TE-type Sample Host Cavity development at Cornell Yi Xie, Matthias Liepe Cornell University Yi Xie – TE cavity developments at Cornell, TFSRF12

Outline Sample host cavity using TE modes; Development of TE pillbox cavity; Development of TE mushroom cavity; Input coupler for both TE cavities; Test insert, thermometry, surface treatments; Baseline r.f. results and conclusions; Yi Xie – TE cavity developments at Cornell, TFSRF12

TE cavities for sample tests Measuring sample surface resistance R s (f,T,H,x,y…) is the crucial step towards thin film application for SRF cavities; Ideally, sample should be tested within/or part of r.f. cavities. The host cavity should have:  ~ 100 mT or larger on sample surface;  ~ n Ω sensitivity;  fast and relative easier measurements to get statistics; TE modes cavities have been used because:  not vulnerable to field emission;  ideally no joint loss between host cavity and sample plate; Yi Xie – TE cavity developments at Cornell, TFSRF12

Preceding sample test cavities From Nathan Pogue, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011 To achieve both high field and high sensitivity is challenging. Yi Xie – TE cavity developments at Cornell, TFSRF12

Previous work on TE cavity at Cornell Yi Xie – TE cavity developments at Cornell, TFSRF12

Previous TE cavity at Cornell (I) Yi Xie – TE cavity developments at Cornell, TFSRF12

Previous TE cavity at Cornell (II) Same coupler for both input and pickup, old style mechanical gear changes coupling, coupler loss; Huge coupler heating problem with input power more than few watts; Test in very small dewar, bad helium cooling; Baseline RF performance Temperature recorded by a calibrated Cernox sensor attached at the side of the pillbox tube Yi Xie – TE cavity developments at Cornell, TFSRF12

New TE cavities design considerations Yi Xie – TE cavity developments at Cornell, TFSRF12

Motivation of new TE cavities In order to test new materials samples such as Nb 3 Sn and MgB 2, the sample TE cavity requires: Flat sample, no grooves; A input coupler which can deliver decent power ~100W; Dedicated insert, class-10 operations; Thermometry system with its sensitivity comparable to single cell thermometry system to add calorimetric function; Same standard treatments as 1.3GHz TM mode cavities; Yi Xie – TE cavity developments at Cornell, TFSRF12

Flange considerations TE 0mn modes are normally used because IDIALLY there is no current across the joints between the cylindrical part and the end plates. Indium Indium height, 0.5~1mm Distance of indium to cavity, 0.1~3mm Indium only reduces cavity Q 0 when it’s very close to cavity Distance larger than 2mm is sufficient in preventing reduction of cavity Q 0. Yi Xie – TE cavity developments at Cornell, TFSRF12

Design of TE pillbox cavity(I)  Only one input power port, use reflected power to phase lock;  No mode separating grooves, easy machining;  f TE011 ~ 5.88 GHz;  H max,sample / H max,cavity ~ 0.8;  Sample diameter ~ 7cm B field distribution Top plate Tube Baseline plate Yi Xie – TE cavity developments at Cornell, TFSRF12

Additional port to keep symmetry TE 011 mode f ~ 5.88 GHz, sample radius = 0.25 cm Small round sample Design of TE pillbox cavity(II) Yi Xie – TE cavity developments at Cornell, TFSRF12

Design of TE mushroom cavity The main design goal is to maximize the ratio R of maximum sample plate surface magnetic field to maximum host cavity surface magnetic field, other considerations are: Sample size (bottom plate of the cavity) should be small (≤ 4 inches diameter). Lower excited modes frequencies (≤ 6GHz) are desirable to avoid global thermal instability as R bcs ∝ f 2. The cavity configuration should be relatively simple and the bottom sample plate should be easily to attach. Yi Xie – TE cavity developments at Cornell, TFSRF12

The design process Yi Xie – TE cavity developments at Cornell, TFSRF12 Cavity geometry file maker Field Solver Objective function Cavity shape parameter set

TE mushroom cavity TE 012 TE 013 s (cm) sample plate host cavity wall H/H max TE 012 TE 013 f (GHz) H max,sample / H max,cavity Sample diameter (cm)9.525 Yi Xie – TE cavity developments at Cornell, TFSRF12

Input coupler for TE cavities Yi Xie – TE cavity developments at Cornell, TFSRF12

An evolution of TE cavity loop coupler tips Original Saclay loop coupler shape Off center Saclay loop coupler shap e H line ║ loop Design A Design B H line ┴ loop How to couple TE modes Yi Xie – TE cavity developments at Cornell, TFSRF12

Q ext of design B Q ext of design A Coupling to the cavity Choose design A, easy to fabricate Yi Xie – TE cavity developments at Cornell, TFSRF12

Input coupler An off-center hook tip variable coupling input coupler for TE 011, TE 012 & TE 013 was designed and fabricated. 3D multipacting simulations using ACE3P have been performed.  Good coupling to magnetic field, Qext ~ 10 6 ~10 10  No multipacting found in 3D simulations and during rf test;  3 section adjustable inner conductor design enables using the coupler for both TE pillbox and mushroom cavities; Yi Xie – TE cavity developments at Cornell, TFSRF12

Insert, Tmap and surface processing Yi Xie – TE cavity developments at Cornell, TFSRF12

Dedicated rf insert A dedicated cryo rf test insert has been built to fit Cornell SRF group rf test area center pit;  accommodates both TE pillbox and mushroom cavity;  enables class-10 cleanroom assembly of both cavities; Yi Xie – TE cavity developments at Cornell, TFSRF12

Thermometry, cavity treatments A ring of 8 thermometers has been mounted near the highest magnetic field region of TE pillbox cavity bottom plate and can successfully detect ~ nΩ surface resistance. TE cavity EP setup TE cavity HPR setup Special small HPR fan jet nozzle Yi Xie – TE cavity developments at Cornell, TFSRF12

So, what’s the baseline performance? Yi Xie – TE cavity developments at Cornell, TFSRF12

Baseline r.f. performance (pillbox) RF performance of baseline niobium sample plate After 120 um BCP, 800C bake and HPR. H max,sample ~ 450 Oe (45mT), Q 0 ~ 6e9; Yi Xie – TE cavity developments at Cornell, TFSRF12

RF performance of baseline niobium sample plate After 120 um BCP, 800C bake and HPR. H max,sample ~ 600 Oe (60mT), Q 0 ~ 3e9; Baseline r.f. performance (mushroom) Yi Xie – TE cavity developments at Cornell, TFSRF12

Can we push the fields even higher? Yi Xie – TE cavity developments at Cornell, TFSRF12

By directing solving thermal feedback model, get thermal run away around 600 Oe. Assuming R res =10 nΩ, T bath =1.6K, Kappa~2 W/(m-K), no phonon peak Global thermal instability? Yi Xie – TE cavity developments at Cornell, TFSRF12

120C bake By 120C bake the cavity, decrease the BCS resistance by half -> can improve the field up to 800Oe. Yi Xie – TE cavity developments at Cornell, TFSRF12

Future plans Assuming our 6GHz TE cavities have approached their global thermal instability limit, we can improve the highest field by Reduce R BCS by 120C baking the cavity; Improve RRR/thermal conductivity by high temperature treatment such as titanium getter; Operate mushroom cavity at its 4GHz mode; By either of above means, we hope to improve the host TE Mushroom cavity breakdown field to above 800Oe (80mT). Yi Xie – TE cavity developments at Cornell, TFSRF12

Conclusions Yi Xie – TE cavity developments at Cornell, TFSRF12

Two dedicated TE sample host cavity systems have been designed, constructed and successfully commissioned at Cornell’s SRF group will become powerful tools capable for new material and niobium characterizations. Currently,  TE pillbox:  TE mushroom: Simulations show that both decrease temperature and 120C bake the cavity will push the field on the sample higher to 800Oe. Conclusions H max,sample ~ 450 Oe (45mT), Q 0 ~ 6e9; H max,sample ~ 600 Oe (60mT), Q 0 ~ 3e9; Yi Xie – TE cavity developments at Cornell, TFSRF12

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