Dielectric tuning of cavities D. Bowring 3rd Workshop on Cavities - - PowerPoint PPT Presentation

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Dielectric tuning of cavities D. Bowring 3rd Workshop on Cavities - - PowerPoint PPT Presentation

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Dielectric tuning of cavities D. Bowring 3rd Workshop on Cavities and Detectors for Axion Research August 21, 2018 On behalf of T. Connolly, M. Kang, M. Ortega, S. Priya, C. Salemi, M. Silezin, A. Sonnenschein, and A. Tollestrup. Headline: We

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Dielectric tuning of cavities

  • D. Bowring

3rd Workshop on Cavities and Detectors for Axion Research August 21, 2018

On behalf of T. Connolly, M. Kang, M. Ortega, S. Priya, C. Salemi,

  • M. Silezin, A. Sonnenschein, and A. Tollestrup.
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Headline: We can tune a 4.5 GHz cavity by a linewidth using an STO crystal.

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Before we start, I want to brag about our students.

Chiara Salemi (MIT, Physics) Marlene Ortega (Brown, Physics) Michal Silezin (UIUC, Engineering) and Thomas Connolly (U.Mass-Amherst, Physics)

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Overview

  • 1. Simultaneous tuning of many cavities is hard.
  • 2. Ferroelectric materials have variable ǫ, depending on

applied voltage.

  • 3. We can tune our prototype cavity via this method.
  • 4. Still some work to be done.

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Concept: Coarse mechanical tuning + fine tuning via ferroelectrics

Tuning cartoon, showing cavity array, coarse-tuning rod assembly, and fine-tuning ferroelectric material.

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Strontium titanate (STO) exhibits dǫ/dV = 0.

O.G. Vendik et al., J. Supercond 12, 2 (1999).

◮ tan δ 10−3 for single-crystal STO at 10 GHz ◮ Significant loss introduced w/ electrodes. We see

this too. (More later.)

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Prior work on 2D resonators

  • S. Priya, M. Kang (Va

Tech)

◮ Prior work on 2D resonators. ◮ Possible to measure ǫ. ◮ Thin film process development

  • ngoing.

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Prior work on 2D resonators

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(Relatively) New Prototype Cavity

◮ f010 4.4 GHz for empty

cavity.

◮ Linear rod path; 25 mm

throw; 6.25 mm rod diameter

◮ TM010 mode has ∼300 MHz

  • f “clean” tuning range.

◮ Can be loaded with STO

samples.

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(Relatively) New Prototype Cavity

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For this work: remove tuning rod & insert crystal

3D-printed L-bracket + single-crystal STO sample. The tuning rod was not installed during the measurements described here.

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Progressively lossy interventions

QL f010 at room temp. (GHz) Empty Cavity 11,000 4.36 Uncoated Sample 10,000 4.28 Ti Coated Sample 1,300 4.36 With Electrodes 900 TBD

◮ Currently large systematic errors. We need a screw

torque standard, e.g.

◮ 10x drop in Q when Ti coating applied. ◮ Straightforward explanation (see next slide). ◮ NB: No effort made to optimize cavity Q (polishing,

RF joint design, etc.)

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Crystal modes hybridize with cavity modes.

◮ Coating w/ metal film turns this into a planar

resonator.

◮ Crystal is 20 x 10 x 0.3 mm (ripe for mode

crowding).

◮ Highlighted mode couples strongly to cavity TM010. ◮ Remediation: spoil crystal mode Qs via thin,

patterned, resistive films.

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Simulate hybridization & effect on C010.

HFSS simulation. Effect on C010 translated into integration time penalty.

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We can demonstrate tuning via temperature control.

O.G. Vendik et al., J. Supercond 12, 2 (1999). Cavity loaded with single-crystal STO sample at wall.

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Qualitatively, simulated and observed mode mixing agree.

∼ 40-50 dB of difference between dark red (high) and green (low).

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Attach electrodes to Ti-coated crystal, tune with voltage.

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Q ∼ 900 means the spectrum colorplot is not as clean.

Leakage current 1 µA below 600 V.

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∆f010 (MHz) vs Voltage

Low-voltage structure probably due to charging. Monotonic above ∼75 V. Approximately one linewidth of voltage-based tuning!

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Still lots of work to be done, of course.

Bandstop

  • n DC

probe “port”

◮ Bandstop filter reduces coupling

losses by 200x.

◮ Patterned electrodes (c.f. A. Dixit)

reduce crystal/cavity hybridization.

◮ Crystal position/size optimization. ◮ Understanding systematics,

improving DAQ &

  • instrumentation. . .

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We have a conceptual design that integrates this into a multi-cavity array.

◮ Via Michal Silezin, summer engineering undergrad ◮ Assume a magnet with a 1-meter bore, 50 cm long. ◮ How many 50-mm dia. cavities can we fit in this

space? How should they be tuned? How cooled? What this design is/is not:

◮ This is not an engineering design ready for review. ◮ First-pass attempt to show various subsystems ◮ Hypothetical fridge scaled from ADMX ◮ Much thermal & electrical design work remains.

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Multi-cavity array: 241 50-mm cavities

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Multi-cavity array: 241 50-mm cavities

“Honeycomb” closed by lid with holes for tuning rods, cabling.

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Coarse tuning via simultaneous motion of all rods

Tuning rods move together as part of a single piece.

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Coarse tuning via simultaneous motion of all rods

Rod assembly moves on linear bearings. One piezo actuator can push the entire rod assembly.

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Single-cell closeups

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Each cell can be loaded with a crystal.

http://www.preperm.com sells stock with 2.5 < ǫ/ǫ0 25.

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Doubling the array preserves L ∼ 5R and maximizes volume.

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Thermal straps and insulations direct heat flow to correct temp. stages.

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Thermal straps and insulations direct heat flow to correct temp. stages.

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Thanks for your attention!

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