim over 40 ms. This results in a rotation frequency of 23 Hz or
1400 rpm, which may be difficult to achieve with rotating seals.
Therefore a larger wheel with a diameter of 58 cm is considered. With a velocity of the rim of 3.25
m/s, allowing for comfortable separation between micro pulses, this leads to a rotation at 1.786 Hz
or 107 r.p.m. which will be much easier to engineer.
With this configuration and taking into account the repetition frequency of the macro pulses of 5
Hz, it results that, in the average, each sphere will be hit every 5 turns, i.e. every 2.8 s, leading to 3
Mio. hits per 100 days of operation.
The total, average power, to be removed from a wheel (and from a stationary target) is about 10
kW. With Helium, pressurized to 1 MPa and with an entrance velocity of 10 m/s, a He mass flow of
30 gr/s is required which would result in an average temperature rise of 84 °C at the exit of the Heflow. Temporary and locally, values of 250 °C in the He may, however, occur at the hottest sphere
just after a micro pulse. Clearly, the wheel must be made vacuum tight and resist to the internal
He-pressure of 1 MPa. Moreover, with the above specified He-cooling, the time constant of the
exponential decrease in temperature of an adiabatically heated sphere is about 100 ms. Thus, a
sphere will be cooled to practically zero before being hit again after 2.8 s.
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P.Sievers/POSIPOL2011/Beijing
A GRANULAR CONVERSION TARGET FOR
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PROBLEM OF EDDY CURRENTS WITH ROTATING WHEEL
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The interference of the magnetic fringe field of the flux concentrator with
the adjacent rotating, metallic structure of the wheel have been studied for
very fast rotating wheels ( I.Bailey et al, Durham ILC e+ Meeting, Oct.
2009). Thus at the above assumed velocity of several m/s these problems
should be much reduced. Moreover, the high electrical resistivity of the
pack of W-spheres and that of the Ti-alloy should help. Further studies will
be required, in particular of eddy currents, possibly induced in the Be-disk
and among the W-spheres.
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A GRANULAR CONVERSION TARGET FOR
THE HYBRID POSITRON SOURCE
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AN ALTERNATIVE SOLUTION TO THE WHEEL: THE PENDULUM
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-To avoid rotating seals, wobbling or trolling targets have been devised (Durham ILC e+ Meeting,
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Oct. 2009) where the displacement of the target structure from the outside into the vacuum is
made via flexible, vacuum tight bellows. The injection of the cooling fluid can thus be ensured
through a rigid, non rotating structure. In the following we consider a “Pendulum Target”, where
the required displacement and velocity is provided by the sinusoidal oscillation of the target.
The width of the target is 13 cm, providing space for 13 micro pulses with a diameter of 1 cm
each and being displaced, when the beam is hitting the target, at a velocity of about 3 m/s over
+/- 7.5 degrees. The total swing is +/- 23 degrees, allowing for comfortable inversion of the
direction of the movement during the “off beam” time of 160 ms. The same range in angle must
be sustained by the bellows oscillating at 2.5 Hz. Since in this configuration the average time
between hits of the same target spot is about 0.2 s as compared to the rotating wheel with 2.8 s,
average temperatures higher by about 100 K will result. This might, if necessary, be
compensated by an improved He cooling. Clearly, prototyping will be required, also to assess the
life time of the spheres, the windows and the bellows, submitted to 40 Mio. cycles over 100 days
of continuous operation.
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A GRANULAR CONVERSION TARGET…
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THE PENDULUM GRANULAR TARGET
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A GRANULAR CONVERSION TARGET…
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CONCLUSIONS
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* The hybrid target with a granular amorphous converter exhibits:
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- a good quality positron beam (yield, emittance) generated by photons from
channeling radiation
- a lower PEDD resulting from the hybrid configuration
- an efficient thermal dissipation process due to the granular character.
In particular, the enlargement of the macro pulse to 40 ms duration in the above
considered « 300 Hz-KEK-System » and leaving a gap of « no-beam » of 160 ms, sufficient
for efficient damping, opens the possibility to reduce the thermal load from the macro
pulse on the target by displacing it laterally through rotation or trooling it. Lateral
velocities of about 3 m/s for this displacement are sufficient to reduce the local energy
deposition density by a factor of 13. Using granular targets, consisting of an ensemble of
Tungsten spheres of 2 mm diameter or below and cooled by a Helium gas stream passing
between the spheres, tolerable temperatures for Tungsten, Helium and the target container
are achieved. In addition to a rotating wheel, a pendulum target is considered, by which
the use of rotating vacuum and He-seals is avoided. Both target types seem to represent
viable solutions for the positron production in the considered ILC scheme.
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