# Presentation: 407877

## Recent Developments in the NIST Cryogenic Thermal Transfer Standard (CTTS) Project

- Thomas E. Lipe, Joseph R. Kinard
- Electricity Division,
- NIST, Gaithersburg, MD
- Carl D. Reintsema
- Electromagnetic Technology Division
- NIST, Boulder, CO

## Outline

- Introduction
- Goals
- History
- Theory of Operation
- Design
- Transmission Line Structures
- Present Performance
- Limiting Factors
- Conclusions and Future Plans

## Goals

- Develop a new primary thermal transfer standard
- Low operating temperature
- Reduce thermoelectric effects
- Increase sensitivity
- Operate at microwatt signal power
- Low-voltage/low-current primary standard

## History

- May 1997 - first CTTS
- 45 cm manganin input transmission line
- Nb transition-edge sensor (TES)
- Horizontal experimental platform
- June 1999
- 45 cm coaxial input transmission line
- NbTa TES
- Vertical experimental platform
- June 2000
- High-Tc input transmission line

## TES Sensitivity

## Theory of Operation

- Electrical substitution calorimetry

- Thermal Conductance

- Rtrim

- Rsig

- Converter Thermal Node

- TES

- Platform Thermal Node

- Ptotal = Ptrim + Psig
- Ptotal = G(Tc - Tplat)
- Ptrim = -Psig

- G

## Experimental Platform

**Circuit boards carry signals**
**under reference thermal shield**

## Experimental Platform

- Cryostat
- Baseplate
**4 K**

- Reference
- Platform
**≈ ****6 K**

- Thermal
- Converter
- Stage
**6 K + ********T****c**

## Sensor Designs - I

**Original **
**Sensor**
**Design**

- Thermoelement

- Ice
- Cube

## Sensor Designs - II

**Sensor Designs - II**

**NbTa TES**

**PdAu Trim**
**Heater - 451 Ω**

**Old Design**

**New Design**

- 1

- 3

## Input Transmission Lines

- Vacuum Jacket

- N Tank

- He Tank

- Normal-Metal Coaxial
- Transmission Line

- High-Tc
- Transmission
- Line

## High Tc Transmission Line

- YBCO

- Au contact pads

- LaO
- Substrate

## Experimental Data - TVC

## Experimental Data - FRDC

**FRDC-dc Difference of CTTS at 14 mV**

## Experimental Data - Level

## Conclusions From Data

- High-frequency errors due to transmission line structures
- FRDC measurements indicate that audio-frequency errors are not due to  thermoelectric effects in the sensor
- Ac-dc differences strongly dependent on input power level
***Probable mechanism: Tc suppression caused by magnetic fields on the chip***

## Magnetic Fields - Present Chip

## Magnetic Fields - New Chip

## Future Plans - I

- Reduce magnetic field coupling from the heater to the TES
- Change heater design to bifilar structure
- Use on-chip shielding or coaxial structure

- Shielding
- Dielectric layer
- Heater
- Silicon

- Cross-section of chip

## Future Plans -II

- Improve high-frequency characteristics by improving input transmission structure.
- Integrate experimental platform and transmission line
- Place transmission line on TES chip
- Improve room-temperature electronics
- Improve resistance measurements
- Improve PID control

## Conclusions

- Cryogenic Thermal Transfer Standard constructed and tested
- High-Tc transmission line installed and tested
- Measurements indicate that audio-frequency errors result from magnetic coupling
- Redesign heater structure and shield TES
- Viable approach to new primary standard of ac-dc difference