ACCELERATOR PROGRAMME

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The bunch symmetry is indicated by ratio of the twobunches. Fig. A.1 0.1 shows typical traces of WCM signal ondigital storage oscilloscope for the case of60% and 92% bunchsymmetry. Fig. A.I 0.2 shows a screen shot of GUI showingresults of online measurement of individual bunch currents

and the bunch current ratio. It also shows plot of bunchcurrents during decay of store beam in Indus-I. Beam life timeof individual bunches can be studied using the logged data.

Bunch Current YS Time

Reported by:S. Yadav (syadav@rrcat.gov.in) and

TA. Puntamhekar

This system is now routinely being used by Indusaccelerator operation crew for adjusting the injection timingsto achieve symmetric filling and recording bunch symmetryfor beam related studies.

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This system uses pickup signal of Wall CurrentMonitor (WCM) installed in Indus-l storage ring formeasurement of the bunch current. The pickup signal ofWCMis fed to a digital storage oscilloscope (DSO) kept in thecontrol room. The digitized signal from DSO is transferred onGPIB bus to a PC for calculation of individual bunch CUlTent.Software has been developed to interface DSO with Pc. Therequired settings of DSO are automatically done by thesoftware during init;alization. During measurement, softwareautomatically changes the volt/div setting of DSO accordingthe input signal level to improve resolution. The softwareprocesses the acquired data to calculate the bunch current fromthe height ofthe pulses. The bunch current, bunch current ratioand total beam current are displayed online and also stored in afile for later analysis. The system was calibrated with DCcurrent transformer (DCCT) of Indus-l to get calibrationfactor for measurement of absolute bunch current. DCCTgives the average current of the storage ring. To calculate thecalibration factor, a linear fitting was done between sum ofindividual pulse heights and average current measured byDCCT. The accuracy of this measurement system is better than2%.

This system operates under sub-atmosphericconditions. Below 2.17 K, liquid helium is in "Super Fluid"state. Properties of liquid helium, amongst others, its lowviscosity, puts stringent conditions for the inner vesselintegrity. Apart from efficient thermal design to minimize theheat in-leak through different mechanisms, there are severaltechnical issues, which are to be tackled in making a cryostatof this type. One ofthem is the problem of cold leaks. These areleaks, which are not traceable using conventional leak

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(b)Fig. A.1 0.1: Typical WCM signal traces captured on DSOfora) 60% and h) 92% bunch current ratio

A.ll: Development of2 K Cryostat

A Pool type Liquid Helium Cryostat for operatingdown to 2 K was designed at Cryo-Engineering and Cryomodule Development Section. In this cryostat, the temperatureof liquid helium, which has a normal boiling point of 4.2K isbrought down further by pumping over the liquid. (Cryostat isa specially designed container to keep liquids of very lowtemperature, typically below -150 Deg C)

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ACCELERATOR PROGRAMME

detection methods at room temperature. However, as soon asthe temperatures go below say 20 K, the insulating vacuumstarts degrading. Due to this reason, the fabrication procedurewith intermediate inspection/ testing steps needs to be workedout carefully.

Basically, there are two methods by which one canreach at 2 K or low using Liquid Helium. The first one is bypumping over the liquid bath. This method is also called"saturated temperature" type. In this method, temperature ofthe liquid remains constant due to the latent heat ofevaporation. Heat loads only results in higher boil-off. In thismethod, the stability of temperature is directly linked with thestability in pressure over the bath. This method is normallyused to meet the temperature requirements in case ofSuperconducting RF Cavities.

The 2 K Temperature was reached in about 73 min afterstarting the evacuation pump at 4.2 K. We started with about 08liters ofliquid in the cryostat, and when we reached 2 K it wasstill left with about 4.5 liters. After reaching 2 K the heliumboil-off was quite low for maintaining 2 K to 1.9 K, as the selfheat load ofthe cryostat to the 2 K liquid chamber is about 70 mW.In next two hours, less than 0.5 litres got boiled. This cryostatwill be used among others, to calibrate cryogenics temperaturesensors in 4.2 to 1.8 K range.

A prototype solid state bouncer long pulse modulatorfor LEP IMW klystrons was successfully designed,developed, tested and shipped to CERN by RRCAT under theDAE CERN Novel Accelerator Technology Project. The Firstacceptance tests were done at RRCAT by CERN engineers.Additional features and modifications requested by CERNteam were then incorporated and tested. The modulator wasthen dismantled and shipped by RRCAT. RRCAT engineerscompleted the integration and commissioning of the

A.12: Prototype solid state bouncer modulatordesigned and developed by RRCAT has beencommissioned at CERN

Reported by:R S Doohan and P K Kush (kush@rrcat.gov.in)

Fig. A.II.2: Experimental setup with 2 K Clyostat

Cryostat designed by us is based on the first methodviz. saturated liquid type. It was fabricated by a localfabricator. It was first tested at normal liquid heliumtemperature (4.2 K). Heat in-leak calculated from the boil-offwas less than 70 mW. These values of heat in-leak match

closely with the designed values.

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In the second method, a portion ofliquid helium at 4.2 Kis separately expanded through a J-T valve, and using heatexchangers, the remaining liquid is cooled. In this method theliquid helium is in "Super Cooled state". Special coolingproperties of Liquid helium at 2 K viz. Super Fluid properties­property to flow through fine capillaries and large thermalconductivity are utilised, at the same time the bulk ofthe liquidin the cryostat remains at pressure above atmosphericpressure. Normally, this method is used to coolSuperconducting magnets operating in 2 K region. Thismethod relaxes conditions for system integrity of theperipheral systems, i.e., in case of very small leaks, helium gascan leak to atmosphere. Limitations of this method are: in caseof excessive heat load, the temperature of the liquid will startrising that gives poor temperature stability. The first case hasvery good temperature stability due to involvement of latentheat, but small leaks in the system will result in air andmoisture entering the system, thereby limiting the reliability oroperation time ofthe system.

12