Rochepeau, G

Thorndahl, L

Vaughan, A

Zettler, C

CERN Document Server

DAMPING OF TRANSVERSE OSCILLATIONS IN THE ISRG. Rochepeau, L. Thorndahl, A. Vaughan, C. Zettler.CERN, Geneva, Switzerland.(presented in summary by M. J. de Jonge)1. AbstractA feedback system to remove horizontal injectionerrors in the ISR as proposed in 1) is described.The system treats the 20 bunches injected from theCPS individually.As the damping process extends over many rev-olutions (typically 100) the power needed to drivethe kicker magnet is modest (30 kW).A different feedback system against resistivewall type instabilities of the stacked beam ismentioned.2. General principle(see Fig. 1)The radial pick-up signals from the 20 success-ive bunches are switched rapidly between 20 filters;each filter forming a memory of the instantaneousamplitude and phase of the 9th azimuthal mode ofcoherent oscillation of a single bunch.A pulsed transmission line deflection structurekicks the kth bunch when the error signals containedin the kth memory have a phase interval such thatthe kick will reduce the error.The time taken by a bunch to pass the kicker(Damping Magnet) is typically 15 nsecs and theduration of a deflecting pulse is typically 30 nsecs.The cross-section of the kicker is such that inthe main the newly injected beam from the CPS isacted upon by the deflecting electric and magneticfields, whereas the stack is kept shielded, seeFig. 2. There is, however, a slight penetrationof the deflecting fields into the shield. Computercalculations show that in the case that all PSpulses are injected with the same error, the dampingprocess will blow-up the stack by an amount corres-ponding to a few percent of this error.3. HardwareThe electronics for driving the damping magnetis shown in block form in Fig. 1, and describedbriefly in the following.The input is taken from a pick-up electrodeand amplified by the normal head amplifier. Asthe information we wish to extract is representedby varying pulse height, we improve the signalto noise ratio by removing the lower portion ofthe pulse in a diode clamp. To reduce the transientson the filters at injection we use a differentialamplifier combining the signals from both platesof the P.ll. electrode. Compensation for unbalancedoutputs caused by changing beam orbits is effectedby varying the gain of one differential channel overa narrow range. The signals are then passed to aparallel group of linear diode gates. These gatesare opened in turn to allow the signal associatedwith a particular bunch to pass into the correctfilter. Switching of the gates is done by a cir-cular shift register, started by a pulse from themachine timing system, and stepped round by a clocksynchronised to the bunches.Each filter unit in turn thus receives theheight varying pulse derived from one particularbunch as it passes the P.ll. electrode. In thefilter unit it is amplified, phase shifted to ensurethat the resulting feedback is negative, and shapedand stored in an L.C. filter. This filter extractsthe 9th mode betatron oscillation (an approximatesine wave) which is converted into a train of TTLpulses by passing through a limiter, a zero crossingdetector and pulse shaperoThe outputs from all twenty filters are thencombined into a single pulse chain by a multiplexingunit. This unit builds up the chain by switchinground in synchronism with the input diode gates.Both the gates and the multiplexer can be thoughtof as two single-throw twenty position switches,stepping round together at the bunch frequency. Themultiplexer output is shaped and amplified by trans-istors up to the 40 volt peak level, then it goesto the driver stage, a six tube distributed amplifielA ferrite core transformer couples this amplifier tothe final stage, a ten tube distributed amplifier.To simplify the output coupling the final amplifieris operated with HT+ on ground, and as a safetymeasure it is enclosed in a special double box.Both amplifiers use capacitor storage voltagemultiplying power supplies as the continuous currentdrain is small.The rise time of the amplifier chain is about5 nsec and the output about 2.5 kV peak on 75 Q.The driver amplifier uses six 4 CX 350 tubes andcomplete with power supply it occupies 13 cm heightin the rack. The power amplifier has ten 4 CX 600tubes and with power supply needs 62 cm rack height.4. Preliminary trialsThe following two pictures show the filteredradial pick up signal after injection of fourbunches into the ISR without and with damping.- 509 -...........jl--....... memories.,....~~_Binaf)multi-pleJer sv.aite.hesbetween '20PoIse"EI"ORV 6ON(H 3synchronoustzO Memories(filter unit's)MEMORY 8UNCH 1Wide bandpre ampliFierlinear rnulhplexerGeneral p-rincie'e----'Wide bandpo\Aler amplifier~I~iRadialpick-upNN~ injectedCps p~lseInner condudorDame!!!.9 magn~~ cross section (?t'.2 trQh~~"uion t,'ne)ShieldStackOu~er conductor• vQCuum chamberThe injection error was of the order of 5 rompeak to peak. The damping magnet was operated forabout 500 ~s and the peak power through the kickerwas about 50 kW.- 510 -5. Feedback damping of coherent instabilitiesof the ISR stackResistive wall instabilities at high currentshave been partly suppressed by the use of feedback.Vertical and horizontal pick-up signals areamplified and applied approximately 1 revolutionlater to a set of clearing electrodes, in the vert-ical case, and to the Q-measurement kicker, in thehorizontal case, to damp the instabilities.So far 15 - 145 kHz filters have remainedinserted to limit the process to the 9th mode ofbetatron oscillations.For machine conditions with vertical instab-ilities, an improvement of the stacking ra£e aswell as of the maximum current has been observed.The long term blow up of the stack due to thenoise in the system remains to be investigated, aswell as the usefulness of damping higher order modesof betatron oscillation.6. AcknowledgementsThe zero crossing detector was developed byP. Brown and S. Hansen. The diode clamp is thework of J. Pett. J. Borer suggested the diodecircuit for the linear gates.7. References1. E. Keil, W. Schnell and P. Strolin: "Feedbackdamping of horizontal beam transfer errors",CERN 69-27, 1969.

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