Design Studies for an 18 MW Beam Dump
at the future Electron Positron Linear Collider TESLA
A. Leuschner, M. Schmitz, A.S. Schwarz, N. Tesch
Deutsches Elektronen-Synchrotron, DESY
Notkestrasse 85, D – 22607 Hamburg, Germany
Corresponding author: Norbert Tesch, e-mail: Norbert.Tesch@desy.de
Abstract
The conceptual design of a water based beam dump for the future electron positron linear collider
TESLA will be presented. Based on a study done by DESY and industrial design approaches, the
problems and difficulties in terms of radiation protection and safety for an 18 MW beam dump
system will be shown.
The whole list of advantages and disadvantages of such a water based beam dump system has been
worked out such as thermo-hydraulic calculations to estimate the heat removal in the complex
water system and to validate the principle feasibility of such a system, formation of radiolysis
gases like H2, pressure waves, activation of the dump medium water mainly in form of 3H and
feasibility of the dump windows (to separate the beam pipe vacuum from the beam dump water
system), radiation protection issues like shielding calculations, air activation, material activation
(mainly primary water system), maintenance procedures and waste disposal.
To overcome the major disadvantages of a fluid based (H2O) beam dump design a proposal for a
gas based (argon) beam dump of considerable length as an alternative has been evaluated. The gas
beam dump will be of cylindrical shape and consists of an argon filled beam pipe and an iron
shielding covered by a water cooling system.
Introduction
The TESLA (TeV-Energy Superconducting Linear Accelerator) project(1) is an electron positron
linear collider with a beam energy of 250 GeV in the first step and 400 GeV in the second step. It
has a total length of 33 km, with 2 accelerator sections of 15 km each. The inner tunnel diameter is
5 m and it will be located 10 – 30 m under the earth surface. In the first stage it has one collision
zone, in the second stage two collision zones for high energy physics experiments with luminosity
up to 5.81034 cm-2 s-1. The 400 GeV electron beam has the following characteristics: 6.81013
electrons per bunch train of length 860 s with a repetition rate of 4 Hz, resulting in an energy of
4.4 MJ per bunch train and a total average power of one electron beam of 18 MW.
At the end of each TESLA linac there will be a beam dump system(2) (Figure 1), which on the one
hand has to absorb the full beam power of 18 MW during the normal luminosity operation (spent
beam) and on the other hand it has to absorb the same amount of beam power from the other side
during machine commissioning. In the beginning the choice of technology for such a beam dump
was the solid (C-Cu) option. Because the heat extraction from a solid beam dump system seems to
be not practicable for the given beam power, the base line design changed to the fluid (H2O)
option. The TESLA water beam dump system consists of a cylindrical H2O volume, with a length
of 10 m, a diameter of 1.5 m and a static pressure of 10 bar. In a first approach the connection
between the vacuum of the beam line and the water of the dump was done by a system of titanium-
graphite sandwich windows(3), but a final solution has not yet been found. To distribute the beam
energy spatially (RMS beam size at the dump is only 0.4 mm2) on the windows and in the water a
fast (within bunch train) circular sweeping system is planned with a sweeping radius of 8 cm(4).
After detailed studies of such a system, which are described in the next three chapters, it was clear
that it might be worthwhile to consider a gas (for example: argon) beam dump to overcome most
of the below described problems of the water beam dump system.
Thermal Aspects: Heat Removal
The internal water system (heat removal inside the dump system) and the external water system
(heat removal from the dump to the general cooling water system) were investigated. The external
water system (Figure 2) is a more or less conventional system, with a primary and secondary
cooling system. The major part of the work was the simulation and development of the internal
water system. The requirements for such a system were: safe removal of the complete dumped
energy under consideration of the given energy density (Figure 10), usability of the beam dump
system from both sides (luminosity operation and machine commissioning), feasible maintenance
and repair procedures and solution of problems due to radiolysis and pressure waves (next
chapter).
Different schemes for the internal water flow have been investigated: vortex flow(5) and
longitudinal/radial flow(6) (Figure 3). For the vortex flow design a bypass system was developed to
increase the internal flow rate from 140 kg/s to 260 kg/s (Figure 4). To study the water flow inside
the dump vessel under the given constraints simulations were done using the CFD program ACE
(CFD-ACE 6.6, CFD Research Corporation) for the vortex flow option and the PHOENICS
program (Version 3.4, 2002, CHAM, Wimbledon) for the longitudinal/radial option. The main
goal of the simulations was to show that it is possible to construct a beam dump system, in which
local boiling (temperature above 180C at 10 bar) can be avoided at any position inside the dump.
The following items have been optimized: characteristics of the incoming water (velocity, mass
flow and distribution), different internal water guide designs as well as inlet and outlet positions. It
was found that both designs fulfill the requirements. The 2d stationary simulation of the vortex
flow design results in a very high water velocity at the inlet of 30 m/s with 130 kW pump power.
Including the instantaneous temperature rise by the energy deposition of the next bunch train the
maximum temperature of this design will be 125C and therefore well below 180C (Figure 5 and
Figure 6). For the longitudinal/radial flow design the 3d simulation with an internal and external
flow of 140 kg/s results in a maximum temperature of 108C and seems to be the better solution at
the cost of more complex water guide installations inside the dump vessel. Therefore in both
studies it could be shown that a water based beam dump system seems to be feasible in terms of
heat removal with the option of further improvements.
Radiolysis and Pressure Waves
Due to the shower of high energy primary electrons in the water dump, the net rate of cracking
H2O molecules by radiolysis is 0.27 g/MJ, which gives 30 ml/MJ H2 and 15 ml/MJ O2 at 20°C and
10 bar. The solubility in water at a temperature of 60°C is 16 ml/l for H2 and 19.4 ml/l for O2. To
exceed the solubility at 10 bar a local energy deposition of 530 J/cm3 for H2 is necessary
respectively 1300 J/cm3 for O2. The given beam parameters lead to a maximum value of 160 J/cm3
by one bunch train (see Figure 10) and thus a safety factor of more than 3 is achieved. But a local
pressure drop or a higher local energy density will easily reduce this margin (see next section).
Overall we expect for a 18 MW electron beam 4.8 g/s H2O to be cracked, which corresponds to
0.54 l/s H2 and 0.27 l/s O2 at 20°C and 10 bar. Without a recombination system the whole primary
water (30 m3) would be radiolysed in about 72 days. Although modern catalytic recombiners can
handle this amount of gas, not all H2 will be recombined because of imperfect water flow (local
pockets). Furthermore, there is a certain amount of dissolved gas, which can outgas and
accumulate at special locations where the temperature is high and the pressure low. Therefore an
efficient recombination should treat 100% of the water flow by a decompress-compress stage in
order to extract and recombine all dissolved gases and additionally by catalytic recombiners being
directly put into the return pipe of the dump vessel.
Pressure wave calculations(7) were done using the CFD program Fluent (Version 6.0, Fluent Inc.)
considering as input one bunch train of length 860 s as direct current. Phase transitions are not
simulated. The pressure rise and drop as a function of radius at the z-position of the maximum
energy density (z=2.5m) can be seen in Figure 7 and Figure 8. The maximum pressure rise for r=0
at 100 s is 3.7 bar, whereas the minimum pressure drop for r=0 at 950 s is -1.6 bar. The latter
effect results in a reduction of the local boiling point and the solubility of the gas, which can
transform into a critical formation of vapour and gas bubbles, especially at higher energy densities
due to beam sizes less than assumed here. The maximum pressure rise at the vessel wall at 650 s
will be 1.8 bar and at the front and rear windows below 0.5 bar, both are uncritical in terms of a
technical solution. The pressure waves decay after 3 ms completely, which is well before the next
bunch train arrives after 250 ms.
Shielding and Activation
Extensive shielding calculations and activation calculation were done for the TESLA beam
dump(8,9,10,11) mainly with the program FLUKA(12,13). The shielding against direct radiation was
studied as well as the activation of soil, groundwater and air to protect the public and the workers
against radiation. Furthermore, the activation of the primary water system, the dump vessel and the
corresponding shielding was investigated keeping in mind the maintenance procedures, repair
scenarios and the final dismantling of the whole beam dump complex.
To reach the planning goal for the public of 100 Sv/year for the dose due to direct radiation
(factor 10 below German legal limit) a radial shielding of 3 m of normal concrete and 7 m of soil
between the dump vessel and a person of the general public will be sufficient. To reach 30
Sv/year for the dose due to incorporation of radioactive water and air (factor 10 below German
legal limit) a radial shielding of 3 m of normal concrete between the dump vessel and the
surrounding soil and groundwater will be adequate.
A critical item is the activation of the 30 m3 water of the primary circuit. Here mostly 3H and 7Be
are the relevant isotopes, the saturation activities for 3H will be 250 TBq and for 7Be 108 TBq,
after one year of operation the amount of 3H will be 8 TBq whereas for 7Be it will be 102 TBq.
The tritium does not contribute to the dose rate due to direct radiation, but represents a
incorporation risk in case of any release due to accidents, maintenance or repairs. Beryllium is the
main contributor to the dose rate due to direct radiation in case of access for maintenance or repair.
It will be more or less equally distributed inside the water, resulting in dose rates up to 300 mSv/h
close to the surface and 50 mSv/h at 1 m distance. Special resin filter systems will accumulate
most of the Beryllium, but accumulation will also happen at other locations such as heat
exchangers. All these locations need extra local shielding to guarantee access. The main steel
vessel will have a dose rate of 400 mSv/h on its axis after one year of operation. This excludes
regular inspections for pressure vessels, which are usually required.
During operation of the dump, the air of the containment, which has to be closed at underpressure, will be activated. It was shown that the best procedure for the 3000 m3 air inside the
containment will be a replacement with an exchange rate of one hour with a buffering time of one
hour (to allow for the decay of short lived isotopes) before the air can be deflated through an
exhaust.
The standard maintenance procedure to work at the primary circuit system (30 m3 water with
about 100 TBq 3H after 10 years), will be flushing the whole water from the primary circuit into
special storage tanks, with about 100 l remaining on the surface (0.1 mm on 1000 m2) inside the
vessel. This amount has to be removed with a special venting system with dry gas, resulting in a
release of about 10 GBq of tritium through a 20 m chimney assuming a 95% efficient condenser.
The whole procedure will take about 42 days.
For the final shutdown of the whole dump system it was assumed that after an average operation
period of 20 years 200 TBq 3H are accumulated, which have to be disposed. A possible scenario
was found by binding the primary water in form of concrete and putting it into 5000 barrels with
200 l and 40 GBq each. The dismantled steel components were estimated at 150 tons with specific
activities of 103 – 106 Bq/g and 1500 tons of concrete with specific activities of 10 – 200 Bq/g. All
components (water, steel and concrete) can be disposed without problems, but the costs are not
negligible and should be taken into account already at the design stage.
A New Idea: The Gas Dump
To overcome most of the above described problems of the water beam dump a gas beam dump
was considered, which is composed of a material with atomic number greater than 20 to reduce the
tritium production and a one-atomic gas to avoid radiolysis. The first attempt was a cylindrical
geometry with an inner tube of radius 4 cm filled with argon at 1 bar, which acts as scattering
target (energy deposition of 0.5%) and distributes the beam energy longitudinally (no sweeping
needed) over a length of 1000 m leading to low power densities in the gas. The energy density
calculated with the FLUKA program can be seen in Figure 9. The main part of the energy is
dumped in the iron shell of radius 52 cm, surrounded by a 4 cm thick water cooling system. The
gas beam dump system with a diameter of 1.20 m and a length of 1000 m might be positioned
inside the collider tunnel. This construction could also be used as a beam dump for a - collider,
where two high energetic photons (instead of electrons and positrons) are colliding.
In comparison to the water dump design (numbers given in brackets) the gas dump design has the
following advantages: The saturation activity for tritium is about 30 TBq, with 0.7 TBq in gas,
29.3 TBq in iron and 0.02 TBq in water (water dump: 300 TBq in water). The window has to
withstand a static pressure of 1 bar and a dynamic pressure of 0.01 bar with a diameter of 8 cm,
these type of windows are already available (water dump: 10 bar static and 0.5 bar dynamic with a
diameter of 20 cm with a huge R&D effort). In case of maintenance or repair the activated gas can
be rinsed out and pressed to a vessel with 5000 liter at 1 bar. A leakage of 0.1% leads to a 0.03
GBq tritium release (water dump: primary water has to be deflated and components have to be
dried; 5% inefficiency for dryer and 20 m chimney result in 10 GBq tritium release). The radiation
risk due to: leakage in the primary cooling system will be very low (water dump: high), broken
beam window will be low (water dump: medium), failure of the main water pump will be very low
(water dump: low). Further advantages are the absence of H2 gas production because the cooling
water is heated by heat conduction (water dump: by ionization).
It will be a challenge to design the dump with nearly equally distributed power density over the
whole length. In that case an average power of 18 kW/m has to be conducted from the inner iron
surface to the water system (temperature difference of 150 K). A disadvantage of course is the
activation of 1000 m tunnel with dose rates in the order of a few mSv/h, hence additional shielding
is needed and maybe because of space problems an adopted tunnel design. But nevertheless the
idea of a gas based beam dump for the next linear collider and even for the - collider seems to be
very attractive and will have to be considered in the early design phase.
Conclusion
It was shown that for the electron positron collider TESLA a water based beam dump can be built
in principle but with certain disadvantages such as risk due to H2 gas production, high tritium
concentration in the coolant, which requires a huge effort in terms of radiation protection and the
complexity of the beam dump window design. Therefore a gas based beam dump with a length of
1000 m was proposed and investigated with the result that most of the above mentioned problems
can be solved within such a design.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
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(editors); TESLA Technical Design Report, Part II, The Accelerator; DESY 2001011 (2001).
M. Maslov et al.; Concept of the High Power e+e- Beam Dumps for TESLA;
DESY TESLA Report 2001-04.
M. Maslov et al.; Concept of Beam Entrance and Exit Windows for the TESLA
Water based Beam Dumps and its related Beam Lines; DESY TESLA Report
2001-07.
V. Sytchev et al.; Concept of the Fast Beam Sweeping System for the e+e- Beam
Dumps of TESLA; DESY TESLA Report 2001-05.
Fichtner GmbH & Co. KG; Projekt TESLA Strahlabsorber - Erstellung des
Basiskonzeptes; Stuttgart, Germany, April 2003.
Framatome ANP GmbH; Projekt 18 MW Beam Dump für TESLA –
Dokumentation; Erlangen, Germany, March 2003.
TÜV Nord Gruppe; DESY Beam Dump – Berechnung des Druckaufbaus;
Hamburg, Germany, July 2002.
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Linear Collider TESLA; DESY Laboratory Report D3-98 (1998).
K. Tesch; Shielding against high energy neutrons from electron accelerators –
A review; Radiation Protection Dosimetry 22 (1988) 27.
K. Tesch; Production of radioactive nuclides in soil and groundwater near the
beam dump of a linear collider; DESY Internal Report D3-86 (1997).
N. Tesch; Soil, groundwater and cooling water activation at the TESLA beam
dump; DESY Laboratory Report D3-114 (2001).
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Proceedings of the Monte Carlo 2000 Conference, Lisbon, October 23-26 2000,
Springer-Verlag Berlin (2001) 159-164.
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Figures
exhaust / chimney
general
cooling water
sand
containment shielding
hall
air treatment
water-system
basin
commissioning beam
water-dump vessel
dump shielding
spent beam
Figure 1 Schematic view of the TESLA beam dump system. The spent beam and the
commissioning beam have to be dumped in the same water dump vessel.
general cooling water
60°C
Static
Pressure
10bar
70°C
70°C
Static
Pressure
10bar
Hydrogen
Recombiner
80°C
Heat
Exchanger
B
Secondary
Loop
Heat
Exchanger
A
30°C
40°C
Pump B
40°C
50°C
Primary
Loop
17.5MW / T=30K
 140kg/s
Pump A
1% to 10% of
total water flow
Water
Filtering
(ion exchanger,
resin filter)
Water Dump
18m3, 10bar
Storage
Container
Scheme of Water System
Figure 2 Schematic view of the water system for the TESLA water dump including the primary
and secondary loop system. The basic components such as water dump, heat exchanger, filter
system, recombiner, pressure system and storage container are shown.
in: =140kg/s, 50°C
out: =140kg/s, 80°C
etube 1
tube 2
Figure 3 Dump design with longitudinal/radial flow. Tubes 1 and 2 have 0.1% and 0.02%
porosity, which results in a radial flow of about 15%. The maximal tube temperatures of tube 1
and 2 are 101C and 95C.
out:
140kg/s, 80°C
130kg/s
260 kg/s
T0=54°C
130kg/s
e-
120kg/s
in: 140kg/s, 50°C
130kg/s
82°C
59°C
z=3m
10kg/s
130kg/s
Figure 4 Dump design with vortex flow. The mixing scheme with cold water from the back part of
the dump is shown to increase the effective internal flow.
velocity(r,
(r,))
velocity
[m/s]
2
1 .5
m
1
0
Figure 5 Velocity distribution in r, at z=3m (side view of Figure 4). Shown are the inlet (left) and
outlet (right) pipe as well as the energy deposition in form of density rings above the outlet pipe.
Teq(r,
(r,))@
@z=3m
z=3m
TT0 0++T
eq
[°C]
91
81
71
max(Teq) = 47K
101
61
T0=54
Figure 6 Temperature distribution in r, at z=3m (side view of Figure 4). Shown are the inlet (left)
and outlet (right) pipe as well as the energy deposition in form of density rings above the outlet
pipe.
p [bar]
p(r)@
@z=2.5m
z=2.5m
p(r)
800s
00t t800s
3
t [s]
2
600
800
1
10 50
0
700
500
400
300
100 150 200
r [cm]
0
10
20
30
40
50
60
70 75
Figure 7 Pressure rise as a function of radius for different times between 0 and 800 s after
beginning of the entrance of the bunch train into the water dump.
p [bar]
p(r)@
@z=2.5m
z=2.5m
p(r)
0.8t t1.6ms
1.6ms
0.8
2
1.2
1.1
0.8
1
1.4
1.3
1.6
0 1.0
1.5
t [ms]
0.9
-1
r [cm]
0.95
0
10
20
30
40
50
60
70 75
-2
Figure 8 Pressure rise as a function of radius for different times between 0.8 and 1.6 ms after
beginning of the entrance of the bunch train into the water dump.
radius [cm]
Air
Water
Iron
Argon
length [m]
Energy Density [GeV/cm3]
Figure 9 Energy density in argon in GeV/cm3 in r-z view for one 400 GeV electron.
3
Energy Density (1 bunch train), dE/dV [J/cm ]
30
0.160
0.285
0.506
0.900
1.60
2.85
5.06
9.00
16.0
28.5
50.6
90.0
160
25
r [cm]
20
15
10
5
0
0
200
400 600
z [cm]
800
1000
Figure 10 Energy density in water in J/cm3 in r-z view for one bunch train.