Electro-magnetic discharges through magnets – studies

and proposal

Sandor Feher, Stoyan Stoynev

Emmanuele Ravaioli

October 2017

IntroductionWe want to study the possibility of superconducting magnet

“preconditioning” by EM discharges

The idea is to over-stress a magnet/coil with over-current ✓ i.e. put a current larger than the quench current at that “instance”

We are using a CLIQ unit (developed by CERN) to provide the charge testing purposes consistent with the current CLIQ system parameters

First we reproduce initial CLIQ tests (commissioning) on a warm magnet

Then we use diodes (part of CLIQ configuration at VMTF) in reverse and do more testing on the warm magnet

Later we test a CLIQ discharge through a superconducting (cold) magnet

In all cases we measure the magnet current and the field in the bore with the idea to detect any delay between the two above few milliseconds

Current-Field relations As presented by Emmanuele (and others) earlier, the magnetic field in the strands

does not change immediately when the current change is applied due to local inter-filament and inter-strand coupling currents. The magnetic-field change delay depends on the coupling current time constants and is typically up to tens of miliseconds

We want to measure what we

see at macro level✓ Ideally we want to compare with calculations

We start with measurement in the bore

We are planning measurements

close to the coil/cabledBa/dt is the applied field change, directly proportional to the current change.dBt/dt is the effective field change, including the contribution of coupling currents opposing the applied field.

From Emmanuele’s thesis

Warm magnet

Magnet: BDA009

Same magnet used for initial CLIQ commissioning in 2016

R = 194 mOhmL = 90 mH at 1 kHz (140 mH at 100 Hz)Save to operate below 300 AHipotted to 500 V

Reproducing results from CLIQ commissioning Initially we calibrated the probes finding

- 1200 A/V for the current meter

- Hall probe calibration varies depending on the meter scale set

(absolute scale is unimportant in the context of our measurements)

Multiple tests at different voltages upto 400 V with CLIQ

discharges from 80 mF capacitor banks

Comparisons with the previous tests show good consistency

Delays between current and field (if any) are below miliseconds

The reproduced tests were performed on April 6th and 7th 2017

Original tests were from May 2016

Reproducing results from CLIQ commissioningTest reproduction Original testBoth at 400 V, 80 mF

+ filed + current

T 700 msI 250 AT 700 ms

I 250 A (1200 *2.2*100/1000) 1. We reproduce the current characteristics

2. We find no significant delay between current and field(as expected)

Warm magnet tests with diodes A set of 20 diodes from VMTF was used and connected to the magnet

in parallel to the CLIQ unit

We tested both polarities but report only one (“reverse”)

which is relevant

Zoomed version on the next slide

CLIQ current sudden drop is a feature of the unit protection design.

Warm magnet tests with diodesIn blue is the ratio between scaled filed and current

As seen there is no systematic deviation from a constant in the field/current ratio – no delay between current and field

As also seen the test configuration allows to keep relatively high field (and hence Lorentz force) for significant amount of time (hundreds of miliseconds) – depends on the time constant

Cold magnet and CLIQ

MQXFS1c used to discharge the CLIQ unit (at the end of testing, 0 A) “One unit” (standard) configuration used in testing

✓ As on the drawing Hall probe inserted in the warm bore

✓ Tightly attached to the shaft for magnetic measurements

One unit CLIQ configuration

MQXFS1 and VMTF

Hall probe

Shaft

Guide to bore

Cold magnet testing

In addition to oscillations some magnetization (and relaxation) effect is also observed

Maximum allowed current from CLIQspecifications is 6 kA

400 V, 80 mF

Cold magnet testing

No current-field delay at few miliseconds level

T/2 24 ms

T/2 23 msDelay < 2 ms

Sinusoidal part of the function

Cold magnet testing proposal Attach Hall probe sensors to the pole and over the coil

✓ Two on poles and two over coils✓ Preferences for which coil (it will not be coil 3)?

Repeat the cold testing with CLIQ discharge and record data✓ 0 A current✓ 400 V, 80 mF

Redo at 20 K to compare frequency (inductance) to predictions

Discharge CLIQ at higher current (~8 kA) with delayed heater and energy extraction and record the data

Repeat the cold testing with CLIQ discharge with copper leads and diodes disconnected and record data✓ 0 A current✓ Up to 400 V, 80 mF (start with low voltage)

Compare delays to predictions (if any)

In future, the idea would be to introduce over-current and over-stress for many milisecondsbefore the protection brings the current down; not for this test

Hall probes

Sensor mounting

Magnet(coil)

Warm

finger

POLE COILCross-sectional view

Face view

Side view

Side view

Sensor views with wires (representations)

Mechanical support(design work on-going)

Two sensors on the pole to be glued from the non-Lead end.A connector to be glued on the end part (not on the coil).

5 mm

The radial length of the sensor is 5 mm and there is 11 mm clearance

Picture of the warm finger (4.5 m long)

Sensors will be in LHe

Air

LHe

Sensor positions

Suggested pole positions

This is the non-Lead end

Only two hall probes will be glued to the pole (over the existing insulation; cleaned).There is no particular requirement where the probes should be on the pole and on which pole. The easiest mounting positions will be chosen.

Connector(s)

A test version of the configurations is being prepared.

Work in progress Designing support for the hall probe on the warm finger

✓ clearance

✓ robustness

Determination of mounting points and mounting procedures ✓ Safety

✓ Reliability

✓ “Matching” to coil positions

Choice of connectors and compliance with procedures

Work on the warm finger is virtually independent on work on the magnet.

The two hall probes mounted on the warm finger are to be “above” coils (likely close to poles), lead end.

Sensor positions – earlier considerations (abandoned)

COIL POLE OR END PARTCross-sectional view

POLE COILCross-sectional view

Face view

Side view

Side view

Sensor views with wires (representations) G10 piece to glue over the metal part

One or two sensors on a coil and one or two on the pole to be gluedG10 piece glued to the pole onlyAttachment from the non-Lead end

5 mm

The radial length of the sensor andsubstrate is 5 + X mm and there is

at least 11 mm clearance (to verify)