E.B. Vossenberg

Design of the injection kicker magnet system for CERN's 14 TeV proton collider LHC

Two counter-rotating proton beams will be injected into the LHC at an energy of 450 GeV by two kicker magnet systems, producing magnetic field pulses of approximately 750 ns rise time and 6.6 /spl mu/s flat top duration. To avoid dilution of the beam emittance during injection, a stringent design requirement of the system is a flat top ripple of the magnetic field of less than /spl plusmn/0.5%. Both injection systems are composed of 4 travelling wave kicker magnets of 2.17 m length each, powered by pulse forming networks (PFNs) and matched to their characteristic impedance. To achieve the high required kick strength of 1.2 Tm, for a compact and cost efficient design, a comparably low characteristic impedance of 5 /spl Omega/ has been chosen. The electrical circuit of the system is being designed with the help of PSpice computer modelling. Most known parasitic elements are included in the model to obtain a realistic pulse response prediction. The present paper reports on design and modeling results of the LHC injection kicker magnet system that has several novel and demanding design requirements.

Solid state switch application for the LHC extraction kicker pulse generator

A semiconductor solid state switch has been constructed and tested in the prototype extraction kicker pulse generator of CERNs Large Hadron Collider (LHC). The switch is made of 10 modified 4.5 kV, 66 mm symmetric GTOs (also called FHCT-fast high current thyristor), connected in series. It holds off a DC voltage of 30 kV and conducts a 5 /spl mu/s half-sine wave current of 20 kA with an initial di/dt of 10 kA//spl mu/s. Major advantages of the switch are the extremely low self-firing hazard, no power consumption during the ready-to-go status, instantaneous availability, simple condition control, very low noise emission during soft turn-on switching and easy maintenance. However, the inherent soft, relatively slow turn-on time is a nonnegligible part of the required rise time and this involves adaptation of generator components. A dynamic current range of 16 is achieved with variations in rise time, which stay within acceptable limits. Important generator improvements have been made with the series diodes and freewheel diodes. A more efficient droop compensation circuit is being studied. It is directly connected in series with the freewheel diode stack and maintains an acceptable flattop variation of 5% of the magnet current during 90 /spl mu/s. This paper presents the complete generator, in particular the solid state switch, and discusses related electrical measurements.

Pseudospark switch development for the LHC extraction kicker pulse generator

CERN, the European Laboratory for Particle Physics, has started construction of the Large Hadron Collider (LHC), a superconducting accelerator that will collide protons at a center of mass energy of 14 TeV from the year 2005 onwards. The kicker magnet pulse generators of the LHC beam extraction system require fast high power switches. One possible type is the pseudospark switch (PSS) which has several advantages for this application. A PSS fulfilling most of the requirements has been developed in the past years. Two outstanding problems, prefiring at high operating voltages and sudden current interruptions (quenching) at low voltage could have been solved recently. Prefiring can be avoided for this special application by conditioning the switch at two times the nominal voltage after each power pulse. Quenching can be suppressed by choosing an appropriate electrode geometry and by mixing Krypton to the D/sub 2/ gas atmosphere. One remaining problem, related to the required large dynamic voltage range (1.7 kV to 30 kV), is under active investigation; steps in forward voltage during conduction, occurring at low operation voltage at irregular time instants and causing a pulse to pulse jitter of the peak current. This paper presents results of electrical measurements concerning prefiring and quenching and explains how these problems have been solved. Furthermore the plans to cure the forward voltage step problem are discussed.

Dual branch high voltage pulse generator for the beam extraction of the Large Hadron Collider

The LHC beam extraction kicker system, MKD, is composed of 15 fast kicker magnets per beam to extract the particles in one turn from the collider and to dispose them, after dilution, on an external absorber. Each magnet is powered by a separate pulse generator. The original single branch generator consisted of a discharge capacitor in series with a solid state closing switch operating at 30 kV. In combination with a parallel freewheel diode stack this generator produced a current pulse of 2.7 /spl mu/s rise time, 18.5 kA amplitude and about 1.8 ms fall time, of which only about 90 /spl mu/s are needed to dump the beam. The freewheel diode circuit is equipiped with a flat top current droop compensation network, consisting of a low voltage, low stray inductance, high current discharge capacitor. Extensive reliability studies have meanwhile suggested to further increase the operational safety of this crucial system by equipping each generator with two parallel branches. This paper presents the re-designed dual branch generator and addresses technical difficulties, and approaches related to this design change, as well as further efforts intended to improve the overall reliability of the system. The final magnet current flat top compensation is also discussed, together with the low impedance transmission line between generator and magnet. This line consists of 8 parallel 18 Ohm coaxial power cables per magnet, each 19 m long, and is an important part of the circuit.

Kick sensitivity analysis for the LHC inflectors

The injection kicker system of CERNs Large Hadron Collider (LHC) will consist of two sets of four kicker magnet systems each producing a magnetic field pulse of 1.3 T.m. with a duration of 6.5 /spl mu/s, a rise time of 900 ns, and flat top ripple of less than /spl plusmn/0.5%. The electrical circuit of the complete system, including all known parasitic quantities, has been simulated with PSpice. Many parasitic elements were determined from Opera2D simulations which included eddy-currents. Equivalent circuits which simulate the frequency dependence of inductance and resistance of the Pulse Forming Network (PFN) have been derived. PSpice has been utilised to carry out a sensitivity analysis of the field to the value of both individual and groups of circuit components. Capacitors for a prototype 5 /spl Omega/ PFN have been purchased and, based on the measured values of these capacitors, the diameter of the PFN coil has been re-optimised. The results of the sensitivity analysis have been used to define component tolerances for a prototype PFN. Low and high voltage measurements have commenced on the prototype PFN, and the results of the sensitivity analysis will be used to determine the source of any excessive ripple. This paper presents the results of both the analyses and measurements.

Construction and 60 kV tests of the prototype pulser for the LHC injection kicker system

The European Laboratory for Particle Physics (CERN) is constructing the Large Hadron Collider (LHC). Two counter-rotating proton beams will be injected into the LHC at an energy of 450 GeV by two kicker magnet systems, producing magnetic field pulses of approximately 900 ns rise time and 6.6 /spl mu/s flat top duration with a ripple of less than /spl plusmn/0.5%. Both injection systems are composed of 4 travelling wave kicker magnets of 2.17 m length each, powered by pulse forming networks (PFNs). To achieve the high-required kick strength of 1.2 Tm, for a compact and cost efficient design, a characteristic impedance of 5 Ohms has been chosen. The design strategy for the magnets and generators has been defined after detailed analysis of existing systems. The electrical circuit has been optimised using the circuit analysis software PSpice. Most known parasitics have been modelled. A prototype PFN has been constructed at CERN and successfully tested at 60 kV. A calibration procedure has been developed and utilised for obtaining correction data for a high voltage probe and oscilloscope amplifier. Measurements carried out with a precision of approximately /spl plusmn/0.1% show that the prototype PFN conforms to the specifications and the PSpice predictions.

Improved turn-on characteristics of fast high current thyristors

The beam dumping system of CERNs Large Hadron Collider (LHC) is equipped with fast solid state closing switches, designed for a hold-off voltage of 30 kV and a quasi-half sine wave current of 20 kA, with 3 /spl mu/s rise time, a maximum di/dt of 12 kA//spl mu/s and 2 /spl mu/s fall time. The design repetition rate is 20 s. The switch is composed of ten fast high current thyristors (FHCTs), which are modified symmetric 4.5 kV GTO thyristors of WESTCODE. Recent studies aiming at improving the turn-on delay, switching speed and at decreasing the switch losses, have led to tests on an asymmetric not fully optimised GTO thyristor of WESTCODE and an optimised device of GEC PLESSEY Semiconductor (GPS), UK. The GPS FHCT, which gave the best results, is a nonirradiated device of 64 mm diameter with a hold-off voltage of 4.5 kV like the symmetric FHCT. Tests results of the GPS FHCT show a reduction in turn-on delay of 40% and in switching losses of almost 50% with respect to the symmetric FHCT of WESTCODE. The GPS device can sustain an important reverse current during a short period. This eliminates the need for an anti-parallel diode stack in the final switch. Extrapolation of the test results onto the final switch result in a turn-on delay of 600 ns and 64 J total conduction losses from turn-on to 20 kA peak current. Further tests on the GPS FHCT at 4.4 kV, 60 kA peak current and a repetition rate of 10 s resulted in a di/dt of 50 kA//spl mu/s with a turn-on delay of 700 ns. These encouraging results, obtained with a slightly modified standard device and based on several hundred thousand discharges, open a wide field of fast high current, high voltage applications where presently thyratrons and ignitrons are used.

Design aspects related to the reliability of the LHC beam dump kicker systems

The two LHC beam dump kicker systems consist each of 14 pulse generator and magnet subsystems. Their task is to extract on request the beams in synchronisation with the gap in the beam. This operation must be fail-safe to avoid disastrous consequences due to loss of the beam inside the LHC. Only a failing operation of one of the 14 pulse generators is allowed. To preserve this tolerance premature beam dumps are forced immediately after early detection of internal faults. However, these faults should occur rarely in order not to be a source of undesirable downtime of the LHC. The report determines first the level of reliability required for the main components of the system. In particular faults which could cause spontaneously non-synchronised beam identified. Then, technical solutions are evaluated on failure behaviour. Those having a most likely failure mode which does not cause dump triggers are favoured. These solutions need redundancy and are more complex but have the advantage to be fault tolerant. The design goal can be achieved with a combination of high quality components, redundant signal paths, fault tolerant subsystems, continuous surveillance and check-list validation tests before the start of the injection of beam in the LHC.

Wideband precision current transformer for the magnet current of the beam extraction kicker magnet of the large hadron collider

The LHC beam extraction system is composed of 15 fast kicker magnets per beam to extract the particles in one turn of the collider and to safely dispose them on external absorbers. Each magnet is powered by a separate pulse generator. The generator produces a magnet current pulse with 3 us rise time, 20 kA amplitude and 1.8 ms fall time, of which 90 us are needed to dump the beam E. B. Vossenberg et al., (2002). The beam extraction system requires a high level of reliability. To detect any change in the magnet current characteristics, which might indicate a slow degradation of the pulse generator, a high precision wideband current transformer will be installed. For redundancy reasons, the results obtained with this device will be cross-checked with a Rogowski coil, installed adjacent to the transformer. A prototype transformer has been successfully tested at nominal current levels and showed satisfactory results compared with the output of a high frequency resistive coaxial shunt. The annular core of the ring type transformer is composed of a relatively low cost commercially available nanocrystalline strip material on an iron base. The characteristic feature of this material is a structure in which a fine-crystalline grain with an average diameter of 20 nm is embedded in an amorphous residual phase. This structure gives the material a high permeability. In addition, the small strip thickness (approx. 20 /spl mu/m) and the relatively high electrical resistivity, result in extremely low eddy current losses and excellent frequency behaviour. With saturation flux density of 1.2 T this material becomes even superior to Permalloys, ferrites or amorphous based alloys. In this particular application the transformer core is exposed to a unipolar induction. With normal magnetic materials this type of flux causes a relative high remanent induction. However this material allows controlling the magnetic properties, so called B-H curve shaping. It is obtained during annealing of the material by an external applied cross field and as a result the remanence ratio is less than 10%, which is excellent for this application. This paper presents the magnetic material, its incorporation in the design of the current transformer, comparative measurements of a prototype with a coaxial shunt precision resistor and explains why this device is an essential part of the LHC beam extraction system.

A High Power Pulse System for the Beam Extraction from CERN's Large Hadron Collider

CERN, the European Organization for Nuclear Research, is close to starting operation of the large hadron collider (LHC). A beam dumping system must protect the LHC machine from damage, by reliably and safely extracting and absorbing the circulating beams when requested. For this purpose a beam extraction system has been designed, built, installed and tested. It is composed of 15 fast kicker magnets per beam line to extract the particles in one turn of the collider. Each magnet is powered by a dedicated pulse generator through special low impedance coaxial cables. The generator charging voltage is proportional to the beam momentum, which is 450 GeV/c at injection and will be 7 TeV/c at top energy. The current pulse has a maximum amplitude of 19 kA with a rise time of 2.8degs and a fall time of 2 ms; the first 89degs of the fall time are used to dump the beam. Each kicker magnet consists of a window frame of Si-Fe tape wound cores and high voltage insulated single turn conductors. They are built around a ceramic vacuum chamber which is metallized on the inside. The measures taken to ensure a high reliability of the system, which was one of the main considerations during the design, construction and testing of the system, are discussed. Results of measurements on the series systems are presented.