Annex 4 - Contracting documentation
Technical specification3
Cryogenic systems for ELI-Beamlines and HiLASE facilities: design, assembly and testing of
cooling loops for repetition rate lasers
The objective of the ELI-Beamlines and HiLASE projects is to build new facilities that will be
employing cutting-edge laser technology for fundamental research and for technology applications.
Diode-pumped multislab laser amplifiers able to deliver pulses with energy 100 to 500 J in a single
beam represent one of the key technologies planned for these centers. These lasers will typically use
Yb:YAG as an active medium and will be cryogenically cooled, with characteristic operating
temperature of 160 K. This document describes the technical requirements for the design,
assembling and performance evaluation of the cryogenic loops for ELI-Beamlines and HiLASE laser
amplifiers.
The subject of this procurement are expert services involving design, assembly, testing, operation
optimization, and commissioning of the cryogenic cooling loop of a diode-pumped multislab laser
amplifier able to deliver pulses with energy 100 to 500 J, and associated expert consultancy. The
subject of the service required is not delivery of hardware of the cooling loop.
1. Required Deliverables
D1
Design of a cryogenic measurement station for complex characterization of optical
materials
The Contractor shall develop for the Client a design of a cryogenic measurement station for the
characterization of properties of optical materials at temperatures within the range of 100 to 180 K.
The station shall make it possible to measure thin samples of crystals, ceramics, glass and polymers in
sizes of up to 25x25 mm. The station shall make it possible, at temperatures ranging from at least
100 to at least 180 K, to accurately measure the following parameters:
-
Refractive index within wavelengths range of 900-1100 nm
Optical quality (2D map of optical homogeneity of the sample in the selected direction)
Amplification of the laser signal in the selected direction
Thermal diffusivity
Thermal conductivity
Coefficient of thermal expansion
The design shall include, in addition to the engineering documentation and drawings, also the
methodological proposal for measuring thermo-mechanical properties of crystals, ceramics, glass and
polymers.
D2
Measurements of material properties at low temperatures according to the Client’s
requirements
The Contractor shall carry out measurement of material properties at low temperatures (ranging
from 100 to 180 K) according to the specific requirements of the Client as may be formulated in
writing, in the extent of approximately 15 samples. Measurements of crystal and ceramics samples
Page 1
(and possibly also glass and polymers) with sizes of 50x5x5 mm will be required as well as the
measurement of the following parameters:
- Specific heat capacity (J kg-1 K-1)
- Thermal conductivity (W m-1 K-1)
- Coefficient of thermal expansion (K-1)
These measurements may potentially be required during the entire duration of the contract.
D3
Expert consulting in the field of propagation of laser beams in the laser amplifier medium
The Contractor shall provide the Client with expert consulting in the field of propagation of laser
beams in the medium of multi-slab laser amplifiers. The content of this consulting shall focus on the
dependence of the beam wavefront on the thermal balance / heat distribution in the amplifier, on
the design of measurement and diagnostics equipment for the characterization of the laser beam
parameters, and on the proposal for system integration of the cryogenic laser amplifiers into the ELIBeamlines and HiLASE facilities.
The expert consulting, which is subject to this part of performance, shall be required over the period
of 24 months, starting on the date of contract signature, and in the maximum extent of 1 500 hours.
D4
Conceptual design of the cryogenic cooling loop for a 100-J laser system
The Contractor shall produce a conceptual design of the cryogenic cooling loop for 100-J laser system
providing 100 J of energy at the output and consisting of one or two multi-slab Yb:YAG amplifier
heads. The designed system has to utilize an active He gas loop with nominal temperature of
160 K (the operation temperature needs to be adjustable between 100 and 180 K) for removal of
dissipated energy of at least 4 kW. The system has to ensure a thermal stability of He within +/-3 K or
better, at the anticipated gas flow of 20 to 40 ms-1 and shall use liquid nitrogen as the primary
coolant. The designed cooling loop has to enable removal of the total dissipated thermal energy
(5 kW) from one head (100 J) or from two heads (50-J class). The Client will specify to the Contractor
the choice of system configuration (one 100 J head or two 50-J class heads).
The result of D4 shall be a technical report including the description of the main components of the
cooling loop, the results of numerical simulations of He circulation in the proposed loop, the
recommendations as to thermo-mechanical properties of materials used to build components of the
loop, the designed technical parameters of the individual loop components, as well as the proposed
procedure for bringing the cooling loop into operation.
D5
Detailed technical design of a cooling loop for a 100-500 J laser system amplifier
The Contractor shall produce for the Client a detailed technical design of a cooling loop for laser
system amplifier consisting of one or two Yb:YAG multislab amplifier heads generating pulses with
the output energy ranging from 100 to 500 J. The cooling loop has to enable removal of total
dissipated heat of at least 18 kW. It is assumed that in the case of a system consisting of two
amplifier heads the ratio of dissipated heat in these heads will be 1:1.
The proposed system has to use an active gaseous He loop with nominal temperature of
160 K (operating temperature needs to be adjustable between 100 and 180 K) and shall ensure
thermal stability of He of +/-3 K or better, at expected flow velocity of 20 to 40 ms-1. It is required
that the basis for the design of this cooling loop for a system generating pulses with output energy
between 100 and 500 J is an extrapolation of the design of the 100-J laser cooling loop developed
Page 2
above under D4.
Performance required within this Deliverable shall include the following activities:
i. Establishing the main components of the cooling loop of the laser system amplifiers, carrying out
detailed numerical simulations of the convection cooling regime for a nominal configuration of
100-500 J system laser heads, carrying out detailed numerical simulations of the circulation of
cryogenic coolant in a closed loop of 100-500 J amplifiers;
ii. Description of arrangement and definition of necessary technical parameters for the testing
(mockup, i.e. potentially using a dummy gain medium and/or a substitute source of thermal
energy) head for the 100-500 J system;
iii. Determination of technical requirements and benchmark criteria for all systems and components
of the cooling loop, elaboration of detailed technical specifications for these components,
producing detailed and complete technical documentation for assembling the cooling loop for a
100–500 J amplifier;
iv. Description of diagnostic elements for measurement of the turbulent flow parameters in the
closed loop and in the amplifier head of a 100-500 J laser system.
Before starting the work defined above under ii), the Client shall provide the Contractor with
information on the selected configuration of the laser heads and their thermal budget. This
information will be provided no later than within 6 months after signature of the Contract for work.
Each activity described above in D5 shall result in a detailed technical report.
D6
Representation of the Client during the receipt of the cooling loop components for the 100500 J amplifier
The Contractor represent the Client during the receipt of the manufactured / supplied cooling loop
components for the 100-500 J amplifier, designed under D5 above. The maximum anticipated
number of hours for the receipt of such components is 200. The Contractor shall perform this activity
for the Client on the basis of Client’s orders in writing.
The Contractor shall represent the Client during receipt of components of the cooling loop from the
supplier / manufacturer selected by the Client. The detailed technical report produced under D5
above, especially under subsection iii), shall form the documentation for the manufacture of the
cooling loop components. The receipt of the components shall take place at the supplier /
manufacturer premises, whereas these premises are expected to be located in Europe. The
Contractor shall be obliged to accept, on behalf of the Client, only those components that fulfill the
technical specifications defined in the technical report produced under D5 above, especially in
accordance with the subsection iii). Client’s representatives will be a part of the team accepting the
manufactured components.
D7
Assembly, testing and optimization of the cooling loop of a 100-500 J laser system amplifier
The Contractor shall assemble the cooling loop of a 100-500 J laser system amplifier (the maximum
laser energy per pulse produced by one laser head is 250 J.), at its own premises. Subsequently, the
Contractor shall examine parameters of this loop during a test operation and shall optimize its
operation parameters. The cooling loop shall be assembled from components supplied by the Client,
produced in accordance with the technical proposal elaborated under D5, and received / handed
Page 3
over under D6.
For the purposes of testing and optimization of the cooling loop parameters, the Contractor shall
provide all necessary diagnostic and measurement equipment. These diagnostic tools shall measure
the temperature of the He coolant, the flow velocity, the flow profile and the heat distribution in the
gain medium. The sensors and the control system for the loop operation, which will be permanently
built-in in the loop, will be provided by the Client.
D8
Handover of the cooling loop of a 100-500 J amplifier for transport to the Client’s
forwarding agent, re-assembly and commissioning of the loop at the ELI-Beamlines center
The Contractor shall disassemble, pack and hand over the cooling loop at its premises to the Client’s
forwarding agent; the Contractor shall notify the Client of the specifications and terms for the
transport at least 60 days in advance.
The Contractor shall re-assemble and commission the 100-500 J amplifier cooling loop at the ELIBeamlines center, and will hand the commissioned loop over to the Client, including the operation
manuals for the loop. For this purpose the Client will ensure transport and/or manipulation of all
heavy components within the ELI-Beamlines building.
The Client shall secure a liquid nitrogen supply system and the supporting infrastructure for the
purposes of installation and commissioning of the cooling loop. The Contractor will provide the
measurement and assembly equipment necessary for the installation of the Technology. For the
purpose of installation the Client will also provide common mechanical tools, mounting and assembly
equipment including fasteners and fittings, cryogenic tubing to connect the cooling loop to the LN2
supply, and vacuum tubing to connect the loop to the roughing and backing vacuum circuits.
Similarly to D7 above, the Contractor shall provide, for the purposes of testing and optimization of
the cooling loop parameters, all necessary diagnostic and measurement equipment. These diagnostic
tools shall be capable to detect the temperature of the He coolant, its flow velocity, the flow profile
and distribution of heat in the gain medium.
The output of this Deliverable is commissioned and accepted cryogenic cooling loop of a 100-500 J
amplifier at the ELI-Beamlines facility.
D9
Post-warranty service of the cooling loop of a 100-500 J amplifier for the period of 5 years
after completing Deliverable D8
The Contractor shall provide post-warranty service of the cooling loop of a 100-500 J amplifier for the
period of 5 years after completing Deliverable D8, in the extent of 25 service days.
2. Internal structure of the ELI-Beamlines facility
Arrangement of the laser halls, of the experimental halls and of the support technology floor in the
ELI-Beamlines building is shown in Figure 1. The cryogenic units will be located in the first floor, in
technology halls situated above the laser halls in the ground floor.
Page 4
Figure 1. Axonometric view of the internal structure of ELI-Beamlines laser building (basement, ground floor,
first floor). The central units of the cryogenic loops will be located in the upper technology floor (first floor),
supplying coolant to lasers located in the below ground floor.
3. Layout of multislab laser amplifier
3.1 Amplifier internal geometry
The active medium consists of an array of slabs. The pumped (illuminated by pump laser diodes)
section of each slab is surrounded by cladding and by a mechanical frame, as shown in Figure 2.
The anticipated dimensions of active medium (i.e. pumped section) of the multislab laser amplifier
(length along z axis direction, width along x direction, thickness along y direction) are
-
60 x 60 mm² (length x width) and 64 mm (total thickness) for 100 J amplifiers,
100 x 100 mm² (length x width) and 64 mm (total thickness) for 500 J amplifiers.
In the latter case the maximum energy generated by one head is 250 J.
The baseline design of the active medium dimensions assumes the damage laser fluence of 5 J/cm²,
laser pulse duration <10 ns, laser intensity spatial modulation <50% and the filling factor 70%.
Page 5
The assumed dimensions of the non-optical parts (cladding and mechanical support) of the laser
head are (length along z axis direction, width along x direction, thickness along y direction) are:
- 300 x 160 mm² (length x width) and variable thickness (*) for 100 J amplifiers,
- 400 x 200 mm² (length x width) and variable thickness (*) for 500 J amplifiers.
The variable thickness (*) refers to the shape optimizing properties of thermo-hydraulic flow
The amplifier has to be actively and uniformly cooled to avoid large thermal gradients in the slabs.
For this purpose sides of the slab are cooled by forced convection of gaseous He.
The amplifier is divided into typically 8 slabs separated by cooling channels, as shown in Figure 2. The
baseline number and thickness of the amplifier slabs and of the cooling channels are:
-
8 slabs 8 mm thick (8x8 = 64 mm) for both 100 J and 500 J amplifiers,
9 cooling channels of 4 mm thick (9x4 = 36 mm) for both 100 J and 500 J amplifiers.
Figure 2. Geometry of a cryogenic Yb:YAG amplifier head indicating the side view (left) and isometric view of
the slabs (right). In a baseline configuration analyzed numerically within ELI-Beamlines and HiLASE the
number of slabs=8, d=8 mm, w=2 mm.
The distribution of heat distribution across the slabs is shown in Figure 3. The dissipated heat
decreases towards the center of the amplifier. Across the slab discontinuity of heat distribution is
observed on the active medium / cladding interface, due to strong absorption of the spontaneous
emission. Beyond this interface the heat deposited in the cladding decreases approximately
exponentially towards the edges
Page 6
Figure 3. Dissipated heat generated in Yb:YAG slabs with Cr:YAG cladding, showing heat
deposition along the beam propagation axis (top), distribution of the heat in one slab with
22.5 mm wide cladding (left) and distribution of the generated heat in one slab with 30 mm wide
cladding (right).
3.2 Amplifier thermal heating
The ELI-Beamlines facility will employ one or several high-energy diode-pumped solid state lasers
(DPSSL), operating at nominal repetition rate of 10 Hz, as pump devices for broadband amplifiers
delivering ultra-short pulses. The nominal time diagram of the pump light (typically at the
wavelength 940 nm) for the active medium based on Yb:YAG is shown in Figure 4. The pump duration
is nominally 1 millisecond and the repetition rate is 10 Hz.
For 10 Hz repetition rate and for 1 ms duration of the pump pulses the average power is equal to 1%
of the peak power which amounts to 5 kW/cm² assuming pump light fluence 5 J/cm2 .
Figure 4. Nominal time diagram for the pump light delivered to Yb:doped active medium from pump
diodes.
Page 7
Simulations and preliminary design of the amplifiers and the cryogenic loops assume:
-
Conversion efficiency from the diode pumping light (typically 940 nm) to the laser light (1031 nm
for YAG) equal to 30 % at 160 K (i.e. 70% dissipated in active medium as thermal heating).
-
Uniform thermal heating in time from the diode pumping light since the duration of 1 ms of the
pump pulses is short compared to thermal timescales.
-
Perfect uniformity of thermal heating across the active medium (plane x-y cross-section in Figs. 2
and 6) dissipated from the diode pump power,
-
Longitudinal profile (y direction in Figs. 2 and 6) of the dissipated pump light (i.e. thermal heat
load) given by:
Pdissipated  A. exp(  . y )  exp( .( y  L)) 
where A = constant (dissipated heat in W.cm-3), = 0.2 (baseline value), y = axial distance along y
axis in the active medium, L = total length of the amplifying medium and arbitrary chosen = 6.4 cm.
In both 100 J and 500 J laser design, the dissipated heat is ~14.4 Wcm-3.
The longitudinal profile of the dissipated heat is shown below in Figure 5.
1.4
Total dissipated power
Dissipated power (a.u.)
1.2
1
Left pump light
dissipated power
0.8
0.6
0.4
Right pump light
dissipated power
0.2
0
0
1
2
3
4
5
6
7
distance[cm]
(mm)
AxialAxial
distance
Figure 5: Longitudinal profile of dissipated heat in the nominal configuration (uniform doping). The
dependence is displayed in relative units without the constant A in the Equation above.
The total pump power and the dissipated power in the active medium for the case of 100 J and 500 J
laser amplifiers running at 10 Hz repetition rate and for 1 ms duration of the pump pulses are:
-
Amplifier 100 J:
Pump power (average) =
Dissipated power (average) =
3.3 kW,
2.3 kW
-
Amplifier 500 J:
Pump power (average) =
Dissipated power (average) =
16.7 kW
11.7 kW
3.3 Cryogenic and vacuum design for amplifiers
Operation employing forced flow of cryogenic He gas requires a vacuum enclosure (cryostat) to avoid
condensation on cold surfaces. Four optical windows are thus required for each amplifier, with two
Page 8
of them for vacuum-atmosphere interface and two others for vacuum-cryogenic cooling fluid
separation.
The size of the windows (vacuum and helium) has to provide a minimal margin of 1 cm on the laser
beam edges (and hence on the pumped region of the active medium). The maximal cumulated
thickness of optical windows in one amplifier has to be lower than 200 mm and has to ensure a
maximal deformation of each window less than 5 m, at the operating He pressure estimated to be
1 MPa or less.
4. Nominal specifications for cooling of the active medium
Besides the need for removal of the heat dissipated in the active medium and the cladding, the
operation of the lasers calls for a possibility of fine temperature control the of the slabs around the
design temperature (~160 K) and for variability in setting the operating temperature between 100 K
and 200 K.
As shown in Figure 6 several thermal gradients will exist in the amplifier slab: between inlet and
outlet of the cooling gas (Tfluid), between the cooling gas and the edge of the gain medium (Twall),
and between the edge of the gain medium and its center (Tmedium).
Tmedium represents the maximal temperature difference inside the active medium and is a function
of the amplifier design, material properties, amount of dissipated heat, and cooling loop operation
conditions. The requirement is that the cooling loop ensures Tmedium lower than 3K.
Figure 6: Summary of the thermal gradients existing in the face-cooled active medium slab.
5. Cryogenic amplifier operation modes
The ELI-Beamlines cryogenic system shall be designed to ensure continuous and automatic operation
for up to 8000 hours without any maintenance.
Figure 7 represents the envisaged working-day duty cycle for the repetition rate lasers at ELIBeamlines, consisting of 4 sessions each with duration of 15 minutes at full performance (100 to
500 J pulses). The time periods needed for warming up and shut-down of the laser must not exceed 2
hours.
Page 9
delay
max 30 min
Laser
preparation
10 min
Warming up
8h
9
10
11
12h
Experimental
run 15 min
13
65 to 110 min
Shutdown
14
15
16
17
18h
Figure 7. Baseline workday operation scheme of the repetition rate lasers using cryogenic cooling; the
lasers are expected running for users in 4 sessions each with duration of 15 minutes.
Besides the daily operation scheme the warm-up and cool-down of the amplifiers should impose low
thermal stress on the gain medium. For this reason the temperature ramp-up (ramp-down) rate is
required not to exceed 1 K/h, with temperature difference across the amplifier at a time not
exceeding 20 K.
In case of any emergency stop or utility losses (LN2, compressed air, water, vacuum, electricity) the
cryogenic system has to ensure safety of the personnel and of the equipment.
The availability of the cryogenic loop for users has to be at least 95% over 200 workdays a year. The
required reliability is at least 98% during 200 working days a year.
Note
Specific dimensions, figures and details concerning the laser amplifier head, amount of dissipated
heat, materials, required thermal gradients, operation mode, utilities and building layout related to
the cryogenic systems will be communicated to the Contractor upon signature of the contract and can
be re-iterated by beginning of works on Deliverables D4 and D5.
6. Cryogenic loop layout
6.1 Generic cooling scheme
A generic layout of the required cryogenic cooling loop for evacuation of heat from the multislab high
energy laser amplifiers is shown in Figure 8. The loop features forced flow of gaseous helium
circulation across the amplifier head; helium as a coolant is chosen for its low refractive index and its
high thermal conductivity. The main components of the cryogenic loop involve:
- laser amplifier head;
- one circulation pump;
- one heat exchanger connected with a cold source;
- temperature control system (not represented in Figure 8);
- He gas management system for discharging and filling the loop;
- transfer lines (cryogenic and “warm”) interconnecting the amplifier head and the cooling
system.
Page 10
Figure 8. Generic scheme of the required cryogenic cooling loop with its major components.
Forced flow of helium gas across the amplifier head, i.e. across the space between the slabs of active
medium, will remove the dissipated heat while maintaining an acceptably low temperature gradient
along the slabs in the flow direction. The flow with an adequate velocity profile shall be supplied at
the inlet of the laser head to ensure cooling that provides the required temperature profile of the
slabs.
6.2 Normal operation
To remove the heat load dissipated in the amplifier and to reach the nominal working temperature of
the slabs (100-200K) the cryogenic cooling capacity could be either provided by a refrigerator
(Brayton cycle) or by a liquid nitrogen (evaporation of LN2 at 77 K). The latter solution using liquid
nitrogen is preferred.
6.3. Cool-down and warm-up
The temperature control system will also be used for the temperature ramp down and ramp up for
the cool-down and the warm-up of the laser amplifiers as specified above.
7. Integration of the cooling loops into the ELI-Beamlines building
As mentioned above, the laser amplifiers are installed on the ground floor of the ELI-Beamlines
building while the central cryogenic cooling units are located on the first floor. The transfer cryogenic
lines connect these two parts through ceiling penetrations with size 500x500 mm.
Page 11
Figure 9. Vertical layout of the cooling loop of a cryogenic amplifiers head. The equipment located on
the first (upper) floor involves He circulation pump, LN2/He heat exchanger, He gas management
system, and temperature control.
Figure 9 shows the required layout of the cryogenic loops in the ELI-Beamlines building. The design
must feature the following distances/dimensions:
-
height of the laser head (optical axis) at 1300 mm from the ground floor level;
elevation of first floor above the ground floor 7000 mm (5400 mm + 1600 mm);
typical distance between the interconnection lines (axis) 2860 mm;
ceiling penetration size 500x500 mm.
Installation of the cryogenic loops in the ELI-Beamlines facility will have to use the main load lift and
service doors that allow transporting equipment with overall dimensions not exceeding 4x4 m and
2.5 m height, and with weight of up to 5 tons.
No overhead cranes will be available on the first floor of the ELI-Beamlines building and consequently
the cryogenic loop design shall involve local handling devices for manipulation and maintenance.
8. Power supplies and services
This chapter gives a preliminary description of utilities which will be available at the ELI-Beamlines
facility and which can supply the cryogenic loops.
5.1 Electrical power supply
All subsystems using electrical power supplies must employ 230 V/ 50 Hz single phase and/or 400 V
/50 Hz 3-phase European standard 400 V (3P + N). Electrical power supply and grounding interfaces
will be available at ELI-Beamlines for pump motors, electrical heaters, electrical cabinets, etc.
Page 12
All electrical components shall be in accordance with the international electrical standards (IEC).
5.2 Compressed air
Clean dry compressed air (pressure 10 bar, oil <0.01 mg/m3, -40°C dew point, meeting ISO 8573-1
standard) will be available across the ELI-Beamlines facility for pneumatic actuators (valves).
5.3 Cooling water
Dedicated cooling water circuit will be available at ELI-Beamlines, providing 19° C de-mineralized and
de-ionized water, for motor or pump cooling. The system can also use standard utility water circuit
(approx. 0.5 MPa pressure, 0.25 MPa max. pressure drop, temperature 20 to 25°C).
5.4 Ventilation / air conditioning
Standard air ventilation will be provided at the first technology floor of the ELI-Beamlines laser
building, ensuring nominal temperature of 20°C and 50% humidity. The laser halls of laser building
will feature cleanliness environment Class 10,000 (ISO 7), at temperature 21° C with long-term
stability +/- 0.5° C, and at humidity 40 to 60% RH.
5.5 Vacuum system
The primary vacuum (about 10-2 mbar) both for roughing and backing purposes will be available at
ELI Beamlines from a central base-build distribution (DN250 and DN100 ISO-K), and consequently
primary vacuum pumping should not be part of the cooling loop design.
5.6 Control system
The ELI-Beamlines cryogenic system will be controlled by central control system with interfaces with
the laser control system. The cryogenic control system shall ensure automatic control and safe
operation and safe shutdown in case of a failure. Each cryogenic loop will be supplied with associated
electrical cabinets and a local Programmable Logic Controller (PLC). The control system architecture
will be specified by the Client within 3 months after signature of the contract.
Page 13