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Transcript
CONDITIONING OF RF CAVITIES FOR INDUS-2 M L Pandiyar, M Lad, P R Hannurkar Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India-452013 Abstract Indus-2 RF system is composed of four RF cavities. The system is of modular construction and each plant is driving a single cell 505.8 MHz elliptical ELETTRA make RF cavity. The cavity gap voltage is set to 150 kV for injection and 375 kV for operation at 2.5 GeV, for which the wasted power on the cavity surfaces is 900 watts, 22 kW respectively. During initial phase of commissioning and whenever the vacuum in RF cavity is broken, the operation of the RF plants is strongly influenced by the RF cavity surface conditions. Multipacting and arcing are two crucial phenomena in RF cavity during the high power RF testing. To achieve ultra high vacuum, firstly baking of the RF cavity is done, then RF conditioning is performed. To bake-out the RF cavities some parts of the cavity are heated to high temperatures, up to 150 ºC, with hot water under pressure flowing in the cavity tubes and stainless steel flanges of the cavity are heated up to temperatures in the range between 150 ºC and 180 ºC by means of electrical heaters. RF conditioning is then performed initially in pulsed mode then slowly the duty cycle is increased keeping the limit on poor vacuum. Once the RF cavity is conditioned at peak RF voltage in pulsed mode with no discharge then CW RF power was increased slowly till final RF power. During this complete RF conditioning process the vacuum was maintained better than 5x10-7 mbar, keeping the reflected power from RF cavity within the limit. This paper describes the baking aspects, RF conditioning and high power RF tests carried out on all four cavities. procedures for commissioning like cleaning, baking followed by RF conditioning Indus-2 SRS consists of four RF cavities resonating at 505.8MHz. These RF cavities provide RF power to the electron beam for acceleration and compensation of synchrotron radiation loss. Fig. 1 shows the view of four RF cavities installed in Indus-2 tunnel. INTRODUCTION Indus-2 is a 2.5 GeV, 300 mA synchrotron radiation source (SRS) with approx 210 kW of beam power. Total photon flux in 360 deg. is 6x1020 photon /sec. This photon flux spectrum ranges from X-rays to infrared range wave length. High beam life time and high photon flux is desirable for beam line users. For reliable machine operation and high beam life time ultra high vacuum is required in the machine. This can be achieved by minimising photon induced desorption yield. The storage ring vacuum envelope is conditioned by beam and photon dose. But in the ring higher volume surface of RF cavity is cleaned by first baking then RF conditioning with RF power and finally by beam. The RF power produces time varying electric field in which electron and ions get trapped and get multiplied. The growth of electron and ion is very fast and the number reaches 104 times in just 100ns. These electrons and ions bombard cavity surface and result in electron and ion induced desorption and severe out-gassing. These problems can be over-come by proper designing RF cavity and following different ____________________________________________ *[email protected] Fig.1 RF cavities installed in Indus-2 tunnel INDUS-2 RF CAVITY BAKING After fabrication and chemical cleaning of the RF cavity, assembly and leak testing of RF cavity is done. Indus-2 ring consists of four Elettra make bell shaped RF cavities. The cavities were delivered to RRCAT after FAT (Factory Acceptance Test). The assembly of RF cavities was performed at RRCAT and after assembly cavities were leak tested using Helium Leak Detector. The cavities were installed in the Indus-2 tunnel. Each cavity has one 270 lit/sec Sputter Ion Pump (SIP) and a 1000 lit/sec Titanium Sublimation Pump (TSP). The cavities have a common Turbo Molecular Pump (TMP) along with a right angled UHV valve. Using TMP, pressure around 1x10-7 mbar was achieved in the cavities. To improve the vacuum level, cavities were baked. During baking, pressurised 1500C hot water was circulated in each cavity cooling tubes and flanges were baked by bend heaters. Fig. 2 shows the view of bake out unit used during baking. Fig.2 View of the Bake out unit The cavities were baked as per recommended Bake out temperature profile. The temperature was increased from 500C to 1500C at a slow rate of about 150C per hour. The 1500C temperature was maintained for 24 hours and then temperature was reduced to 500C by same rate. At the temperature 1200C the bend heaters were put on. The SIP and TSP were operated when cavities temperature were above 130 deg. centigrade. Fig.3 shows the bake out temperature profile along with cavity pressure profile. Fig.3 Bake out temperature and Cavity pressure profile As the temperature was increased to 1500C, the cavity pressure reached 5x10-5 mbar. With 24hours baking pressure reduced to 1x10-7 mbar. After completion of baking cycle, the pressure in the cavity reached to about 2x10-9 mbar. To minimise RF driven multipacting to run the accelerator in reliable mode and to further improve the vacuum in RF cavities, RF conditioning was done. INDUS-2 CAVITY CONDITIONING The RF conditioning is required for obtaining good treatment of the cavity inner surface such that RF power can be delivered to the cavity for long time with no sparks and with a vacuum level below a certain threshold, typically 10-9 mbar. These are the optimized operating conditions before the beam conditioning in the storage ring for sustaining RF power which are required for the operation of accelerator and cavity vacuum integrity. RF conditioning should be performed after cleaning, assembling and baking the RF cavity for 24 hours at 150 deg. centigrade. The RF conditioning procedure consists is a slow increase of the RF power level in the cavity until the final RF input power is reached. A good conditioning procedure minimizes the number of discharges in the cavity, even if the cavity design is multipacting free: this kind of phenomena may occur in the main coupler, where different levels of multipacting can be observed. There is RF burn of the particulates at surface area of the cavity and there is induced controlled gas layers desorption. The objective of this cleaning to clean the surfaces from contaminates (molecular or particulate). First, for short pulses, the RF voltage in the cavity is increased by increasing the forward power; then for a given, fixed RF voltage in the cavity the pulse duration is increased until continuous wave operation is reached. The cavity is kept tuned to the source frequency all the time. Amplitude modulation is done to feed RF power as follows: a) The RF signal is modulated with a pulsed signal, in the beginning with the lesser RF cavity gap voltage in short pulses. b) At the lesser RF voltage amplitude increase the pulse width. c) Increase the of RF voltage amplitude in short pulses to achieve required voltage. d) Increase the pulse width to reach to CW mode. During RF conditioning, the vacuum level of the cavity degrades due to adsorbed gases coming out of the cavity surface; the vacuum interlock is implemented at vacuum level of 5x10-8 mbar. In Indus-2, RF conditioning was started in pulse mode, with small pulse of 0.1 ms at 10 Hz at low amplitude. This was followed by increasing the pulse width up to 20 ms. and increase in amplitude up to max voltage. Then duty factor was increased till CW is reached. After this CW processing at maximum working RF field level was done for at least 8 hours. The RF processing was performed in a controlled manner by fast vacuum interlock to avoid arcing in the cavity. During RF conditioning pressure rise was monitored and RF power was increased carefully. After RF conditioning the pressure level of about 5x10 -10 mbar was achieved in the RF cavity. This process was followed for all four RF cavities of Indus-2 CONCLUSION Conditioning of RF cavities for Indus-2 was done during initial commissioning and future when coupling factor (beta) was adjusted for higher current operation. After baking the pressure level in the cavities reached to about 2x10-9 mbar which was further improved to 5x10-10 mbar after RF conditioning. RF conditioning also helped in resolving the multipacting and arcing phenomena and hence enabling the smooth and reliable operation of RF cavities to the required RF power level. The RF cavities are operating smoothly for past seven years. Indus-2 is being regularly operated in user mode with 100mA and also a milestone of 157mA, 2.5 GeV has been achieved with all four RF cavities operating at total gap voltage of 1220 kV with total RF powers including beam power to 150 kW. At 157mA, 2.5 GeV beam the vacuum level in cavities is of about 2x10-9 mbar. The snapshot of RF panel at this beam storage condition is shown in Figure 4. ACKNOWLEDGEMENT Authors are thankful to entire Indus-2 team, especially RF team for their support. REFERENCES [1] Mircea Stirbet et al, “RF conditioning: Systems and Procedures” HPC Workshop, 2002. [2] Indus-2 RF cavities Manual Figure 4: Snapshot Indus-2 RF parameter panel