G.H. Schroder

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.

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.

Coaxial 30 kV connectors for the RG220/U cable: 20 years of operational experience

The RG220/U cable connector has been used very successfully for 20 years under difficult conditions. Its design principle can be applied to a wide variety of other cable types and voltages. The principle of the tool for machining the cable can be adapted to many different cable types. The main manufacturing difficulty is the execution of the finger contacts to tight tolerances. These contacts must be handled with care and checked individually before assembly. The radiation resistance of Scotchcast/sup R/ 815 is better compared to polyethylene and therefore does not limit the use of this connector design. Finally, the design was successfully adapted to mass production and the price can be compared favorably with a commercial 30 kV connector. For the Large Hadron Collider, two large injection systems and two extraction systems are under construction requiring more than 800 coaxial high-voltage connectors. In view of our very positive experience and the excellent long-term behavior, we conclude that the design of these connectors is also perfectly adapted for the new project.

Operation modes of the fast 60 kV resonant charging power supply for the LHC inflectors

CERN is constructing a Large Hadron Collider (LHC) to be installed in the existing LEP tunnel of 27 km circumference. The LHC will accelerate two proton beams, injected at 450 GeV, in opposite directions and will collide them at a centre of mass energy of 14 TeV. The injection kicker systems will consist of four travelling wave type magnets and four pulse forming networks (PFNs) for each beam, discharged by thyratron switches. Resonant charging systems (RCS), located with the switches and PFNs in a gallery parallel to the LHC tunnel, are employed to charge the PFNs within 1 ms to 60 kV. The aim of this fast charging is to minimise the number of spontaneous firings of the thyratron. The stability and pulse to pulse reproducibility of the charging voltage must be maintained to a precision of /spl les/+0.1%. Each resonant charging system consists of a 2.4 mF primary capacitor bank, charged to 2.5 kV, and connected via a gate turn-off thyristor (GTO) and a 1:23 step-up transformer to two PFNs of 5 /spl Omega/ characteristic impedance, each with a total capacitance of 0.96 /spl mu/F. The PFNs are discharged 400 /spl mu/s after the end of the charging period into the kicker magnets. The GTO switch is used in gate assisted turn-off (GAT) mode and the pulse transformer has a particularly low leakage inductance. In this paper special attention is paid to analogue circuit simulations of the RCS showing both normal and abnormal operating modes. Furthermore the choice of electrical components is presented and discussed. These RCSs are designed, constructed and tested at TRIUMF in collaboration with CERN as part of the Canadian contribution to the LHC project.

Design and testing of a high voltage coil for the kicker magnets of CERN's Large Hadron Collider

A prototype LHC magnet has been built and tested at voltages of up to 35 kV. This paper describes the design of the high voltage excitation coil of the magnet, the different options for the coil insulation material and the adopted manufacturing process. It discusses the loss-factor measurements that have been carried out as part of the acceptance tests. Finally, it reports on endurance tests of the coil when it is mounted in the yoke and operated in a pulsed mode. The two dumping systems are furthermore used to remove the counter-rotating beams safely during the setting up of the accelerator and in case of emergencies. Reliable and accurate dumping is of prime importance. Uncontrolled beam losses will cause damage or destruction of adjacent superconducting magnets and will induce unacceptable heat losses in the cryogenic cooling system.