Furnace inserts are physically accessible for exchange after opening the process chamber lid and removing the cartridge and the intermediate support plate.

The following are photographs of a typical Furnace Insert:

Hot core of (LGF) furnace prototype with five heating zone, and integrated in its water-cooled jacket

Low Gradient Furnace (LGF)

The Low Gradient Furnace (LGF) will be the first European furnace insert in the MSL and is developed to support research in the field of Bridgman crystal growth. The LGF consists of two heated cavities separated by an insulating, "adiabatic" zone. It is intended to provide restricted but well-controlled gradients between two stable temperature plateaux. Crystal growth is performed by translating the furnace over a stationary sample; thermomechanical stresses in the grown crystal are minimised by maintaining the solidified part of the sample in the heated cavity throughout processing. Alternatively, the insert can be used for thermodiffusion experiments by keeping the furnace stationary and using only the gradient capability. Isothermal experiments can be carried out with suitable sample/cartridge configurations.

Performance Characteristics

The LGF consists of seven separately controlled heating zones. The heater elements are based on pyrolytic boron-nitride/pyrolitic graphite heater technology with carbon composite diffusers to provide thermal uniformity. The LGF provides high stability thermal control by sapphire optical fibre thermometers (OFT) and a high performance control algorithm. Sample gradients are dependent on cartridge design and thermal conductivity; typical gradients obtained in prototype tests (with a boron-nitride dummy sample) ranged between 5 and 50 K/cm. An electromagnet provides for controlled stirring of the melt by a rotating magnetic field.

LGF Technical Performance Data

Solidification and Quenching Furnace (SQF)

The Solidification and Quenching Furnace (SQF) is the second European furnace insert. The SQF consists of a hot cavity separated from a water-cooled cooling zone by an insulating "adiabatic" zone. It is primarily intended for metallurgical solidification research under intense temperature gradients, with the possibility of quenching the solidifying interface at the end of processing (by quick displacement of the cooling zone). Directional solidification is achieved by translating the furnace over a stationary experiment cartridge. High gradients are established by coupling the cartridge to the cooling zone with a liquid metal contact. The furnace is re-configurable to different sample diameters and gradients by exchange of the components providing the adiabatic zone.

Performance Characteristics

The SQF hot cavity consists of four separately controlled heating zones: one booster heater, two main heaters and one guard heater. The intended heater technology is graphite-based, allowing for very high operating temperatures. Heaters are controlled by thermocouples and a high performance control algorithm to a thermal stability of + 0.1 K. The achievable temperature gradients depend on cartridge design and sample thermal conductivity, with a design aim being 150 K/cm in a nickel sample.

SQF Technical Performance Data

Quench Module Insert (QMI)

The Quench Module Insert (QMI) is a NASA furnace insert to MSL, which will be available also to European scientists. It is intended for metallurgical research with the requirement to quench the solidification interface at the end of processing (by release of helium gas). Its performance will be similar to the SQF but with a lower maximum operating temperature.

Performance Characteristics

The QMI hot cavity consists of three heated zones: booster, main and guard heater. It will have an isolating zone and a cooling zone for the generation of the temperature gradient. In addition to the standard cartridge instrumentation it will provide sample resistance measurement as experiment-dedicated electronics.

QMI Technical Performance Data

Diffusion Module Insert (DMI)

The Diffusion Module Insert (DMI) is also a NASA developed insert, to which European scientists will have access. The DMI is intended for thermodiffusion experiments in semiconductors by Fickian or Soret process, supporting both isothermal and gradient temperature profiles. Special shear cell cartridges allows to align two columns of sample material once molten; after diffusion is complete all cells are sheared and separated from each other to freeze the diffusion profile. The sample is cooled quickly by gas injection.

Performance Characteristics

The DMI is a multizone furnace with five heating zones: booster, hot main, hot guard, cold main and cold guard.

DMI Technical Performance Data

Floating zone furnace with rotating Magnetic Field (FMF)

The Floating zone furnace with rotating Magnetic Field (FMF) is considered for development by DLR as a German national furnace insert and is expected to be available in the MSL from 2002. It is aimed at crystal growth by floating zone or travelling heater method, with the additional capability of magnetic stirring of the melt. It is a design objective to include optical access to the molten zone for real-time control of process parameters, provided that technical feasibility can be demonstrated. The experiment cartridge can be rotated for good thermal symmetry.

Performance Characteristics

The FMF will have 7 resistively heated heating zones, based on boron-nitride/graphite heater technology. It will provide high thermal stability by use of sapphire optical fibre thermometers. If included, optical observation will be supported at a frequency of approximately 1 Hz.

FMF Technical Performance Data

Experiment Cartridges

Scientific samples to be processed in the MSL will be contained in experiment cartridges of a maximum diameter of 48 mm (subject to furnace design). The cartridge assembly will generally consist of a cartridge tube, crucible (and if applicable other internal components), sample probe(s) and a cartridge foot, which constitutes the mechanical and electrical interface to MSL.

The cartridge is normally a metallic tube, which creates the leak tight barrier between the sample and the furnace environment; this is required for safe processing of materials in space. A chemically inert crucible may be implemented to avoid reaction between melt and cartridge. Further internal components may include, for instance, expansion compensation mechanisms (for thermal expansion effects and to avoid the creations of voids) or other mechanical parts required to control or condition the melt in the absence of gravity. The cartridge foot houses the interfaces and cables to the experiment cartridge instrumentation. MSL cartridges with toxic sample materials will be pressurised with krypton gas to allow detection of cartridge leaks by a mass spectrometer.

An example of a space worthy cartridge assembly is shown here.

SQF type experiment cartridge with liquid metal ring

Scientific thermocouples provide the sample temperature profile of the microgravity experiment; this profile should differ slightly from the ground-based one, due to the lack of convection in space. Peltier pulse indexing is mainly used in crystal growth experiments: a high-current pulse creates an artificial striation band, which by later etching gives the shape of the solidification interface. Shear cell activation is intended for thermodiffusion experiments: to terminate the experiment and preserve the concentration profile, a motor is activated that separates the cells of the diffusion column from each other. Reservoir heating provides the means to maintain a controlled vapour pressure of volatile elements, for instance for arsenic in gallium-arsenide crystal growth; this may be required for stoichiometry of the grown crystal.

There may be limits to the combination of sample diagnostics and stimuli that can be implemented in one cartridge (due to restricted space in the cartridge foot and limited compatibility of instrumentation). This will be evaluated on a case-by-case basis.

Technical Interface Data for Experiment Cartridges

LGF Experiment Cartridges: LGF cartridges will have an outer diameter of up to 28 mm. (Smaller cartridges are supported, but without changes to the furnace geometry.)

SQF Experiment Cartridges: SQF experiment cartridges will have discretely selectable diameters in the range 10 to 28 mm (with corresponding exchange of the adiabatic zone and liquid metal adapter). They will generally have a metallic cartridge tube and a ceramic crucible. If intense gradients are required, cartridges will be equipped with a liquid metal ring coupling to the cooling zone.

QMI Experiment Cartridges: TBD

DMI Experiment Cartridges: DMI experiment cartridges will normally be of shear cell type. In this case the crucible is a ceramic tube, in sections arranged axially. The sample material is contained in thin columns in the tube. The sample cells can be aligned and later separated by the activation of a shear motor to investigate diffusion effects.

Seebeck Voltage and Resistance Measurement

Seebeck diagnostics for directional solidification experiments were first developed within the French Mephisto programme. The diagnostic is based on the differential measurement of the thermoelectric voltage between two solid-liquid interfaces: a static reference interface and a solidifying interface. If both interfaces are planar, the differential thermoelectric voltage is proportional to the temperature difference between them, allowing a measurement of interface under-cooling during solidification. Interface microstructure and curvature, dislocations, melt concentration gradients and melt flows also contribute to the Seebeck signal, which means that the evolution of such effects during solidification can to some extent be monitored. Seebeck measurement is to date the only real-time and in situ diagnostic of the solidifying interface used in space.

Seebeck diagnostics can be combined with a measurement of the sample resistance. If the geometry, resistivity dependence on temperature and temperature profile are known, this value can be translated into a measurement of interface position.

The Seebeck technology is presently being modified from a "double-gradient" configuration used in Mephisto (Seebeck electrodes at both ends of the sample), to the "single-gradient" arrangement of MSL (electrodes only at one sample end). In the MSL configuration the reference interface will be established in a pure material, with the "scientific" sample being an alloy.

The following plot features the measurement of the electric resistance of a sample along with the furnace position, showing quantitatively the thermal lag of the furnace.

Ultrasonic Measurement of Solidification Velocity

An ultrasound diagnostic instrument based on guided waves was developed to measure the growing rate especially in opaque alloy systems. Experimental tests demonstrate that a high local resolution of the moving solid-liquid interface position down to 0.01mm can be achieved. Therefore this ultrasonic diagnostic technique is a proper and desirable tool to measure the solidification velocity both during steady state and unsteady solidification processes. The aim is to investigate quantitatively the role of this process parameter on the resulting materials' properties. Controlled unsteady growth seems to become a main research topic for the near future especially with respect to industrial application.

The ultrasonic measurement of solidification velocity is a diagnostic tool for directional solidification experiments, developed within ESA's Technological Research Programme. An ultrasonic pulse, transmitted from the sample's cold end and reflected by the phase boundary of the solidification interface detects the position of the solid-liquid interface. By accurate measurement of the time-of-flight with an auto-correlation technique and knowing the ultrasound velocity in the sample, the interface position can be obtained. The gradient of this parameter with time gives the solidification velocity with high accuracy.

The instrumentation consists of the ultrasound transducer, excitation and read-out electronics (Figure 1). Special software packages average and correlate the ultrasound echoes. Excitation voltages are in the order of a few volts, with a limited portion of the signal being transmitted into the melt. These low energies are not expected to influence the solidification. An off-line characterisation measurement allows determination of the local distribution of sound velocity in the sample before and after processing. This feature can be used for calibration of the technique and supports sample analysis.

The ultrasonic diagnostic has been characterised in conventional ground furnaces and with commercial ultrasonic equipment. As an example Figure 2 shows typical measurement of the real interface position by ultrasound during a directional solidification experiment in comparison with the furnace movement. In tests a spatial resolution of about 10µm and a velocity resolution of better than 1µm/s have been obtained.

The instrument has proved to be particularly valuable in determining the transient behaviour at start of solidification and at changes of the solidification parameters and to measure slight variations in solidification velocity at constant furnace speeds.

More detailed information about this diagnostic can be found here.

The plot above features measurements performed during directional solidification experiments in a Bridgman-Stockbarger type high temperature gradient furnace using a CuMn27.6wt% alloy. It shows two experiment runs, each with two different furnace velocities: v=0.4mm/min and 1.0mm/min, respectively v=1.0mm/min and v=0.4mm/min. The real position of the solid-liquid interface can be seen to always lag behind the furnace position. A marked discrepancy occurs at velocity changes. In total the solidification length in each sample is several mm shorter than the furnace movement, a result confirmed by the metallographic analysis of the processed samples.

The ultrasonic technique has been demonstrated for copper-manganese samples and is considered applicable to most materials. A good acoustical coupling to the transducer has to be obtained and acoustical coupling between sample and cartridge has to be avoided. The method requires only very low power and therefore does not disturb the solidification process. Preparatory ground-based investigations will be required for this diagnostic and needs to be supported by the institutes proposing to use this tool for MSL experiments.

The development of space hardware for ultrasonic measurement of solidification velocity has not yet been initiated and will depend upon scientific interest in this diagnostic after the first Announcement of Opportunity.

Experiment Selection and Development

The lead time of an experiment development programme is typically 18 to 24 months.

MSL Flight Operations on International Space Station

The International Space Station will be serviced every three months and together with the subsequent operational period, this set of operations is called an increment. The interval between uploading of new experiment cartridges to MSL and the return of the processed ones to ground will consequently be (at least) three months. It is intended to run the MSL by experiment campaigns for the different furnace inserts. Several furnace inserts may be used within one increment, supporting different fields of research.

The execution of the space experiment itself starts with the astronaut loading the experiment cartridge into the MSL. After evacuation of the process chamber an Experiment Sequence is initiated; it consists of a number of process steps with pre-defined parameters (for instance heat-up, directional melting, homogenisation, pulling/directional solidification, quenching and cool-down). The transition from one step to another can either be time- or event-controlled. As a baseline, the Experiment Sequence is executed automatically from beginning to end, but the MSL also provides the possibility to adjust process parameters and step sequences in real-time ("telescience"), both from ground and by the crew. Such actions, though, have to be verified as being safe before their execution. After processing the cartridge is removed from the MSL and stowed until it can be transported back to earth.

Facility Responsible Centres

The experiment development programmes, ground operations and the reference experiments under gravity will be supported by what are called Facility Responsible Centres (FRC’s). The MSL will most likely have three FRC’s: one in France, one in Germany and one in the USA. The FRC’s will be in charge of the MSL Engineering Model (EM) and/or the Science Reference Model (SRM) with their furnace inserts, and will thus be the locations for any high fidelity tests. The FRC’s will also be responsible for distributing the experiment data to the experimenter’s institutes.

MSL Safety Philosophy

The MSL incorporates instrumentation to guarantee a safe working environment for the crew under all possible conditions and events. Hazards that may be anticipated include:

Overheating in the furnace insert is detected by various temperature sensors, in the heaters themselves, in the furnace insert cooling loop and in the experiment cartridge. A hard-wired safety circuit, which is independent from the facility electronics, compares measured values to experiment-specific set points and switches off power to the furnace insert if a critical value is exceeded. The water-cooling system is designed to withstand possible over-temperatures for the time between switch-off and actual cool-down; any damages shall be confined to the furnace insert, which can be exchanged.

Loss of cartridge integrity during processing is a hazard for toxic samples. These may release particles into the core facility and ultimately expose crew to toxic material. Experiment cartridges containing toxic substances will therefore be required to include an amount of krypton tracer gas. Cartridge integrity is monitored by mass spectrometry analysis of the krypton peak for evacuated gases throughout processing. In case of ambiguous results, inspection of the cartridge through a glove-bag is anticipated. If a cartridge is believed to have leaked, the whole process chamber can be sealed off and transported to ground for decontamination.

Overheating in electronics boxes due to malfunctioning components is a fire hazard. All electronics boxes will therefore be equipped with temperature sensors. Anomalous temperatures will result in a switch off of the facility and alerting of the crew.

MSL Supporting Sub-Systems

The MSL Supporting Sub-Systems distribute and condition the resources from the International Space Station for the Core Facility. In addition they provide the facility control and intelligence.

Electronics Drawers

The MSL electronics are contained in two main components, the Power Supply Unit and the Facility Control Unit.

Power Supply Unit (PSU)

The MSL Power Supply Unit (PSU) provides the conversion and conditioning of primary power from the International Space Station into suitable power for heaters, magnetic field generation, drive units, and Peltier pulse generation. All power supplies include filtering of ElectroMagnetic Interference (EMI) and current limitation.

The PSU has its own processor and houses certain control boards for specific equipment. The main task of the processor is the calculation of new power values for the heater control, based on the inputs from heater temperature sensors. The PSU processor communicates with the Facility Control Unit (FCU) through a standard data bus (MIL 1553).

The main components of the PSU supply secondary power with the following characteristics:

Heater Power Converters:

Magnetic Field Generator and Controller:

Peltier Pulse Current Sources

Facility Control Unit (FCU)

The Facility Control Unit, as its name indicates, provides for overall control of the MSL facility. It contains the main processor, mass memory and the communications interfaces towards the rack and different subsystems. It is responsible for data acquisition and storage of housekeeping and experiment data. Further it controls all of the facility operations, supported by ground or crew commanding.

The main functional blocks of the MSL FCU are:

Two electronics boxes with 28 V DC supplies and RS422 interfaces may be added to the FCU: a fibre-optic pyrometer box for read-out of the optical fibre thermometers (OFT) and a box for Experiment-Dedicated Electronics. Both these electronics packages can be replaced in flight. The current design baseline is to accommodate the Seebeck voltage and resistance measurement electronics as experiment-dedicated electronics.

The MSL software is designed for autonomy, but with a great deal of flexibility. It can be commanded either from the ground or by the crew. The software architecture has different modes for the running of an experiment, for maintenance activities and for trouble-shooting, all with appropriate means for interaction with the operator. A safety supervision system is continuously running and will inhibit hazardous commands. The operator can always request information on the status of the facility and the software (for instance SW version, SW mode, furnace insert configuration) or the status of an experiment (experiment number, step under execution, remaining experiment time et cetera). An important module of the experiment processing SW is the furnace insert control algorithm, which can be changed between experiments. The MSL software will also be responsible for the timelining of resources.

Gas Supply Drawer

The MSL gas supply provides inert gas to establish a process atmosphere up to a pressure of 300 mbar and to flood the chamber to ambient pressure after an experiment. With gas being a consumable, the entire drawer is exchangeable and has to be replaced after 15 – 20 experiments. The gas supply drawer contains three gas bottles of 3 liters capacity each. They will as a baseline contain argon at 99.9995 % purity, with traces of krypton (for verification of cartridge integrity monitoring). The total gas volume of the gas supply is 1500 NL.

Gas / Vacuum Distribution System and Water Pump Package

The gas and vacuum distribution system controls the pressure of the core facility, using the MSL gas supply and vacuum interface from the International Space Station. A turbomolecular pump is used to achieve a vacuum level better than 10^-4 mbar. The gas/vacuum distribution system also includes the mass spectrometer used for monitoring of cartridge integrity.

The drawer of the gas/vacuum distribution system also contains the water pump of the water cooling system.

Water Cooling System

MSL heat removal will be performed entirely by water-cooling. A dedicated water-loop, with parallel branches, interfaces to the water-cooling system of International Space Station through heat exchangers. The water loop is designed for a total heat load of 6 kW. The branches of the water-cooling system and their respective flow rates are:

Project Timeline

MSL in US Lab

MSL in the Columbus Laboratory

For more information on the MSL please contact:
Peter Behrmann (pbehrmann@estec.esa.nl)
Project Manager for the MSL facility
Microgravity Facilities for Columbus Division (MSM-GF)
ESA ESTEC Keplerlaan 1 - NL 2201 AZ Noordwijk ZH