# Presentation: 741835

## Quench Protection of the 50 T HTS Solenoid

- Steve Kahn
- Muons Inc.
- 3 November 2006

- S. Kahn -- Quench Protection

- 3 Nov 2006

## Preliminary Calculations

- I am presenting some preliminary calculations using the quench calculation program QUENCH.
- This program was written by Martin Wilson at Rutherford Lab in the 1970’s. The current version of the program is marketed by B. Hassenzahl of Advanced Energy Analysis.  BNL has a license to use it.
- There are other codes available:
- QUENCHPRO at FNAL Technical Division.
- SPQR from CERN.
- QLASA from INFN-Milano.
- QUABAR.
- Vector Fields is developing a quench propagation code.
- Being beta tested now.

- S. Kahn -- Quench Protection

- 3 Nov 2006

## Basic Design Parameters of the 50 T Solenoid

| Parameter | Kahn
EPAC06 Design | Palmer Current Design |
| --- | --- | --- |
| Length | 70 cm | 1 m |
| Inner Radius | 2.5 cm | 2.0 cm |
| Outer Radius | 23.5 cm | 36.5 cm |
| Total Stored Energy | 20 Mega-Joules | 57 Mega-Joules |

- S. Kahn -- Quench Protection

- 3 Nov 2006

## Conductor and Insulator Description

- The conductor is BSCCO 2223 which is 30% HTS filaments and 70% Ag matrix and Ag-Mg sheath (for strength).  We assume the matrix/sheath is all Ag for the calculation.
- The insulator is assumed to be the Stainless Steel interleaving.  A minimum thickness (0.07 mm) is used since we are describing the inner layers.
- In practice we will likely add a ceramic coating or kapton wrap as insulator.  This will inhibit the transverse quench propagation.

| Component | Area | Fraction |
| --- | --- | --- |
| HTS conductor | 0.3483 mm2 | 0.238 |
| Ag matrix/sheath | 0.8127 mm2 | 0.556 |
| SS Insulator | 0.301 mm2 | 0.206 |

- S. Kahn -- Quench Protection

- 3 Nov 2006

## Conductor Material Properties Necessary for Quench Protection

- We need the material properties of all the components of the conductor and insulation.
- The important properties are
- Heat capacitance (Specific Heat)
- Resistivity
- Thermal conductance
- Obtained from resistivity with Wiedemann-Frantz law.
- CV and  are parameterized as in up to four temperature ranges:

- S. Kahn -- Quench Protection

- 3 Nov 2006

## Critical Current Measurements

- Measurements of critical current as a function of B and temperature are from EHTS (another provider of BSCCO 2223).
- The measured data is used to determine parameters of the following equation:

- Used only high field part of data to determine Bc20

- This formula is used for NbTi and Nb3Sn.  Is it valid for HTS??

- S. Kahn -- Quench Protection

- 3 Nov 2006

## HTS Characteristics: JC vs. B for Constant T

- 0.85
- Contingency
- factor

- S. Kahn -- Quench Protection

- 3 Nov 2006

## Quench Propagation Velocity

- Quench protection calculations depend on the *quench propagation velocity.*
- The quench propagation velocity can be calculated from the formula below.
- This is what I did.
- Experience for NbTi shows that the formula does not reproduce the measurements.
- Typically the experimentally determined value is used.
- We need to measure this for HTS.
- One of the weaknesses of the velocity calculation is that the specific heat (CV) varies as T3 and is rapidly varying at the quench front.

- S. Kahn -- Quench Protection

- 3 Nov 2006

## Slide 9

- S. Kahn -- Quench Protection

- 3 Nov 2006

## Slide 10

- S. Kahn -- Quench Protection

- 3 Nov 2006

## Comments on J. Schwartz Quench Velocity Measurements

- The quench propagation velocity in this experiment is ~4 cm/sec.  In NbTi it is approximately sonic.
- If a quench occurs and the heat can not be dissipated, the local area will heat up to a very high temperature.  It will be destructive.
- The slow propagation velocity is largely due to the fact that at 4.2°K most of the SC is far away from the critical temperature.
- The closer we are to the Tc, the faster the quench velocity.
- Restated:  A conservative design with a large safety factor built-in against quenches occurring could be self-destructive.
- The typical approach used with NbTi and Nb3Sn of firing heaters to make the magnet go normal faster *won’t work.*
- It takes too much external energy to make the magnet go normal and would likely increase the destruction.

- S. Kahn -- Quench Protection

- 3 Nov 2006

## Circuit for Quench Energy Extraction

- Quench circuit components:
- Solenoid represented by inductance L.  Also there is an internal resistance (not shown) which is about 10 ohm.
- RPR represents the energy extraction resistance.  This will take the large share of quench energy.
- Switch will be activated by quench detection system.
- Could even be a diode system.
- REXT represents the resistance associated to leads, power supply, etc.

- Solenoid Magnet

- S. Kahn -- Quench Protection

- 3 Nov 2006

## Circuit Parameters

- QUENCH treats the whole magnet.  It does not provide for segmenting the magnet into separate coupled systems.
- The total inductance can be calculated from the stored energy:
- U=1⁄2LI2 where U=20 Mega-Joules and I is the total amp-turns(=2.97107).
- There are 444 layers  175 turns/layer = 77625 turns.
- This gives 280 henrys (big!)
- The resistance associated with 61 km of Ag is 8 ohms.
- We certainly will need to trigger an external resistance into the circuit with a quench is detected.

- S. Kahn -- Quench Protection

- 3 Nov 2006

## Quench Parameters as a Function of External Resistance

- The figures show the following parameters
-  as a function of an external resistance for
-  energy extraction.
- Maximum temperature on conductor
- Time constant for decay
- External voltage on external resistance
- Note that as one increases the external
-  resistance one decreases temperature, but
-  increases the external voltage.

- S. Kahn -- Quench Protection

- 3 Nov 2006

## A Better Approach

- It may be a better approach to divide the magnet radial into separate thermally and electrically isolated systems on separate power supplies.  These systems would be coupled through mutual inductance.
- Each system would contain less stored energy and could have different time constants and different start times.
- Different sub-systems would have different critical currents since they are at different magnetic fields.  Some may not quench at all.
- QUENCH can not simply handle this *complex* system.
- Do the other codes handle this or do we have to write our own?
- QuenchPro may be able to handle this since it treats each turn separately.

- S. Kahn -- Quench Protection

- 3 Nov 2006

## Quench Detection

- Typical quench detection circuits used for LHC and the 25 T NHMFL Solenoid(with HTS insert) trigger at 200-250 mV.
- This corresponds to ~4 cm of “Ag resistance” or 1 sec detection time.
- A “back of the envelop” calculation of T gives ~150°K.
- Caveat:  Cv varies as T3 so one needs to do a proper integration over time which can change this significantly.
- We would like to keep T < 200°K if possible to avoid potential damage to the conductor (from micro-cracking)
- If we can detect a quench at 0.1 sec (trigger at 20-25 mV) we would gain significantly.
- We anticipate that the time constant to remove the field to be ~1 sec.

- S. Kahn -- Quench Protection

- 3 Nov 2006

## What Do We Need to Measure?

- It is clear from this initial calculation that some parameters are not well known.  We should try to measure them.
- Electrical resistivity and heat capacity of HTS conductor as a function of temperature.  This should be done above critical current.
- Same measurements of Silver as a control.
- Determine Ic(B) at high field.  Verify that the critical current relation that we used (which was developed for NbTi, Nb3Sn) works for HTS.
- Measure the quench propagation velocity.  This is important.

- S. Kahn -- Quench Protection

- 3 Nov 2006