Plasma Cleaning of SRF Cavities Program Status J. Mammosser, S. Ahmed, L. Phillips Jlab S. Popovic, J. Upadhyay, L. Vuskovic, M. Nikolic ODU

What is the plasma processing program? The program goal is aimed at developing techniques for improving cavity operational performance while installed in cryomodules (field emission reduction mainly) Main focus is on developing plasma processing techniques that has shown promise Additionally, take a look at helium processing to see if there is a way to improve this technique

What is Plasma Processing Plasma processing is a commercial process developed for the semiconductor silicon etching (dry chemistry) –The process is straight forward and can be applied to any vacuum tight volume and especially RF resonant structures Ashing – removal of organics Reactive ion etching µm/min nW/cm3 100’s mW/cm3 Surface Chemistry Noble gases Ar,He,Ne,Kr,Xe Reactive gases SF6, CF4,CH3Cl Free radicals!

How is it applied to cavities? Process gas in Vacuum Pump RF in Best applied at room temperature! Can be applied at 4.2K, with Helium gas Plasma systems change if operating pressure changes!!

Helium processing (HP) and plasma processing (PP) are different in the following ways: HP injects a gas at very low pressure (10-5 Torr) and uses RF fields to heat field emitter sites to generate electron. Electrons ionize gas and back sputter RF surface –Very localized and random –Typically performed at 2K in vertical test PP injects gas at higher pressures (mTorr to Atm) and uses RF to generate and sustain ions in a large volume –Global process, typically fills volume of container

Helium processing (HP) and plasma processing (PP) are different in the following ways: HP RF drives resonant mode in cavity –BW –10’s of hertz at low temperature PP RF drives resonant mode in cavity to generate the plasma then it’s the plasma resonance not the cavity mode therefore you can drive frequency over a large range –BW - 8MHz for CEBAF five cell at room temperature

Mapping out the plasma parameters Plasma Not Sustained Increasing Plasma Intensity

What is the best process? HP Pros: Its easy to apply with very little risk HP Cons: Very random process –doses not always work Best results first time processing, most likely affect is breaking up emitter or melting it, but does not remove it!

What is the best process? PP Pros: Ions bombard all surfaces, can create atomically clean surface, works great on reducing multipacting Removes contaminants as a gas Can be developed to target known contaminants and is therefore promising PP Cons: Can etch areas you don’t want (copper plating) Can thermally stress objects in cold plasma Process is slow, time consuming if soft etching procedure (safe approach) Careful choice of parameters and technique is required

Current Status We have gained experience with the five cell cavity, single cell cavity and have demonstrated: 1.Ability to generate plasma in cryomodule configuration 2.Ability to avoid window discharges Vertical testing of a five cell cavity in VTA is underway to develop process procedures and demonstrate performance improvements

What is needed for cryomodule application? Need experience on real cryomodule (FL02) Demonstrate we can routinely generate plasma in-situ –Plasma fills the cavity, this is tricky –Understand signals that can be used to monitor process and know when a window discharge occurs That we can control process parameters such as gas pressure and RF power over period of shifts (can be automated)

What’s Needed? Need to develop instrumentation for process to develop an understanding and monitoring: –Spectrometer (Borrowed from FEL) –RGA (on pump cart) –Data acquisition (developing now) –Measure plasma parameters under various conditions(Langmuir Probe needed will purchase)

Plans For FL02 1.FL02 was tested at 2K, cavity 8 and cavity 6 had a baseline RF test performed (Q0, gradient and radiation measured) 2.FL02 will be moved out of the test cave for room temperature PP –Process will be ashing process (gentle cleaning, removal of organics) with Argon gas –Attempt at window cleaning with plasma 3.Will demonstrate the following –Plasma in cavity 8 and cavity 6 –Stable processing over periods of hours and maybe shifts 4.Requalification of performance at 2K 5.Warm-up to 4.5K –Attempt to perform PP and HP –if time permits

Plans for R&D 1.Develop and understanding for process parameters and their affects on field emission reduction –Small Nb samples utilizing existing SRF infrastructure (SEM, Fe scanning system, FEV) 2.Develop and understanding of cavity RF performance in VTA –Affects on Q0 and field emission onset (radiation) –Correlation of plasma parameters with RF power level, vacuum pressure, RF frequency, process time and gas type –4.2K tests PP and HP 3.Compare cold and room temperature processing performance –Look for helium processing improvements and plasma options Not Started! Started! Not Started!

What has been done in the PP at other labs? ORNL/SNS processed a full HB cryomodule, 805MHz, pulsed RF at 4.5K (Helium) –H01 was the worst performance  middle of the pack University of Wisconsin (Bob Legg) processed a SRF gun cavity, 200Mhz, CW at room temperature (Argon/Oxygen) INFN processed single cell cavities, 6 GHz, CW at room temperature, (Helium)

SNS Setup (S-H Kim) AAC Review, February 24-25, 2009 Radiation/electron activity diagnostics in the Test Cave Ionization Chamber Internal Ionization Chamber Phosphor Screen, Camera, Faraday Cup IC0 IC1 IC2 IC3 IC4 IC5 IC6 IC7 IC-int Phosphor Screen & Faraday Cup Phosphor Screen & Faraday Cup Cavity A Cavity B Cavity C Cavity D

SNS -Changes to Radiation (S-H Kim)

UW Setup (R. Legg)

Test Results – UW (R. Legg)

INFN Setup (N. Patron,E.Palmeri)

Test Results – INFN (N. Patron,E.Palmeri)

What we have done at Jlab We have mapped out parameters for 5cell and single cell cavity “Resonant-Frequency Discharge in a Multi-Cell RF Cavity”,under review for publication A demountable single cell was fabricated for measuring plasma parameters such as: –Ion density –Electron temperature and density A test cart was assembled with all necessary hardware to perform plasma processing (mobile)

Plasma in CEBAF 5 Cell Plasma Fills volume Plasma at RF input Probe discharge The End Thank You!