Cathodic Arc Deposition of superconducting thin films of MgB 2 for RF cavities*

Transcription

1 Alameda Applied Sciences Corporation Cathodic Arc Deposition of superconducting thin films of MgB 2 for RF cavities* Mahadevan Krishnan, Andrew Gerhan, Kristi Wilson and Brian Bures Alameda Applied Sciences Corporation, San Leandro, CA, USA Anne-Marie Valente-Feliciano Thomas Jefferson National Accelerator Facility, VA, USA David Larbalestier, Jianyi Jiang, Applied Superconductivity Center, Florida State University, Tallahassee, FL, USA Sami Tantawi, Stanford Linear Accelerator Center, Palo Alto, CA, USA 3rd International Workshop on "Thin films applied to Superconducting RF and new ideas for pushing the limits of RF Superconductivity - Jefferson Lab, VA - USA * This research was supported by DOE Grants #DE-FG02-05ER84139 and DE-FG02-07ER84742

2 Cathodic arc deposition of MgB 2 High vacuum achievable: 5x10-9 Torr base pressure Heater implemented for substrates Substrates: 1cm φ sapphire and 6.3cm φ Nb (for RF tests at SLAC) Easy to incorporate elbow magnetic filters in future Substrate Filtered Plasma Plasma & debris Macroparticle Filter 90 Magnetic Filter Coil Anode Cathode Plasma Pulsed Arc Source PFN

3 Potential advantages of cathodic arc deposition (with pulsed bias)

4 Can cathodic arc deposition be simpler than conventional methods: single step vs. multi-step? Torr pr range to grow MgB2 films at >1125K P-T diagram for MgB 2 Three groups of temperatures used to grow MgB 2 films: (<575K, K and >1125K (Wang et al) In order to improve the stoichiometry and grain size, a high temperature processing step is required after deposition with a low or intermediate substrate temperature. Once the precursor film is deposited, the film is annealed, often either in an inert gas or Mg vapor environment Torr pr range to grow MgB 2 films at >1125K S-F Wang, Y-L Zhou, Y-B Zhu, Z Liu, Q Zhang, Z-H Chen, H-B Lu, S-Y Dai, G-Z Yang ŅProperties of chemicalvapor-deposition-prepared MgB 2 thin filmsóthin Solid Films 443 (2003) 120 Š123

5 SQUID data for MgB 2 film deposited at AASC, annealed and analyzed at ASC A B A T c of approximately 30K is apparent from the SQUID data, figure A (left). Although this is below the ideal value of 40K, figure B (right) shows no offset in the peak of M indicating that the MgB 2 grains are well-connected, with no MgO or other contaminants between the grains interrupting current flow

6 Resistivity data for MgB 2 film deposited at AASC, annealed and analyzed at ASC T c 30K A T c of approximately 30K is apparent from both the SQUID data and these resistivity data

7 SEM images of annealed MgB 2 film 1µm 100nm Cathodic arc deposited MgB 2 films show promise AASC has demonstrated a T c of 34K in MgB 2 thin films by depositing at 550K from a stoichiometric MgB 2 source using cathodic arc deposition, improving upon it to where a superconducting MgB 2 film was successfully deposited over a continuous 2 diameter circular area on a niobium substrate for RF evaluation using a TE 011 test cavity at SLAC RF tests at SLAC are a necessary next step

8 Is cathode source stoichiometry important? When a MgB 2 cathode is ionized, the Mg ion flux is much greater than the B ion flux. Cathode stoichiometry is NOT preserved in the films The sticking coefficient onto unheated substrates is large for both materials and films are Mg rich with a Mg to B ratio of as high as 5:1 To drive the Mg:B ratio to 1:2, substrate heating to 550K is required: with increasing temperature the sticking coefficient of Mg decreases more rapidly than that of B At 675K, Mg was totally absent from the films Deposition from Mg-rich cathodes may form MgB 2 films at higher substrate temperature, yielding better superconductive properties from a single step

9 ASC can achieve high purity sintering of MgB 2 Comparison of material from traditional (left) and improved ASC (right) sintering process White flakes in as received B image indicate contamination at grain boundaries in the sintered MgB 2 cathode. Despite this, Tc 38 o K was measured. ASC has developed a new method to purify the material prior to sintering (image on the right)

10 Oxygen contamination due to H 2 O in MgB 2 films A=Å

11 Future plans Future work will address the following questions: Can cathodic arc deposition produce high quality MgB 2 thin films in a single step: correct morphology, T c, RRR? Are there better alternatives? (Nb 3 Sn, other candidates)? What are the RF properties of high quality MgB 2 thin films and how are these properties affected by the MgB 2 thin film microstructure? Is a cathodic deposition process that is compatible with deposition of MgB 2 onto Nb and/or Cu RF cavities a possibility?