Photon Absorbers (Crotches)

Introduction: In INDUS-2 (2.5 GeV, 300 mA), approximately 85% (158 kW) of synchrotron radiation power from dipole chambers is absorbed by total 64 water-cooled photon absorbers (crotches) scattered around the ring on 16 dipole chambers. There are 4 numbers of crotches mounted on each dipole chamber. The normal incidence power density to be absorbed, ranges from 10 to 12 kW/cm2. These crotches were designed and developed indigenously.

Design: Salient design features of the crotches are : Heterogeneous structure of OFHC copper bottom & stainless steel flange top, no water to vacuum joints, UHV compatibility, vacuum brazed joints, rectangular finned type water channels for enhanced heat transfer, wedge shaped profile for grazing incidence of photons facilitating substantial decrease in heat flux density & for directing photon induced desorbed gas molecules towards sputter ion pumps installed below the crotches. Designing grazing incidence profile, power density drastically decreases to the range of 800 to 1000 watt/cm2.     3-D sectional view of a typical crotch is shown.

3D sectional view of a crotch

Finite element analysis was done to get the temperature profile of the crotch body absorbing the heat of incident photons in operating condition. Maximum localized temperature at hot spot, where photons are directly incident is 130° C.  Maximum temperature on the cooling side wall is 80° C.

Fabrication of crotches involves various stages like, CNC profile machining, vacuum furnace brazing, quality assurance etc.

5-axis CNC machining center was used to machine the profiles of OFHC copper components, especially for the wedge shaped outer profile of heat absorbing body.

In order to get bright, oxide free, degassed and UHV compatible leak tight assembly, vacuum furnace brazing was used to braze the entire assembly of the photon absorber. Entire assembly was brazed in two stages. Two-stage brazing facilitates advantage of leak detection as well as rectification of leaks. Quality assurance plays an important role in the fabrication of crotch assembly, meeting the required technical specifications. Before machining, ultrasonic test of the raw material was done to detect any internal casting defects like: blow holes and porosity. Only defect-free materials are used for machining. Machined components are inspected by Co-ordinate Measuring Machine (CMM) for checking the profile tolerances and flatness. This is very much needed to ensure the required capillary gap all around in the brazed area. Leak detection was done for all the crotches before the ultimate vacuum testing. Leak detection was done using ALCATEL make leak detector machine Model ASM 180 /TD+, having sensitivity <1x10-10 std cc/sec for Helium. The leak detection was carried out in two stages. In the first stage, hydraulic circuit brazed joint is checked for detecting leak tightness. In the second stage, vacuum to air brazed joint is checked for its leak tightness. In both the cases leak tightness achieved is <2x10-10 std cc/sec for Helium.

These crotches are supposed to work in UHV environment with ultimate vacuum ~2x10-10 mbar without Synchrotron  Radiation (SR)  beam  and   ~ 1x10-9 mbar with SR beam.. All the crotches are tested for ultimate vacuum in cold condition (without SR beam), followed by residual gas analysis.

Power Test: A strip type electron beam gun was developed for power testing of these crotches. Installed capacity of electron beam energy and current are 60 KeV and 100 mA respectively. Corresponding beam power is 6 kW. Operating pressure in gun chamber is <1x10-6 mbar. The e-beam irradiates the strip on the central surface of the crotch in an area of 5 mm x 151 mm. Testing was done in increasing order of beam power starting with 500 watts upto 2.8 kW. The crotch was cooled by water with flow rate of 4.8 l/min and inlet water temperature of 20°C.
The temperature measurement was performed for the rear side of crotch for different electron beam power, and it was observed that the temperature rise was within tolerable limits.

Ultimate Vacuum with electron-beam: Power testing of crotch was done in unbaked condition. Initially vacuum was ~1X10-8 mbar without electron beam. With electron bombardment vacuum level comes down due to the energy transfer to the gas molecules residing on the surface. The vacuum level improves again with respect to time with accumulative beam dose and resultant cleaning. Ultimate vacuum achieved with beam is ~3X10-9 mbar with a 270 l/s SIP.

Surface Inspection: With the aim to see the irradiation effect on the surface, crotch absorber was taken out from the chamber and surface was visually inspected for melting/surface erosion. It was observed that there is no any melting effect and surface was intact.

 e-beam irradiated crotch