A capacitor discharge, quasi-trapezoidal pulse generator for particle extraction

In the CERN SPS Accelerator two methods for particle extraction are used. One of these methods, called Slow Extraction, delivers extracted beams with a duration of up to several seconds to the majority of experiments. The other one, the Fast Resonant Extraction, providing particle bursts with a duration of a few milliseconds, is used for neutrino experiments. For the latter kind of extraction a quadrupole magnet is installed, which is connected to a high voltage pulse generator delivering quasi-trapezoïdal current pulses. The pulse generator is a capacitor discharge system generating current pulses, with a rising slope having 2 different gradients, of which the second one is approximately zero. The falling slope is obtained through natural decay in a freewheel circuit. The use of modern GTO (Gate Turn Off) power switches resulted in a much simpler circuit than the use of standard thyristors would have permitted

Darcian permeability constant as indicator for shear stresses in regular scaffold systems for tissue engineering

The shear stresses in printed scaffold systems for tissue engineering depend on the flow properties and void volume in the scaffold. In this work, computational fluid dynamics (CFD) is used to simulate flow fields within porous scaffolds used for cell growth. From these models the shear stresses acting on the scaffold fibres are calculated. The results led to the conclusion that the Darcian (k 1) permeability constant is a good predictor for the shear stresses in scaffold systems for tissue engineering. This permeability constant is easy to calculate from the distance between and thickness of the fibres used in a 3D printed scaffold. As a consequence computational effort and specialists for CFD can be circumvented by using this permeability constant to predict the shear stresses. If the permeability constant is below a critical value, cell growth within the specific scaffold design may cause a significant increase in shear stress. Such a design should therefore be avoided when the shear stress experienced by the cells should remain in the same order of magnitud

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

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 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

Control Loop for a Pulse Generator of a Fast Septum Magnet using DSP and Fuzzy Logic

A prototype of a fast pulsed eddy current septum magnet for one of thebeam extraction's from the SPS towards LHC is under development. The precision of the magnetic field must be better than ±1.0 10-4 during a flat top of 30 µs. The current pulse is generated by discharging the capacitors of a LC circuit that resonates on the 1st and on the 3rd harmonic of a sine wave with a repetition rate of 15 s. The parameters of the circuit and the voltage on the capacitors must be carefully adjusted to meet the specifications. Drifts during operation must be corrected between two pulses by mechanically adjusting the inductance of the coil in the generator as well as the primary capacitor voltage. This adjustment process is automated by acquiring the current pulse waveform with sufficient time and amplitude resolution, calculating the corrections needed and applying these corrections to the hardware for the next pulse. A very cost-effective and practical solution for this adjustment process is the integration of off-the-shelf commercially available boards into an active digital control loop. A 16-bit fixed point, 33 MIPS, DSP together with a 12-bit, 500 kSPS, ADC (total cost of under 250 $) has been used for this control process. The correction algorithm developed for the DSP uses Fuzzy Logic reasoning

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