Status Report of the Review of Vacuum Systems and Procedures Tug Arkan, Allan Rowe 2/21/2011 1

Introduction Charge from Mark Champion: – As a result of recent disappointing performance results of 1.3 GHz cavities in the horizontal test stand at MDB, I've charged Allan Rowe and Tug Arkan with the task of reviewing the vacuum equipment and procedures in use at the following facilities: Argonne clean room IB1 VTS MDB HTS MP9 clean room IB4 clean room NML – In particular, they will be looking at evacuation and venting equipment and procedures, cleaning procedures, and procedures for making & breaking vacuum connections to cavities. The outcome of this investigation will be a report of their findings long with recommendations for improvements where needed. – The goal is to ensure that we are not inadvertently contaminating cavities due to lack of proper equipment or procedures. 2

MDB HTS RI-018 dressed cavity was tested in HTS; during the first test, it achieved 35 MV/m gradient with minimal FE As a part of the review, it is decided to re-test RI-018 in HTS. The tasks outlined below were performed (in order) before the second test: – Warmed up the cavity to room temperature – Backfilled the cavity to ~820 Torr absolute pressure. (Cavity vacuum was at 5 x 10^-9 Torr before backfilling, the specified backfill pressure is ~780 Torr) – Opened the HTS cryostat end domes – Closed the cavity valve (RAV) slowly, did not remove the cavity outside of the HTS cryostat – Conflat flange connection between the HTS gooseneck flange and HTS vacuum manifold hose Break Remake Pump down & Leak check Backfill to ~780 Torr absolute pressure – Opened the cavity RAV slowly Manifold pressure rose from 780 Torr to 820 Torr in approx. 1 second – Pumped down the cavity 3

Backfilling Manifold Setup (HTS) LN2 gas supply line 0.5 psi Pop-off Burst Disk 0.01 micron particle filter Variable Leak (needle) Valve Isolation valve for the roughing & turbo pump HTS vacuum manifold hose Ion Pump Ion gauge RGA Pressure gauge 4

Backfill Procedure (HTS) A portable Nitrogen dewar is situated outside of the HTS cave The nitrogen boiled off gas was supplied into the cave with a black Tygon tubing (~30 feet long) While the nitrogen is flowing from the Swagelok connector at the end of the tubing, attach the tubing to the HTS backfilling manifold (Needle valve is closed at this time, the burst disk starts to leak gas at 0.5 PSI gauge) Ensure that isolation valve is closed and the ion pump, ion gauge and RGA are turned off The pressure gauge is reading 0 at this time because the cavity the manifold are under vacuum Open the needle valve slowly to ensure a backfill rate of 20 mTorr/second until 1 Torr Increase pressure by ~30 Torr/min above a 1 Torr vacuum line pressure Close the needle valve once the pressure gauge reads ~780 Torr. (There is no automatic shut off. System relies on a 0.5 PSI, 28 Torr pop off burst disk) 5

Backfill Procedure (HTS) - Recommendations Write formal procedure/checklist that includes more detail than current general checklist Add RGA scan data for both cavity and vacuum manifold. Spectra should be saved each cycle Replace the Tygon tubing with clear cleanroom compatible flexible PVDF tubing Make the Swagelok connection of the nitrogen supply to the backfilling manifold inside a portable cleanroom Install another inline particle filter after the needle valve Install isolation valves so that ion pump, ion gauge and RGA are not backfilled during the cavity backfill process It is not recommended to manipulate the needle valve in order to increase the gas flow during the backfill, because the needle valve will create particles Install a pressure transducer and automatic shutoff valve to ensure a backfill to exact 788 Torr (1050 mbar) absolute pressure 6

Conflat flange connection between the HTS gooseneck flange and HTS vacuum manifold house 7

Break the Flange 8

Remake the Flange 9

Pump Down & Leak Check & Backfill The newly made Conflat flange connection needs to be leak checked: – Pump down the line (fast pumping, isolation valve is fully open, cavity RAV is closed) – Start RGA, spray helium and conduct leak check – Backfill slowly as done for the cavity backfill 20 mTorr per second until 1 Torr 30 Torr per minute until ~780 Torr is reached 10

Recommendations for the flange connection inside the portable cleanroom Adopt the Particulate Free Flange Assembly (PFFA) procedures for the Conflat flanges assembly & disassembly Use virgin electro-polished stainless steel and silicone bronze hardware from IB4 inventory for every new cavity installation in HTS Clean assembly hardware should be packaged individually at A0 to minimize cleaning time in the portable cleanroom The nitrogen blow gun should be kept all the time inside the cleanroom Install a rack inside the portable cleanroom in order to store clean lint free clothes, clean Class 10 gloves Use copper gasket holders to hold the seal and do not drop the seal vertically in place during assembly Change the order of assembly such that the vacuum connections are tested before the RF coupler is installed. 11

Pump Down the Cavity RI-018 was pumped down slower than usual before the first horizontal test. – It is very difficult and inaccurate to throttle the pump down speed with the isolation VAT right angle valve (RAV). – There is a potential of creating particles even though the isolation RAV is away from the cavity. The cavity RAV is fully open before the pump down starts We decided to pump down the RI-018 cavity with usual pump down speed as it was done for previously tested cavities (~2 Torr per second until 100 Torr and then open the isolation RAV more so that ~5 Torr per second pumping speed is reached) After mid 10^-6 Torr pressure is reached, the isolation valve is closed. The ion pump is started and the ion gauge is turned on. RGA is also turned for overnight scanning Next day morning, RGA scan was observed No RGA anomalies identified—i.e., the system and cavity are clean. 12

HTS Summary Minor hardware changes to N2g system would improve cleanliness. Formal procedures required for venting, pump-down, and assembly sequence. Current checklist is too general. Improve the control of the pump-down and venting with additional hardware. Consider isolating certain components during venting. Modify assembly procedure to limit operator’s hands from leaving clean space. Keep vacuum line and cavity pressures above 1 Torr as equal as possible---with slight positive pressure in cavity. Hardware used in assembly should be electropolished, new, and individually bagged. Adopt standard clean flange assembly technique used at MP9 and SCSPF. Will shorten disassembly time and improve cleanliness. 13