Transmission-line difficulties can be notoriously difficult to solve, especially when the load end is not easily accessible. Although there are a number of different methods available, this chapter discusses time-domain reflectometry (TDR). It works on the principle that states that the waves propagating along an unmatched transmission line reflect from the load back toward the source. The waveform seen at any given point along the line is the algebraic sum of the forward or incident waveform and reflected waveform. When they combine, the incident and reflected waveforms produce an analyzable pattern. Although a professional TDR is an expensive piece of equipment, some TDR methods can be used by anyone who has access to an oscilloscope and a pulse generator. The chapter also presents a diagrammatic setup of an impromptu TDR, including an oscilloscope and a pulse generator: a pulse generator, or some other source of 1-MHz square waves, is applied simultaneously to the vertical input of an oscilloscope and the input end of the transmission line.

Time-Domain Reflectometry

The planned FAIR synchrotron SIS100 has to deliver high intensity beams. Following the JINR Nuclotron, GSI has chosen to build this machine using fast-cycling 4 T/s, 2 T superferric magnets. Cycle repetition rates of about 1 Hz cause high AC power losses in the magnet components. Eddy currents deteriorate the field quality. After millions of operation cycles and high radiation flux the coil support structure can loose mechanical stability. Therefore these effects were analyzed to warrant the magnet's performance over its lifetime. Following intensive experimental and computational investigations on short test models three full size dipoles and a 1.2 m long quadrupole with a maximum field gradient of 27 T/m in the aperture of 135 mm 65 mm will be manufactured until the end of 2007. Two of the dipoles are straight 2.7 m long 2.1 T magnets with a beam aperture of 130 mm 60 mm (width x height), while the third one is a curved 1.9 T dipole 3.0 m long with a reduced horizontal aperture of 115 mm. All the models use a superconducting hollow NbTi/Cu composite cable cooled with two-phase helium flow. The main design features and estimated operation parameters are given. In parallel preliminary tests of a 2.8 m SIS100 equivalent dipole system was performed. The cooling stability of the system at different SIS100 operation cycles was checked experimentally for the first time. These results are discussed as well as possible design options to fulfil additional operation requirements.

Full Size Model Magnets for the FAIR SIS100 Synchrotron

MIITs, a zero-dimensional concept to study hot spot temperature, has been previously used to estimate hot spot temperatures and quench heater delays in NbTi and Nb 3 Sn magnets. However, quench behavior is completely different in high-temperature superconducting (HTS) magnets due to the slow normal zone propagation velocity and the high temperature margin. Because the MIITs concept does not take into account thermal diffusion in the magnet, opposite to the finite-element method (FEM) analysis, the difference of these concepts is studied in this paper. Here, we have taken the approach to compute the hot spot temperatures for a future HTS magnet, designed to be built from REBCO Roebel cable, with MIITs and FEM simulations. The magnet protection is accomplished with a dump resistor, and the effect of quench detection threshold voltage on the hot spot temperature has been studied. Furthermore, the inductance of the magnet increases with the magnet length. Thus, there exists a maximum inductance of the magnet, which should not be exceeded to be able to protect the magnet only with a dump resistor. The hot spot temperatures with different values of inductance are also studied in this paper. Our simulations show that the hot spot temperatures computed with MIITs are from 60 to 150 K higher than those of FEM analysis. Thus, the MIITs concept seems unreliable when considering hot spot temperatures in HTS magnets protected with only dump resistors. However, the MIITs concept might be a usable tool when comparing different magnet designs. If 400 K is the upper limit for the hot spot temperature and the protection scheme includes only a dump resistor, the length of the investigated magnet can be increased to only such value that the magnet inductance is at most 50 mH.

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Hot Spot Temperature in an HTS Coil: Simulations With MIITs and Finite Element Method

The SIS100 synchrotron for heavy ion research is based on fast-ramped superconducting magnets. These superferric magnets are being designed, built, and tested. The first 3-m-long series dipole (maximum field of 1.9 T; ramp rate of 4 T/s) has been built. We present the test results obtained (quench training, magnetic field, ac loss, and repetition rate) and compare them with the predicted values and the required operation parameters.

Fast-Ramped Superconducting Magnets for FAIR Production Status and First Test Results

A significant cryogenic load is generated in rapidly-cycling superconducting magnets due to ac losses This load should not be excessive, otherwise cryogenic system will not be able to evacuate heat and magnet will quench To verify magnet design and to know real cryogenic load the test of ac losses has to be performed Electrical and calorimetric methods can be used for this purpose The calorimetric method is slower because it requires stabilization of cryogenic system The electrical method called V-I is faster and it can be applied to any magnet independent of its cooling schemes With this method ac losses are measured directly and results are provided on-line However, the electrical method is considered more error prone, mainly due to the fact that power factor of superconducting magnets is close to zero

Sources of Error in AC Losses Measurement Using V-I Method

For testing rapidly-cycled superconducting magnets in the framework of the FAIR project (Facility for Antiprotons and Ion Research) a test facility was set up at GSI to test model and prototype superconducting magnets for different machines (SIS100, SIS300, Super-FRS). We are able to perform the following magnet tests: quench training, magnetic measurements, measurements of AC losses (calorimetric and V-I methods), temperature distributions in the magnet, and long-term stability tests. The forced-flow and bath-cooled scheme can be applied for the superconducting magnets. The facility consists of the following components: a refrigerator (400 W at 4 K); a distribution box for different cooling schemes (supercritical and two-phase flow); 2 feed-boxes providing electrical and cryogenic supply for the magnets; an all-purpose horizontal magnet cryostat which can be used for all kinds of different magnets (max. 3 m length and 0.8 m diameter); a power converter (11 kA) including the quench protection system; a quench detection system adapted from the LHC-series test facilities. The results of the AC loss measurements and the quench training for the first tested superconducting magnet are presented.

Commissioning of the Prototype Test Facility for Rapidly-Cycling Superconducting Magnets for FAIR

Testing the different superconducting magnets for the Facility for Antiproton and Ion Research requires testing capabilities beyond the necessities of dc superconducting magnets: stable power converters with fast cycles, capabilities for measuring the loss, and abilities to measure the field on the ramp for the magnets of the heavy-ion synchrotron SIS100. On the other hand, the Super Fragment Separator (Super-FRS) magnets are of large aperture and large size. The high-current design of the SIS100 main dipole and quadrupole magnets necessitated upgrading the power converter, the current leads, and different measurement systems. Facilities are being built or refurbished for measuring the SIS100 quadrupole units and the Super-FRS magnets. We describe the abilities of these facilities and the status of preparation. The upgrade of the existing facility at GSI has been completed. We present the performance of the main systems and their accuracies based on the measurements made for the SIS100 dipole.

Low-Temperature Test Capabilities for the Superconducting Magnets of FAIR

Identifying the location of an electrical fault (mainly short-circuit or circuit break) in close systems, such as superconducting magnet, is very difficult, especially when a problem appears only at cold conditions (no resistance). The time domain reflectometry (TDR) method could be useful notwithstanding that there is a commonly known limitation on the use of TDR to test the inductive circuit results of the high attenuation of incident and reflected pulses. This paper describes a method for locating electrical faults in superconducting magnets by means of measurements of the pulse response of magnet coil circuits (TDR principle). As a result of adapting TDR to test a circuit characterized by high values of equivalent capacitance and inductance, which is a winding of superconducting magnet, a study of the relationship between the frequency response of magnets and the injected pulse transient parameters is presented. The cutoff frequency of the investigated circuit is crucial information for pulse shaping. A TDR reflectometer with a pulse shaping circuit is described. The instrument consists of commonly available laboratory-measuring equipment: a pulse generator and a 50-MHz oscilloscope, which significantly facilitates the performance of the measurements. Test measurements were carried out for both the undamaged magnets and the magnets with various locations of coil shorts. Advantages and limitations of the proposed method are discussed.

Locating Electrical Faults in Superconducting Accelerator Magnets Using Time Domain Reflectometry

The paper shows some of the most notable difficulties connected with constructing a superconducting magnets frequency characteristics test system. The hardware and software of the system was shown, and the method of calculating the model of the magnet coil parameters. The paper shows some of the test results of example superconducting magnets.

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Testing of the Superconducting Magnets Frequency Characteristics

The Facility of Antiproton and Ion Research (FAIR) will feature two superconducting fast ramped synchrotrons (SIS 100, SIS 300). The core components of this new facility are two-phase forced cooled superferric magnets ramped with 4 T/s up to 2 T and single-phase forced cooled costhetas magnets ramped with 1 T/s up to 4 T. The GSI test facility consists of 2 test benches. One of them is equipped with a universal cryostat and second one is dedicated to cryostated superconducting (SC) magnets. So far, one SC coil and one SC magnet have been tested. We report on the training, quench behavior of magnets and results of AC loss measurements.

A. Stafiniak

Papers

Commissioning of the Prototype Test Facility for Rapidly-Cycling Superconducting Magnets for FAIR

Low-Temperature Test Capabilities for the Superconducting Magnets of FAIR

Locating Electrical Faults in Superconducting Accelerator Magnets Using Time Domain Reflectometry

Testing of the Superconducting Magnets Frequency Characteristics

The GSI Cryogenic Prototype Test Facility—First Experience Gained on 2-Phase-Flow Superconducting Prototype Magnets of the FAIR Project