Electronics and Data Processing for the KATRIN Experiment
Contact: Dr.-Ing. S. Wüstling, Institute for Data Processing and Electronics (IPE)
The KATRIN Neutrino Experiment
The KATRIN experiment is intended to measure the mass of the electron anti-neutrino by an ultra-precise measurement of the energy distribution of Tritium decay (beta) electrons.
The most prominent component of the experiment is a very large electrostatic retarding spectrometer being formed by an ultra-high vacuum stainless steel vessel with a diameter of 10m. The decay electrons are guided by high magnetic fields from a windowless gaseous tritium source (WGTS) through differential pumping sections into the spectrometer. By applying a high voltage (ca. 18.6 kV), a decelerating electric field is formed, that bounces back most of the electrons. The electrons with the highest energies penetrate through the potential barrier and are counted by a silicon semiconductor detector.
An experiment of this complexity requires a large number of highly specialized components, also in the domain of electronics. These components can only be designed in a close interdisciplinary environment of the experiment collaboration.
The IPE is currently contributing in the following areas
- KATRIN task leadership of electronics and data processing-related tasks
- Slow Control (experiment control and monitoring) and data management
- Detectors and related electronics
- Fast data aquisition of detector signals
- Superconducting magnet safety and quench detection
- IT infrastructure
- High voltage stablilization
- EMC planning
- Consulting on electronics-related topics
KATRIN Task Leadership at the IPE
The KATRIN Task Leadership "Electronics and Signal Processing" and "High Voltage", the Deputy Task Leadership "Data Aquisition", and a Co-Leadership of the task "Slow control" are currently held by members of the IPE.
Slow Control and Data Management
The KATRIN main components like the electrostatic spectrometers and the tritium source are highly complex apparatuses. Many challenges like ultra high vacuum, high magnetic fields, high voltages, cryogenic temperatures or bake-out temperatures of 300°C have to be managed, sometimes a number of them at the same time. In the domain of electronics, KATRIN not only requires low-noise detectors for charged particles, but also needs complex automation and control technology, called “Slow Control”. To obtain safe operation on the one hand and on the other allow for adequate flexibility in the scientific environment, the slow control consists of two structures: The basic machine control and the scientific monitoring and control system. The first one is implemented based on robust industrial programmable logic controllers. This part of the experiment control is being developed by the KIT institutes ITEP and IK, who have extensive experience from instrumenting other safety-critical experiments before. The scientific monitoring and control is being designed under the general management of the IPE. This part of the control system comprises of e.g. a 3D temperature monitoring, control of the distributed high voltage system, magnetic field monitoring and the control of the focal plane detector.
The monitoring system is based on the software package ZEUS (“Central Acquisition and Control”), which was developed at Forschungszentrum Karlsruhe (now part of KIT). This software package offers modules for data acquisition, processing and distribution of process images, a data logging feature, a messaging system and functions to create operator control panels. All these modules are implemented in National Instruments’ LabView®. The whole package is on a configurable basis, with informations on the data sources and the associated data processing being held in a MySQL data base. The tools for configuration and data access are implemented in LabView® as well. Thanks to the built-in support for the OPC protocol and the freely programmable TCP/IP communications, PLC (programmable logic controller) systems can as easily be accessed as own in-house developed measuring devices. Thanks to the modular software design, system functions can be distributed onto several computers within a network, which gives a large degree of scalability.
In order to make all the data from the various parts of the experiment available in a centralized fashion, the ADEI (Advanced Data Extraction Interface) system was created. This system accesses the raw data of the various subsystems, checks for consistency and data quality and applies standard data pre-processing steps. The complete data repository is being made available to the worldwide experiment collaboration via the KATRIN data portal. A fast interactive navigation is provided by an intelligent caching strategy.
The data management systems described above have been employed successfully and further improved in the so-called pre-spectrometer experiment.
Visualization of the 3D temperature distribution of the KATRIN main spectrometer vessel
Central control and monitoring window of the so called pre-spectrometer experiment
Example for the web browser based data visualization in the IPE ADEI system
Control Room with operators’ desks
As a detection method for the tritium beta electrons, segmented silicon PIN detectors turned out to be the most viable principle for KATRIN.
At the IPE, the electronics and mechanics for the detector of the so-called pre-spectrometer experiment was designed. This experiment is a smaller, evaluative version of the final KATRIN experiment. It makes use of a spectrometer vessel that will serve as a pre-spectrometer in the final KATRIN.
The pre-spectrometer detector has 64 elements (pixels, pads). During the design phase of the electronics, experience was gathered that helped in the design of the electronics for the KATRIN main detector.
The electronics for the KATRIN main (“focal plane”) detector (FPD) has also been designed at IPE. This detector has 148 elements, arranged in a “dartboard” pattern with a 90mm diameter. Its electronics, which reside partially in vacuum, are completely isolated from the electrical ground potential in order to allow an electrostatic post-acceleration of the beta electrons.
Also for a number of auxiliary KATRIN experiments, the detector electronics was designed at IPE:
- In the so-called TRAP-Experiment Tritium was to be detected using a relatively large Si PIN detector (300mm²) under difficult mechanical constraints.
- The so-called Forward Beam Monitor is exposed to the full electron flux of the tritium source at high magnetic fields and has to be mechanically moveable across the flux tube.
- In the so-called Monitor Spectrometer, the apparatus of the Mainz Neutrino Mass Experiment is going to be reused, the detector electronics has to be refurbished however.
- For the commissioning of the KATRIN Differential Pumping Section DPS-2F, a 14-element Si detector for operation under ultra high vacuum and at high magnetic fields is required.
Common to all these electronics designs are the requirement to operate in high magnetic fields (up to 6T), vacuum compatibility, and spatial restraints.
To satisfy the requirements of ultra high vacuum technology, thermal ruggedness and also to obtain sufficiently small building blocks, thick film technology is often used for electronics circuit boards.
Cutaway of the 148 pixel KATRIN "focal plane" detector (FPD)
Test stand ("Iron Bird") for the electronics and the data acquisition system of the KATRIN focal plane detector
Ceramics circuit board with FPD preamplifiers
The 64 pixel-pre-spectrometer detector
Preamplifier front-end for the "TRAP" experiment detector, thick film technology
Preamplifier for the "Forward Beam Monitoring" detector (in operation with the KIT Institute for Nuclear Physics).
Electronics boxes for the KATRIN veto detector. Very weak light pulses consisting of few photons only are converted into electrical signals.
Detector Data Acquisition System (DAQ)
The raw data rate produced by the KATRIN detectors (focal plane detector, veto detector, beam profile monitoring detector, etc.) is formed by more than 148 channels with a signal bandwidth of several MHz each. Processing this amount of data without losing information content requires a suitable multi-channel data acquisition system.
For the Pierre-Auger-Observatory , the IPE designed a flexible DAQ system featuring a large number of channels (>400 per equipment rack), a high sampling rate (20...40 MHz), and a freely programmable FPGA-based data pre-processing. Today, the system is used by several different experiments and also lends itself as detector DAQ system for KATRIN.
It is able to cope with the high dynamic range of event rates (mHz…MHz) at the KATRIN detectors and the digital pre-processing can be reconfigured in order to adapt to different experiment modes within 10ms.
Operator interface and data transfer to mass storage is performed by the software package ORCA (Object-oriented Real time Control and Acquisition), that was designed by the KATRIN collaborators at the University of Washington at Seattle and at the University of North Carolina. With the support of our US colleagues, the IPE software group provided the suitable libraries and software drivers to read out the IPE DAQ hardware at KATRIN. ORCA also provides interfaces to the slow control system (see above). The integrated scripting mechanism allows for an easy automation of complex procedures. This makes ORCA an ideal tool for the design of measuring systems in physics.
IPE DAQ rack, version 4. On the rear side, 440 detector signals can be fed in. There is an integrated DAQ computer residing on the central plug-in board.
Operator window of the ORCA control software, providing control and monitoring of the KATRIN focal plane detector (FPD).
High Voltage Supply and High Voltage Safety
The retarding voltage in the KATRIN main spectrometer will be up to 35 kV in some calibration modes. This voltage is one of the most important magnitudes in the KATRIN experiment, as it directly determines the final neutrino mass measurement accuracy. The voltage must be stable to the ppm range, which is not only required for the average DC value, but also into the radio frequency range. Moreover, there are difficult EMC (electro-magnetic compatibility) constraints due to the electrically unshielded arrangement of the spectrometers.
The University of Muenster has designed an ultra-precision voltage divider for the monitoring of the DC component of the retardation voltage. The IPE is responsible for the dynamic stabilization of this voltage.
The drawing below shows how these two systems will work together to obtain the required overall KATRIN retarding voltage accuracy.
Moreover, due to the large spatial dimensions of the experiment, special attention has to be dedicated to the reference potential concept. Despite possible interference pick-up, it has to be made sure that the reference potential is accurate to the mV-range at all relevant places.
High voltage safety must of course also be addressed due to the large exposed parts of the spectrometer that are connected to hazardous voltages.
Block diagram of the high precision post-regulation arrangement for the main spectrometer retardation voltage
Reference potential concept for the KATRIN experiment
The proper functioning of the KATRIN main retarding spectrometer not only relies on electrostatic fields, but also on a very precise shaping of the magnetic field in the spectrometer area. A Low Field Coil System (LFCS), composed of large (12m) air coils, and an Earth Magnetic Field Compensation System (EMCS) are responsible for the field forming. In order to monitor the correct functioning of these systems, the IPE designed a multi-point 3D magnetometer. Significant cost savings have been obtained by tailoring the magnetometer system to the special KATRIN main spectrometer requirements.
Magnet Safety for Superconducting Magnets and Quench Detection
The axial guidance of the beta electrons emerging from the tritium decay is accomplished by high axial magnetic fields (3…6T) which are produced by superconducting coils.
In the WGTS (Windowless Gaseous Tritium Source”) and CPS (“Cryo Pumping Section”) sections, these coils are operated in the so-called “Driven Mode”, i.e. the power supplies for charging the coils remain permanently connected, keeping the coil currents constant.
In order to protect the coils from damage in case of loss of superconduction (“Quench”), special monitoring and protection measures are required. In the WGTS and CPS sections, a quench detection system developed and built at the IPE is employed. It performs its task in conjunction with a safety oriented PLC controller and a set of custom-built electronic hardware.
IPE is coordinating the installation and the maintenance of the KATRIN IT infrastructure. To fulfil the different safety and security requirements for the various IT services (from networking of safety-related process control up to simple PC office and internet applications), a hierarchically organized ring structure with several parallel fiber-optic ring lines is employed.
General IPE tasks within KATRIN
- General consulting for the KATRIN collaboration in all areas of electronics/electrical engineering (EMC, grounding, high voltage, etc.)
- Mentoring of bachelor, master and PhD students, assistance in electronics and computer science fields