Surface Treatment & Testing of 4R Cavity
March 30, 2012 (Updated, Oct 31, 2012)
1 MOTIVATION
This document summarizes the surface treatment and cold testing of the 4-rod UK cavity
potentially take place at CERN. The cavity will operate at 400 MHz and be tested at 4K and
2K for validation of deflecting fields in a Niobium cavity.
2 TEST LOCATION AND TIMELINE
The cavity will undergo heavy BCP at Niowave in the U.S. under the contract from Lancaster
University and shipped to CERN. Additional surface treatment will be needed at CERN.
Following chemistry, high temperature bake and HPR, vertical testing could take place at
CERN. The SM18 tests will have to take place latest by August to finish testing before the
refurbishment of the cryoline estimated for September 2012. Else, testing will have to take
place early 2012. After refurbishment, a dedicated 2K cryostat will be available for testing.
Checklist:
1. Leak tightness for the cavity (done by Niowave). Details to be provided by Niowave
to CERN. In addition complete details of EDM and EB-welding is also required for
the purposes of tracking the cavity performance through the treatment and test cycles
at CERN. Optical inspection, endoscopic or x-ray will be done upon reception at
CERN.
2. Heavy BCP to be performed at Niowave (150 microns removal) using acid flow. The
cavity can be expected at CERN by mid or end of July. Recipe to be checked with
Jlab on cavities made of solid Nb.
3. Special flanges (see BCP section for link) with degassing capability to be made for
BCP to protect NbTi flanges and ports. Niowave will employ a vertical setup with
chillers to control the temperature.
4. Sketch of the CERN BCP setup, flanges, material specs, piping, water tank and other
equipment is shown in BCP section. All parts that can be adapted to the CERN
systems should be recuperated, awaiting Niowave quote.
5. Degreasing, inspection and High temp bake at 600-800 deg C for hydrogen degassing
will has been completed before a light BCP. HPR system available at CERN will be
used after the light BCP and dried by pumping.
6. Beam pipe flanges with external SS are compatible with NbTi cavity flanges using
copper gaskets. The thickness of the seal should be checked with CERN. It is strongly
recommended to add NbTi flanges to cavity ports with high fields.
7. Collar for attachment to CERN support structure (both for BCP & Testing
manipulation) should be made from SS316. CAD model of the structure is designed
by LU engineers. Fabrication should be performed LU/Niowave
8. The input and pickup antenna geometry and exact penetration were defined and the
antenna’s not available at CERN will be fabricated by LU/CERN. Pickup probes, two
available at CERN, but need a change to the antenna shape to an L-shape. Ceramic
material details for Q calculations were provided by CERN.
9. Two hybrid flanges, one for the beam pipe port to be used for pickup (CF100 to
CF16) and second for top input coupler port (CF63 to CF16). One hybrid flange for
the bottom beam pipe port (CF100 to CF35) for pumping port/gate valve
10. Test support structure used for SPL cavities (2.4m total length) can be used with a
modification to the pumping port. Mathieu confirms that it fits but with very little
clearance. Therefore, 2K helium level should be actively monitored to ensure proper
level. If tested at 4.2K, removal of bubbles from the void of bottom side of rods
should be conceived.
2.1 CAVITY ASSEMBLY
A test program of the 4R cavity in the SM18 facility at CERN will potentially imply a surface
treated and vacuum sealed cavity assembly. Clean room facilities are available in case of
reassembly but it is preferred not to open the cavity vacuum processing and baking in a clean
room. Therefore, input and all pickup probes are to be prepared in advance for assembly
immediately after the cavity processing at the same site. Probes from the LHC 400 MHZ
main RF cavities can be recuperated for these tests if there are not provided with the cavity
assembly. Cavity assembly after the HPR can be performed in class 100 with laminar flow.
Cavity Weight
35 kg
Total Length (Flange-to-Flange)
800 mm
Total Width (Flange-to-Flange)
322.5 mm
Beam pipe diameter
Beam Pipe Flange
84 mm
6 inches (100 CF, 152 mm)
Side pickup ports diameter
64 mm
Top pick up port diameter
40 mm
Pickup port flanges
4.5 inches (63 CF, 113.5 mm)
A schematic of the cavity profile (top and cross sectional view) are shown below with
associated dimension in mm and inches. The total flange to flange length is 800mm. The
cavity width with side ports is 322.5 mm. The beam pipe ports are 84mm in diameter with a
152 mm 100CF flange. The three ports on the cavity body have a port diameter of 64 mm on
the side and 40mm on the top with a 113.5mm 63CF flange. All cavity flanges are made of
NbTi and will be blanked off with stainless with copper gaskets. The exterior flanges will be
coated with Niobium or copper at CERN for testing.
2.2 SURFACE TREATMENT & BAKING
Due to the complexity of the structure and lack of experience on such devices, especially
with EDM on solid Nb, it is believed that a significant amount of material needs to be
etched for reaching high performance. The cavity is to undergo heavy BCP at Niowave.
An initial degreasing with ultrasonic de-ionized water will take place before a surface
treatment for degreasing and surface residue. The standard chemical mixture of
HF+H2NO3+Phosphoric acid (1:1:2) can be used with the phosphoric acid as the buffer
to stabilize the rate reaction. An initial etching of the surface of approximately 150
microns using some rotational mechanism could be used to ensure uniform removal of
material on the surface. It should be noted that approximately 50 microns were already
removed from the 4rod portions prior to welding according to Niowave. Frequency
tracking during the removal and weighting the cavity before and after should be
performed. If available, ultrasonic measurements from point to point should be carried out
according to JLab suggestion. The cavity will be vertically oriented with acid flow rate of
approximately 5 gallons/min and it might be reduced to 2gallons/min to further slow the
material removal. The cavity will be cooled with external chiller sprayed on to the surface
acting a heat exchanger with temperature kept between 5-9 degrees centigrade. The BCP
is performed in one complete go. All flanges will be covered with standard plastic (viton)
material and appropriate seals. After the chemistry, the cavity will undergo an ultra clean
water rinse and a leak tightness of about 10-10 torr.
The cavity surface fields (Left: Electric & Right: Magnetic) are shown below for input to
chemistry experts and also placement of thermal sensors for testing. Note that the peak
electric field is located between the rods facing each other along the beam direction and
the peak magnetic field is located at the base of each rod slightly offset from the beam
pipe region.
Electric Field
Magnetic Field
Upon reception of the cavity at CERN, a degreasing with ultra pure water and endoscopic
inspection will take place to note the surface features as received from Niowave after
heavy BCP. A high temp bake at 600 deg C for hydrogen degassing has been performed
in the UHV furnace of the bare cavity (see pictures below). This step should help alleviate
the Q-disease and a generally accepted step in the SRF community. The exact
temperature and time period is variable but past experience show 600 deg of approx 48
hours or more is sufficient.
The cavity will then undergo an additional light chemical etching of 20-50 microns and
rinsing. Following this light removal of the surface, a high pressure rinse with de-ionized
water should follow to remove all impurities help improve the Q-slope at high fields and
improve field emission properties. An additional UHV bake of at 120 deg for approx 48
hrs should take place either which is believed to help with high field slope. Safety aspects
should be studied in detail with careful planning to avoid overlap with other chemistry
activities. The chemistry team will require additional equipment to adapt the cavity to the
BCP system at CERN (see schematic below). Any equipment used at Niowave to treat the
cavity could be recuperated if it can be directly adapted to the CERN chemistry facility.
This includes all plastic flange material, piping, water tank, and the support system to
manipulate the cavity. Niowave will provide an estimate to procure this material. The
plastic material specification can be similar to that given in the link below:
www.es-technologies.com/pdfs/ENCM2080.pdf
Information on the EB welding is also vital to avoid accidents that may damage the
cavity during chemistry if the leak tightness of the weld is not adequate. A thorough
inspection at CERN via optical and x-ray will take place to ensure all welds are adequate.
A recipe as an initial place holder is described. This recipe can be replaced with a one
after consultation from JLab. Niowave has informed that the EB-welding is 100%
penetration.
2.3 CAVITY PICKUP PROBES
A picture of in input probe used for the tests of the LHC main RF cavities. Two such probes
exists which are mounted on a CF16 flange (see picture below). Either these pickup probes
can be mounted on the 4R cavity assembly or similar probes be made to adapt to the existing
flanges on the 4R cavity. The Qext for the measurements have to calculated and also
measured with a network analyzer for calibration immediately after the cavity assembly. The
appropriate Qext for a maximum amplifier power of 200 W and preferably well below this
value to reach the deflecting gradient of 3 MV must be specified.
The lengths of the antennas have been calculated for critical coupling at 2K both for the
pickup (left picture below) and input (right picture below). The pickup and input have a
calculated Qext of 6x1010 and 1.4x108 respectively. The antennas are approximately 7mm in
diameter and are made of stainless steel.
The orientation of pickup probe with a hook shape should be approximately -450 in the z-y
plane which is sketched below. The flange has 6 anchors giving a freedom of approx. 600 per
bolt, so the orientation of the pickup probe will only be approximate.
-45 deg
2.4 TRANSPORT, MANIPULATION & TEST STRUCTURE
Since the cavity body is prone to deformation, four stubs on the top and the bottom as shown
in figure below is needed to ensure rigid support which is attached to two racetrack collars. A
rotational degree of freedom is added for transport and manipulation of the cavity from the
different stations. Lever arms to the same specification of the HIE-ISOLDE support structure
will be designed to use the same transport carts between the different stations. The overall
dimensions are shown below. All parts of the support structure should be made of SS316
non-magnetic material.
350 mm
802.54 mm
409 mm
Since the contraction of SS and Nb are vastly different during cool down, a modification of
this structure maybe needed to relieve any stresses on the cavity from differential contraction.
A simple clamp structure on the beam pipe will be used for test purposes to avoid the issue of
the differential contraction. The support structure used for the recent SPL cavity tests could
be adapted for the UK 4-rod cavity for the insertion into the cryostat. A picture of the thermal
shielding and the support structure is shown below. The exact fixation to the support has to
conceived and fabricated for rigid support of the cavity. See for example the collar design
used for the anchoring the ISOLDE cavity with the same support structure. The pumping
ports should be located at the bottom ends of the beam pipe with a gate valve at the bottom
which will enable the connection to the pumping system of the cryostat insert. A minor
modification this support structure will be required to allow for pumping from the bottom of
the cavity. This will also allow for the cavity to be transported directly from the rinsing
facility, dried to the testing w/o opening the cavity vacuum. The test collar and support
structure with the cross section is shown below. The integration into the cryostat is verified
without any issues. Four transducers facing at an angle of approximately 45 deg at the top
and bottom of the faces of the cavity will be used to record any second sound effects from
cavity quench. Additional resistors maybe attached to the cavity surface to record the
temperature during the tests. If the shutdown schedule doesn’t permit the testing at CERN in
2012, the test will take place in SM18 early 2013 in a dedicated 2K cryostat planned to be
fitted during the SM18 shutdown.
2.5 RF POWER AND CONTROLS
Amplifiers from the LHC 400 MHz cavities can be used to power the cavity. The power
requirements from the coupling should be verified as the available power is limited to 200W.
A new power supply with 500W capability is purchased and should be available for tests in
2013. Higher power would require modification of input adapters for power compatibility if
string multipacting barriers limit the maximum the cavity field with the existing 200 W input
power.
Check the RF wiring for power transport.
2.6 CRYOGENICS AND TEMPERATURE MONITORING
Thermal sensors are already available (how many) on the support structure. The placement of
these sensors is likely to be on the lateral beam pipe region close to the rods where the
magnetic fields are maximum. Four such sensors should suffice, but some additional sensors
if available could be placed on the cavity body for additional temperature measurements
during the measurements.
Second sound acoustic transducers could also be used for quench detection. Such sensors are
already used in the cryolab setup. Kitty has placed the order for the use 2nd sound equipment
in the SM18 cryostat for 4R cavity testing. The T-mapping system however requires
additional work which is not foreseen at CERN. Therefore, if this is required, LU should take
the responsibility of fabrication of the T-mapping flexible tapes to be fitted for testing.
2.7 VACUUM AND ANCILLARIES
The cavity after the surface treatment and UHV baking will undergo a high pressure rinse
with all coupler probes, flanges and valves installed. The drying is done with active pumping
and therefore can be directly connected to the cryostat support structure via the bottom valve
for testing without opening the cavity vacuum. Pictures of the pumping port valve used to be
used for the crab cavities are shown below. The appropriate hybrid flanges are made for
mating CF100 on the beam pipe port to that of the pumping port setup and the integration
into the cryostat was also verified without any issues.
3 SCHEDULE OF THE TEST PROGRAM
3.1 CAVITY RECEPTION & INSPECTION
Since a light surface treatment and testing is foreseen at CERN, an optical inspection at
various stages of the cavity processing is desired. A minimum of three inspections, one at
reception, one after main BCP and the third after the UHV bake is highly desired.
AS the cavity will undergo heavy chemistry at outside CERN, it is strongly recommended to
assemble all flanges with appropriate blanks to minimize the cavity contamination during
transport. The coupler probes and the pumping port valves will be assembled at CERN
immediately after the final HPR. The cavity can be assembled to the support collar and
transported directly to the SM18 facility where it can be tested.
3.2 CRYOGENIC TESTING
All testing should be foreseen before the end of August 2012 to avoid conflicts with other
testing and upgrade of the cryo line foreseen for September 2012. If all ancillary parts are
fabricated and delivered to CERN with the cavity by mid-July, a surface treatment and
cryogenic testing could be foreseen before the end of August or early August. However, this
schedule doesn’t take into account any conflicts with other testing ongoing already at SM18
which may affect the planning.
4 RESOURCES & REQUESTS
4.1 MANPOWER
All activities foreseen to take place at CERN will be coordinated and organized within the RF
group and the vacuum group. Support from engineering for finalizing the structure support
and additional clamps for cold testing will be required. Participation of a student/fellow
during cold testing in SM18 from LU/DL is foreseen.
4.2 MATERIAL AND SUPPLIES
The cavity and its support structure will be provided by LU/Niowave to the appropriate
design specifications given by CERN. All support structures should be constructed out of
non-magnetic SS-316. The cavity EB-welding and leak tightness information from Niowave
should be transmitted to CERN with the cavity. Prior to BCP, the cavity should be welded
with stiffeners to guarantee its structure integrity throughout the test cycle. CERN will be
able to provide a limited amount of resources upon approval for the addition of stiffeners.
The support structure rods on the exterior maybe threaded at the extremeties to allow for
bolting on to the support plate of the test stand if this structure is used.
All BCP related items that can be adapted to the CERN chemistry facility will be recuperated
from Niowave. The exact list of items is pending a quote from Niowave. Additional
equipment required both for chemistry and testing will be procured by CERN.
All flanges and hybrids adapters for probes is foreseen to be procured from Niowave. Any
additional flanges and adaptors required can be used from the CERN inventory. It is highly
recommended to use NbTi flanges on the port of the cavity body to minimize the heat losses
due to high fields. The flanges on the beam ports can be of stainless steel.
4.3 PLANNING
Based on the Jun 30 meeting at CERN with the LU, an optimisitic planning is outlined.
But delay in delivery anticipated due to addition of stiffeners (2 weeks) according to
Niowave which will push the above schedule 3-4 weeks. Conflicts at the chemistry and
testing are also taken into account and a new schedule is given below.
New Planning as of Oct 19, 2012
Aug 31:
Cavity arrives at CERN
Sep 13-20: Vacuum H2 outgassing (48 Hrs, 600 deg C), no degreasing as Niowave
confirmed that the cavity was degreased and packed in class100 clean room.
Sep 23-Oct 23 (Weeks 39-42): Cavity inspection, Flanges and seal fixed on the cavity,
RF measurements, leak tightness, optical/endoscopic measurements
Oct 24-Nov 2 (Week 43-44): Light BCP, high pressure rinsing and handing to RF
Nov 5- Nov 12 (Week 45): Transport, RF test stand preparation, vaccum & cooldown
Nov 12- Nov 23 (Week 46-47): First RF tests at 2K
Old June 30 Planning: Obsolete, see below for current planning
Jul 16:
Cavity arrives at CERN (Arrived Aug 31, 2012 due to delays)
Jul 16-20:
Cavity inspection, leak tightness, optical/endoscopic measurements
Jul 23-27:
Cavity degreasing (if needed), preparation of vacuum H2 outgassing
Jul 30-Aug3: Light BCP, high pressure rinsing and handing to RF
Aug 6-10:
Transport, RF test stand preparation, vaccum & cooldown
Aug 13-17:
First RF tests at 2K