Improved Turn-on Characteristics of Fast High Current Thyristors

The beam dumping system of CERN's 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 ms rise time, a maximum di/dt of 12 kA/ms and 2 ms 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 test an asymmetric not fully optimised GTO thyristor of WESTCODE and an optimised device of GEC PLESSEY Semiconductor (GPS), GB. The GPS FHCT, which gave the best results, is a non irradiated 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 6 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/ms 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

Upgrading the Fast Extractions Kicker System in SPS LSS6

A fast extraction system, located in the LSS6 region of the CERN SPS accelerator, transfers 450 GeV/c protons, as well as ions, via the TI 2 transfer line towards the LHC. The system includes three travelling wave kicker magnets, all powered in series, energised by a single Pulse Forming Network (PFN) and terminated by a short circuit. The specification for the system requires a kick flattop of $7.8 \mu$s with a ripple of not more than ±0.5%. Recent measurements with beam show that the ±0.5% kick specification is achieved over $7.1 \mu$s of the kick flattop; however the ripple over $7.8 \mu$s is ±0.75%. Initial electrical measurements have been carried out on each of the three magnets; more detailed comparisons of the beam measurements and the contribution of each magnet to the detailed shape of the flattop kick will be carried out. This paper reports the results of initial measurements and plans for future measurements to permit modifications to the PFN for reducing flattop ripple

Wideband current transformers for the surveillance of the beam extraction kicker system of the Large Hadron Collider

The LHC beam dumping system must protect the LHC machine from damage by reliably and safely extracting and absorbing the circulating beams when requested. Two sets of 15 extraction kicker magnets form the main active part of this system. A separate high voltage pulse generator powers each magnet. Because of the high beam energy and the consequences which could result from significant beam loss due to a malfunctioning of the dump system the magnets and generators are continuously surveyed in order to generate a beam abort as soon as an internal fault is detected. Amongst these surveillance systems, wideband current transformers have been designed to detect any erratic start in one of the generators. Output power should be enough to directly re-trigger all the power trigger units of the remaining 14 generators. The current transformers were developed in collaboration with industry. To minimize losses, high-resistivity cobalt alloy was chosen for the cores. The annealing techniques originally developed for LEP beam current measurement in collaboration between CERN and industry allowed to extend the frequency response beyond that of traditional core materials. The paper shows the results obtained, exposes the problems encountered with shielding, conductor position sensitivity, load resistor technology and their solutions. The know-how acquired during the collaboration was further applied by the industrial partner to cover a wider range of sensitivity, size and frequency

High power semiconductor switches in the 12 kV, 50 kA pulse generator of the SPS beam dump kicker system

Horizontal deflection of the beam in the dump kicker system of the CERN SPS accelerator is obtained with a series of fast pulsed magnets. The high current pulses of 50 kA per magnet are generated with capacitor discharge type generators which, combined with a resistive free-wheel diode circuit, deliver a critically damped half-sine current with a rise-time of 25 ms. Each generator consists of two 25 kA units, connected in parallel to a magnet via a low inductance transmission line

The Beam Screen for the LHC Injection Kicker Magnets

The two LHC injection kicker magnet systems must each produce a kick of 1.2 T.m with a flattop duration variable up to 7.86 ìs, and rise and fall times of less than 0.9 ìs and 3 ìs, respectively. Each system is composed of four 5 Ù transmission line kicker magnets with matched terminating resistors and pulse forming networks (PFN). The LHC beam has a high intensity, hence a beam screen is required in the aperture of the magnets This screen consists of a ceramic tube with conducting ?stripes? on the inner wall. The stripes provide a path for the image current of the beam and screen the magnet ferrites against Wake fields. The stripes initially used gave adequately low beam impedance however stripe discharges occured during pulsing of the magnet: hence further development of the beam screen was undertaken. This paper presents options considered to meet the often conflicting needs for low beam impedance, shielding of the ferrite, fast field rise time and good electrical and vacuum behaviour

Emittance Growth at LHC Injection from SPS and LHC Kicker Ripple

Fast pulsed kicker magnets are used to extract beams from the SPS and inject them into the LHC. The kickers exhibit time-varying structure in the pulse shape which translates into small offsets with respect to the closed orbit at LHC injection. The LHC damper systems will be used to damp out the resulting betatron oscillations, to keep the growth in the transverse emittance within specification. This paper describes the results of the measurements of the kicker ripple for the two systems, both in the laboratory and with beam, and presents the simulated performance of the transverse damper in terms of beam emittance growth. The implications for LHC operation are discussed

Upgrading of the SPS injection kicker system for LHC requirements

The present SPS injection kicker system is composed of 12 travelling wave magnets connected in pairs to six pulse generators. The eight most upstream magnets ('S'-type) have a kick rise time (2-98%) o f 145 ns and the remaining four ('L'-type) of 215 ns. The flat top ripple of the kick is ±1%. In the future, this system will also inject protons and ions for the LHC, with a bunch spacing of respect ively 220 ns and 125 ns, and a flat top ripple requirement of at most ±0.5%. Important modifications, concerning both magnets and generators, are then required to meet these goals. For ion injection only 'S'-type magnets will be used. The reduction of the kick rise time will be achieved by shortening the magnet length and increasing the characteristic impedance. To compensate for the loss in tota l kick strength, four new magnets and two new pulse generators will be added. At the moment it is not intended to modify the 'L'-type magnets. Most of the pulse forming networks (PFN's) must be adapt ed to the higher characteristic impedance of 16.67 W. The internal structure of all PFN's will be upgraded to reduce the flat top ripple and improve the turn-on characteristics

The future of the SPS injection channel

The SPS accelerator will be used as injector for the LHC and has to be adapted to the LHC requirements. The tight specification on beam blow-up in the SPS requires a reduction of the magnetic field ripple of the SPS injection kicker magnets to less than ±0.5 %. The bunch spacing of the LHC ion beam requires a reduction of the kicker magnets' rise time from 145 ns to less than 115 ns. To obtain the shorter rise time the existing kicker magnets have to be reduced in length and the characteristic impedance has to be increased. The resulting loss in magnetic field has to be compensated by the installation of additional magnets. Results of studies on the required kicker strengths and physical apertures for the different types of beam and corresponding operational modes are shown. Changes to the Pulse Forming Network (PFN) and the option of using Pulse Forming Lines (PFL) are presented. Results of first magnet measurements are shown