# Presentation: 947704

## Quench Protection for Very High Field Magnets using HTS Conductors

- Stephen Kahn
- Muons Inc.
- NFMCC UCLA Meeting
- 1 February 2007

- S. Kahn -- Quench Protection

- 1 Feb 2007

## HTS Magnet Protection—Why is this different from normal Quench Protection?

- In our scenario for a muon collider we (casually) use very high field magnets that can not be achieved using normal Niobium superconductors.
- The NbTi alloy is a ductile material that has reasonably decent mechanical properties.
- Reasonable current to 7T at 4.2 K
- Nb3Sn superconductor is a more brittle material.  Fields to ~17 T.
- It has been demonstrated that HTS conductor can carry significant current at fields as high as 45 T
- HTS conductor is very brittle.  There is a major concern about damage from strain.

- S. Kahn -- Quench Protection

- 1 Feb 2007

## Recent Model of 50 T Solenoid (LTSW06)

- S. Kahn -- Quench Protection

- 1 Feb 2007

## Comparison of High Field Solenoid Parameters

| Quantity |  | EPAC06 | LTSW06 | NHMFL (Insert) |
| --- | --- | --- | --- | --- |
| Central Field |  | 50 T | 45 T | 25 T |
| Insert Field Contribution |  | 50 T | 30 T | 5 T |
| Outer Insert Radius |  | 23 cm | 39 cm | 4.8-8.2 cm |
| Outer Radius |  | 23 cm | 58 cm |  |
| Total Energy |  | 20 MJ | 229 MJ | 16 MJ |
| Total Insert Energy |  | 20 MJ | 51 MJ | 10 kJ |

- S. Kahn -- Quench Protection

- 1 Feb 2007

## Two Concerns with HTS Magnets

- One concern is how to remove this large amount of energy from the magnet in case of an incident.
- The second concern is that the quench propagation velocity for HTS is very slow.  The superconductor can heat up significantly before the quench energy can spread.
- This can cause local irreversible damage to the magnet.

- S. Kahn -- Quench Protection

- 1 Feb 2007

## 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

- 1 Feb 2007

## 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

- 1 Feb 2007

## 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

- 1 Feb 2007

## Critical Current, Field, and Temperature Measurements on BSCCO 2223 Conductor

- 4.2 K

- Data from EHTS Bi-2223 Data Sheets.
- Measurements were performed at GHMFL, France.

- S. Kahn -- Quench Protection

- 1 Feb 2007

## 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

- 1 Feb 2007

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

- 0.85
- Contingency
- factor

- S. Kahn -- Quench Protection

- 1 Feb 2007

## 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

- 1 Feb 2007

## Slide 13

- S. Kahn -- Quench Protection

- 1 Feb 2007

## Slide 14

- S. Kahn -- Quench Protection

- 1 Feb 2007

## Comments on Quench Velocity Measurements

- The quench propagation velocity in this experiment is ~4 cm/sec.  In NbTi it is approximately 1 m/sec.
- If a quench occurs and the heat can not be dissipated, the local area will heat up to a very high temperature.  It can 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 critical condition, the faster the quench velocity.
- Restated:  A conservative design with a large safety factor built-in against quenches occurring could be self-destructive if a quench does occur.
- 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

- 1 Feb 2007

## 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

- 1 Feb 2007

## 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

- 1 Feb 2007

## Circuit Parameters

- QUENCH treats the whole magnet.  It does not provide for segmenting the magnet into separate coupled systems.
- This means that the current has the same time-dependence throughout.
- We shall ignore the Nb3Sn outer coils.
- We assume that they are sufficiently away from critical that they won’t quench.  Also that the Nb3Sn coils will not affect the HTS (this may be a bad assumption).
- The total inductance can be calculated from the stored energy:
- U=1⁄2LI2 where U=66 Mega-Joules and I=129 amps is the single turn current.
- There are 672 layers  250 turns/layer = 167821 turns.
- This gives 8000 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

- 1 Feb 2007

## Quench Simulation

- We want to ramp the magnet down with a single time dependence.  (This is all that *Quench *can do) we would like to segment the magnet into separate circuits with diodes.
- This provides greater sensitivity to quench incidents.
- This will prevent unreasonably large voltages across external resistances.
- Each sub-circuit consists of two adjacent conductor layers.
- Quench detection circuitry can be put on one end of the magnet.
- IGBT switches to include external resistance circuit when the quench is detected.

- S. Kahn -- Quench Protection

- 1 Feb 2007

## 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

- 1 Feb 2007

## 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 10-25 mV) we would gain significantly.
- We anticipate that the time constant to remove the field to be ~1-10 sec.

- S. Kahn -- Quench Protection

- 1 Feb 2007

## An Alternate Approach  to Remove Energy

- When a quench occurs, the detection circuit triggers *some* of the dump resistors.
- Energy is transferred to adjacent coils inductively such that they will induce a quench in those coils.
- The dump resistors of the remaining coils are turned on.
- This approach would spread the quench throughout the magnet without adding energy from external sources as heaters.  This should permit a faster ramp-down of the magnet in a controlled manner.

- This idea was suggested by R. Flora.

- S. Kahn -- Quench Protection

- 1 Feb 2007

## A Program to Study Quench Protection for HTS Magnets

- Muons Inc has submitted an SBIR proposal with Fermilab to study protection of HTS magnets.
- We would like to setup a test station to study quench detection of HTS magnets.
- We would like to investigate techniques to detect quenches in ~10 ms with amplitudes of ~10 mV.
- 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

- 1 Feb 2007