This issue of the X-IFU newsletter aims to provide an update of the main I-PRR (Instrument Preliminary Requirement Review) actions, as they were described during the last Consortium Meeting #10 (CM10) held in Toulouse. It should be noted that CM10 was the first meeting with Vincent Albouys acting as the X-IFU project manager. Vincent will here take up the challenge of giving his perspective on the event. We have several articles related to the technical aspects of the instrument. Alice Pradines will report on the status of the Passive Cryogenic SIM/X-IFU Interface (PaCSXI) working group activities. Jean-Michel Mesnager will discuss on the definition of interfaces as the core of system engineering. Lastly, Irwin Maussang will discuss the activities carried out by the microvibrations working group, one of the working groups set-up following the I-PPR.

We keep trying to improve the efficiency of our CMs, even more so now that we have decided to move to only one CM per year. I will describe how the next meeting will be shaped, hopefully in a format that will satisfy the largest possible audience. As it was introduced during CM10, the X-IFU consortium will aim to reduce its travel footprint, I will come back to the topic in this issue of the newsletter.

Finally, we will give the floor to Stéphane Paltani, X-IFU Co-investigator and Swiss representative in the X-IFU Consortium Board. Stéphane will report on the overall contribution from the University of Geneva, the home university of Michel Mayor and Didier Quesloz, the recently awarded 2019 Nobel prize for the discovery of exoplanets back in 1995. Stéphane will also play the game of summarizing his outstanding career which led him to the point where he now stands.

I wish you a good reading.

Didier Barret
X-IFU Principal Investigator

A successful CM10 in Toulouse
The 10th Consortium meeting was held in Toulouse, hosted by IRAP/CNES from September 16th to 20th. As expected the meeting was perfectly organized for both the splinters and the plenary sessions. This was my first experience participating in a CM as Project Manager. I was very impressed by the quality of the presentations by all speakers and the completeness of the addressed topics. Congratulations to all!
The meeting started with splinters on Monday afternoon: XSAT (X-IFU Science Advisory Team), XCAT (X-IFU Calibration Team), performances, and Project Manager Meeting ran in parallel. Day 2 and 3 were dedicated to plenary sessions, with very detailed presentations giving a global insight into the actual design of the instrument and of all its subsystems. Post I-PRR discussions were key and specifically the first conclusions of the CryoSIM Working Group (PaCSXI) which willl be presented later by Alice. The programmatic aspect of the cryostat development was also addressed during plenary discussions and several splinters. Discussions on this topic are still currently going on. ESA presented the status on the ongoing Mission Formulation Review (MFR), whose objective is mainly to confirm the feasibility of Athena and thus to move from Phase A to Phase B1. The conclusion is expected at the MFR board meeting scheduled for November 12th. The forum highly appreciated Marcos Bavdaz and Max Colon’s talks about the Athena optics Development. We will try to continue to include external presenters in the next Consortium Meetings to improve our knowledge of the Athena System.
A significant decision was taken during CM10 to reduce the global carbon footprint of the Consortium by having only one consortium meeting a year. Thanks a lot to Didier for this extensive work in developing the first X-IFU carbon model which will probably help us a lot to optimize our travels in the future.
As a final word, CM10 will also be remembered for the Cuvée X-IFU wine tasting organized by Didier and Jérôme Jaune (Vineoc).

We are now waiting for the next tasting in Liege during CM11 to be held in April 2020!

Vincent Albouys
X-IFU Project Manager

What is PaCSXI 2?
This barbaric acronym refers to the second phase of activity of the Working Group PaCSXI, standing for “Passive Cryogenic SIM/X-IFU Interface”. This working group was set up after the I-PRR (Instrument Preliminary Requirement Review) in order to implement one of its recommendation: studying an alternative configuration of the instrument. It would reduce the risk related to microvibrations as well as reduce the complexity of the instrument by considering the number of active coolers and compressors as the main quantitative figure of merit. Among others, one driving idea behind this recommendation was to modify the SIM/X-IFU configuration and interfaces to make a more extensive use of the radiative cooling offered by the cold space, and implement two stages of passive cooling for the Instrument (170K, 80K), in order to reduce the active cooling needs in the hottest stages.

After a first phase during the spring of 2019, the joint ESA/CNES/CEA working group presented early July a new SIM/X-IFU cryochain configuration complying with these requirements, including 6 active coolers instead of 9, for a total of 15 compressors instead of 22. This new configuration, called CryoSIM and implying the switching of the two payload positions, showed no obvious design show-stopper at that stage. It was considered beneficial for microvibrations management and mechanical design, and neutral in mass with regard to the baseline.

However, in addition to some unavoidable losses in phase-A design maturity, in particular with the introduction of new 3K-JT coolers, and possible impacts on the project schedule, this configuration would have induced major changes on verification activities, logic and resources, both at Instrument and SIM level, due to the cryogenic temperature of the Dewar outer vessel in operations.

Consequently, the configuration was not considered sufficiently beneficial in terms of microvibrations to justify the added complexity in integration and testing. This solution will not be further analysed. Nevertheless, the work done by the working group has highlighted some interesting ideas for the mechanical and thermal improvement of X-IFU cryochain, which could be worth studying in a “non-CryoSIM” configuration, i.e. not implying a cryogenic temperature for the Dewar external skin.

In conclusion, for the second phase of the study, the working group was charged with studying the following ideas in the context of the SIM/X-IFU baseline:
   • Accommodation of the coolers’ cold heads into separate cryostats, which would give more flexibility for microvibration management, and potentially for AIT/AIV activities;
   • Implementation of a single 3K-JT coolers’ stage instead of 2K-JT + 4K-JT, which would reduce the number of kinds of coolers, the number of coolers themselves, and possibly the number of compressors. This could alleviate the development and integration processes, and reduce the cryochain complexity, at least from an industrial point of view;
   • Various mechanical and thermal optimisations, aiming at removing one of the five PT15K coolers.

These different topics are currently under assessment within the X-IFU and SIM teams, with conclusions to be submitted before the Mission Formulation Review board mid-November.

Alice Pradines
CNES Project Team

Interfaces: The core of system engineering
In system engineering, the interface of an object can be defined as the way it interacts with its external environment. If we consider the upper-level system, the interface can also be seen as all the interactions existing between two subsystems or more. It covers a very wide range of features, from physical properties like mass and size, to functional properties (what kind of service is provided by the object), or susceptibility to environmental conditions, such as electromagnetic compatibility or vibrations.

One of the earliest disciplines that used and developed the concept of interface was computer science: the need to breakdown complex pieces of software into smaller and simpler modules (or subroutines for Fortran adepts!) became mandatory to master the complexity of the whole system. Later, the need to formalize the interaction between the user and the computer gave birth to the Man Machine Interface (MMI) notion.

This concept is now extensively used in system engineering, since the benefits are the same for most kinds of systems: breaking down the X-IFU into smaller cooperating subsystems not only results in smaller and simpler elements, but also enables independent development processes. By focusing on subsystem functions and interfaces (the external point of view), system engineering also separates the subsystem properties expected at upper level from its design. This brings more latitude in subsystem design and enables a more efficient customer-provider relationship.

On X-IFU, like on most other complex systems, the foundation of the system-level design is the product tree which identifies all the products that, once put together, will make up the instrument. Whereas the main specifications (aligned on the product tree) gather the requirements that are specific to one product, interface requirements describe how products will interact with each other to make instrument functions and meet the performances. One specificity of interfaces is that when one end changes, the other has, in general, to be adapted accordingly. This is why interface requirements are, in general, captured in separate specifications that are applicable to several products at the same time.

As for the management of all other requirements, good practices recommend to separate the user’s need (Interface Requirement Document or IRD), from the description of the actual object (Interface Control Document or ICD). The first describes an early user’s need. It enables the start of the subsystems design (the mass shall be less than 100kg) and is, in general, frozen at the Preliminary Design Review. The second describes the product as designed/manufactured (the mass is 95kg) and is released at the Critical Design Review, or later.

Verification of interfaces is one of the earliest phases of verification and is sometimes named “compatibility testing”. Both the conformity of the design to the need (IRD), and the compatibility of the actual design (ICD) need to be checked to guaranty the compatibility of the interface.

This is the theory. Let’s now apply it to X-IFU!

Jean-Michel Mesnager
CNES Project Team

Image by macrovector_official / Freepik
Microvibration – Macroscopic approach
Athena X-IFU instrument is deemed to be sensitive to microvibrations. Indeed, to reach the very low temperature required at detector array level, several mechanical coolers are accommodated near the Dewar. Consequently, the vibrations generated by these mechanical coolers are transmitted through the Dewar structure to the Focal Plane Array (FPA). These vibrations can generate thermal fluctuations, included at low frequencies, that can disturb the detection of photons.

In our case, microvibrations handling is challenging in its three constitutive components: disturbance sources, mechanical transfer of disturbance, and susceptibility of sensitive device. The important number of mechanical coolers leads to tackle a lot of complex scenarii of disturbance. The uncertainty of the mechanical transfer of the Dewar may be atypical due to specific structures (straps to limit parasitic conductive heat loads, shields with a high number of sheets of Multi-Layer Insulation, and important parasitic mechanical path) and the fact that these structures will work at low temperature. Lastly, FPA’s susceptibility to microvibrations has to be assessed precisely and this may involve dedicated numerical tools/methodology of work at instrument level.

Thus, and as an outcome of the I-PRR, a micro-vibration working group was initiated and is currently running. Due to the specific nature of its subject, this working group involves an important number of participants: CNES (as X-IFU and X-IFU end-to-end microvibration budget responsible / DCS global test plan responsible), ESA (as both spacecraft responsible and cooler provider), SRON (FPA responsible), CEA (Sub-Kelvin cooler responsible), and INTA (DCS Cryostat responsible).

The first phase of this working group (May-July 2019) has allowed to understand FPA physics and characteristics and to identify specific post-processing to be implemented at instrument level or models to be implemented at 2K core level. It has also been checked that, as far as allowed by its representativeness, DCS testing plan takes into account the need to consolidate X-IFU microvibrations knowledge.

Adding to this, assumptions concerning disturbance sources and key parameters for the FPA were agreed in order to start the second phase of this working group. Indeed, this second phase consists of short loop micro-vibrations analyses to identify:
   •  Potential constrains to be applied on disturbance sources to minimize microvibrations risk;
   •  Mechanical frequencies of interest and as far as possible establish qualitative budgets.  The ability to control and limit microvibrations risk by design will be evaluated accordingly;
  •  Microvibration simplified criteria to allow fast loop analyses, budget cross check at spacecraft level, separation of Dewar microvibration transmission studies and 2K core susceptibility studies if possible.

As a summary, current short-term activities of the microvibrations working group will provide a global view of the risk associated with  microvibrations at the beginning of 2020. It will also identify manageable methodologies to work with several partners at different scales with several schedules of development.

Irwin Maussang
CNES Project Team

CM11: April 6th-10th in Liège
The upcoming Consortium Meeting will be held in Liège from April 6th to 10th. It will be hosted by the University of Liège and the Centre Spatial de Liège. The meeting will take place at the premises of the University of Liège. It should be reminded that the local organizers have committed during the last CM social dinner in Toulouse to organize a special event around beer: possibly by releasing a 2020 X-IFU beer. Let us see how far up they can take the challenge, but it is already clear to everyone that the bar was set high with the X-IFU cuvée released this year. More seriously, we plan to use CM11 to test a new format for the meeting. The idea is to shorten the plenary sessions to less than a day (Monday PM and early Tuesday AM), during which general presentations focusing on broader topics will be given. Each presentation should summarize the latest developments since the last meeting, and leave some room for a few “external” presentations, similar to the one on optics that we had during the last CM. Then, we will split in sub-plenary sessions on four themes: sensor and readout, cryogenics and mechanics, system and avionics, and science. The X-IFU Science Advisory Team (XSAT) will invite the X-IFU Calibration Team (XCAT) and the performance team to join in the last part of the XSAT’s Tuesday meeting. The objective of these changes is to have a better audience for each meeting, instead of requesting everyone to attend talks which have increasingly become too specialized and technical. 

This part will end on Tuesday evening and will be followed by the social dinner. Wednesday will be dedicated to group meetings (e.g. project manager meeting, Consortium Board meeting, science ground segment…), as well as the first set of face-to-face meetings organized by the CNES project team. These meetings will fill all of Wednesday. On Thursday morning, we will reconvene for a couple of hours in plenary sessions to listen to a set of short presentations, summarizing the discussions held over the previous days and the actions to be carried out until the next meeting. Starting from Thursday afternoon, the CNES project team will organize a second series of face-to-face technical and progress meetings, on selected topics with restricted attendance. The meeting will end on Friday around noon. At all times, a waiting room will be available for people to discuss or wait in between meetings they are expected to attend.

Although the overall format will be as described above, we still have to work on optimizing the organization of group and face-to-face meetings so that key members can attend all their meetings without any risk of overlapping. We believe that this new format will maximize the efficiency of the meeting and will make the best use of people’s travels, thus also being a good way to further reduce our travel footprint.

Although we will now move to one consortium meeting per year (most likely in April), there will still be two project manager meetings per year: one in the January-February timeframe and another between October and November. In order to ensure the best attendance possible, we will try to avoid vacation or holiday periods, and this may imply to have these meetings located outside academic institutions (which only have free or cheap rooms during vacation periods).

Didier Barret
Key measures to reduce our travel footprint
Legend: Applied to the audience of the last consortium meeting, this figure shows the benefits of traveling by train versus by plane, whether flying is considered only if the distance to the meeting location is larger than 300 km (red) or 1000 km (green). As can be seen, if the meeting were held in Grenoble, which is the site of minimum emission (76 tons), we could save up an aditionnal 28 tons of CO2eq if flying were only considered for travel distance superior to 1000 km (as opposed to 300 km). Refraining using short haul flights as advocated elsewhere (e.g. by scientists from the Max Planck Society) is indeed a very efficient way of reducing the footprint of travels.
The travel footprint of the X-IFU Consortium is large because X-IFU is an international project. This being said, we must take sensible actions to reduce our travel footprint. There are ways of doing so without impacting the development of the project. The most obvious one was to reduce the number of consortium meetings to one per year. This was unanimously agreed by the X-IFU Consortium board, which met during CM10. This decision means optimizing each CM and Liège will provide a test bench for a new format which we believe will optimize the efficiency of the meeting. The other obvious solution is to make the best use of people's travels, by organizing dedicated face-to-face meetings along either consortium meetings or project manager meetings (twice per year). By doing so, no additional travels may be required, if progresses can be tracked on a quarterly basis. A third solution is switching, whenever possible, from face-to-face meetings to virtual meetings. We have carried out a survey of the softwares used within the Consortium. The results of the survey will soon be circulated, but it is already clear that there are some tools that seem to satisfy a large number of Consortium members. We should encourage the members of the Consortium to use those tools and equip their institute with video-conferencing facilities. A fourth solution is to organize all meetings at locations minimizing the travel footprint, and privilege trains over planes, whenever possible. If you are interested by your travel footprint, a tool is available for that purpose. The tool enables you to see the benefit of traveling by train, the impact of flying in seat class other than economy, and can also compute the meeting location that minimizes the travel footprint. The form will be soon replaced by an automated and anonymous system currently under development. Not of negligible impact, we should also collectively optimize the way X-IFU is presented in conferences, for instance by reducing the number of attendees, while maximizing the coverage of topics. Other measures could be taken to reduce the environmental impact of the consortium as a whole. For instance, by reducing the number of single-use items (e.g. plastic cups and bottles) during meetings or selecting hotels with an official eco-label.

The above solutions apply at Consortium level, but this should not prevent all of us, as individuals, to think about the way we want to handle the issue of reducing our environmental footprint. We ought to do it, first because there is a clear urgency to act, and second because we should lead by example, even more so as we work in a field from which we are competent to talk about the vulnerability of our beautiful planet.

Didier Barret
The Swiss contribution to the X-IFU

The Swiss contribution to Athena X-IFU is led by the University of Geneva and consists of two main components. First, Switzerland will be providing the Filter Wheel subsystem. This subsystem consists of a mechanism able to place in and remove from the optical beam some filters used to optimize the X-IFU observations, especially in the case of particularly bright sources, in the optical/UV or in the X-rays. In addition, the driving electronics will also pilot the active X-ray calibration sources developed by SRON. This contribution is based on the heritage from the JAXA missions ASTRO-H/Hitomi and XRISM, for which Switzerland has provided and is still providing similar equipment. The expertise acquired in the development of mechanisms in space has also been put to use for the Euclid mission, for which Switzerland provides the shutter of the visible (VIS) instrument, and for the future mission candidate THESEUS.

The other Swiss contribution to the X-IFU is the responsibility of the development of the X-IFU scientific analysis software. This is based on a long tradition of development of ground-segment activities for space mission established in 1995 with the INTEGRAL Science Data Centre, the center responsible for collecting, packaging and distributing the INTEGRAL analysis software, and for the processing and archiving of its data, among other tasks. Ground-segment activities have later been conducted for several other space missions, in particular Gaia, CHEOPS and Euclid. The Athena activities will constitute another major effort in this area.

The University of Geneva ranks among the major European universities, and is a member of the League of European Research Universities. The Department of Astronomy, the largest institute for research in astronomy in Switzerland, is one of the most prominent research institute of the University, now hosting two Nobel Prizes in Physics for the discovery of the first exoplanet. The high-energy astrophysics group was established in 1990; it first concentrated on the study of active super-massive black holes, and then on the science of the INTEGRAL mission. It has now developed into a group active in most areas of high-energy astrophysics, from X-ray to TeV astronomy, as well as in observational cosmology.

Stéphane Paltani
X-IFU Co-Investigator
Swiss representative in the X-IFU Consortium Board

Image by Ruag Space AG/University of Geneva

Bio: Stéphane Paltani

Stéphane Paltani is Professor and lead of the high-energy astrophysics and cosmology group at the Department of Astronomy of the University of Geneva. He is the Swiss member of the X-IFU Consortium Board, and is leading all Swiss contributions to the Athena mission. He first got involved in hardware activities through the participation of his group in the analysis software for the Eureca (European and Japanese Calorimeter Array) project, a prototype development of a Transistor Edge Sensors (TES) array led by SRON. In 2008, he was invited by JAXA to join the ASTRO-H/Hitomi mission by contributing the filter wheel mechanism and electronics for the SXS micro-calorimeter, a collaboration that continues with the future XRISM mission, and which shaped the Swiss hardware contribution to X-IFU. In addition to these hardware development activities, he is leading the development of significant ground-segment activities in particular for ESA's cosmology Euclid mission and for Athena. His main domains of research are closely related to these two missions, namely the environment of super-massive black holes and the evolution of galaxies. He is also the Swiss PI of the candidate mission THESEUS and Co-PI of the MOONS instrument for the Very Large Telescope.

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