Download conditioning of rf cavities for Indus-2

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
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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