# Presentation: 487211

## Development and testing of in-vessel magnetic sensors for ITERprogress on in-Vessel pick-up coils

- presented by G. Chitarin (Univ. of Padova and Consorzio RFX)
- acknowledgements to:
- S. DalBello, S. Peruzzo, C. Taccon
- L. Lotto, L. Ranzato, A. Sottocornola

- Development and testing of in-vessel magnetic sensors for ITERprogress on in-Vessel pick-up coils

## Outline

-  background on magnetic measurements at RFX
-  design targets for ITER in-Vessel pick-up coils
-  proposed design concept for equilibrium pick-up coils
-  design rationale & weak points
-  prototype construction procedure
-  prototype tests procedure & results
-  present alternatives and conclusions
-  experience with JET-EP Halo sensors

## JET-EP (Enhancement Projects):

- JET Magnetics Enhancements (2002-2006) :
- in-Vessel sensors (upper coils and outer poloidal limiter coils)
- ex-Vessel sensors (pick-up coils, Hall probes, flux sensors)
- JET Halo Current measurement system: (2001-2006)
- Halo rogowski coils (behind first wall CFC tiles)
- Halo TF pick-up coils (behind first wall CFC tiles)

- background on magnetic diagnostics design at RFX

- ITER :
- Re-evaluation and optimization of the sub-system for measuring Halo currents in ITER (2004)
- Design study of the ITER magnetics diagnostic (2005)
- Inner-vessel tangential and normal coils
- External flux loops
- Halo current sensors
- Steady-state sensors
- Development and testing of Magnetic Sensors (2006)
- Inner-vessel tangential and normal coils
- Blanket and Divertor Halo rogowski sensors

## outline of ITER in-vessel magnetics and pick-up coil design targets

- tangential pick-up coils:
- maximum field			≈ 1 T
- measurement error target 		± 0.7 %
- frequency bandwidth		 10-4 - 104 Hz
- overall sensor dimensions		90mm x 50mm x 45mm

## ITER in-vessel pick-up coil requirements and design parameters

- typical voltage output				300 mV
- peak output voltage @ 100T/s 			30 V
- max flux					0.3 Vs
- allowable flux error				2 mVs
- allowable voltage offset for 3600s operation 		< 1 V
- typical RIEMF/TIEMF temperature coeff. *		 ≈0.5  V/K
- allowable T within the coil due to nuclear heating		< 10°C
- RIEMF = radiation induced electromotive force; TIEMF =  thermally induced electromotive force
- (see papers by G. Vayakis, T. Sugie, R. Vila, E.R. Hodgson, R. Van Nieuwenhove and L. Vermeeren in references)

- operating temperature 				≈150-200 °C
- maximum temperature				600 °C
- temperature excursions (500 cycles)			200°C
- neutron flux					< 3 1018 n/m2 s @blanket gaps
- max nuclear heating 				≈1 W/cm3  (in steel)
- vacuum outgassing rate (one sensor)			<10-11 Pa/m3s
- typical electric field in the insulation			< 250 V/mm
- rated voltage for insulation to ground 			 (*tbd) 500 **V *(JET required 1 kV)
- tolerable neutron fluence	for permanent sensors		 (*tbd) **10*24 * **n/*m2

## JET in-vessel pick-up coils

- Original design concept of ITER magnetics (DDD 5.5) based on Mineral Insulated Cables (MIC), similar to those successfully used for JET in-vessel magnetics [http://users.jet.efda.org/pages/mags/index.html].
- However:
-  MIC can have large RIEMF/TIEMF  coefficient
-  nuclear heating and difficult thermal contact between MIC imply large temperature gradient in the MIC winding pack larger.
-  improved thermal anchor and copper felt around the winding pack are not sufficient to reduce T below 100 °C
- 	integration drift due to RIEMF/TIEMF voltage exceeds acceptable measurement error
- 	improved design with low thermal gradient required for ITER in-Vessel pick-up coils

## Qualitative analysis for thermally improved in-vessel pick-up coil

- worth being tested first

## Outline of the improved ITER in-Vessel pick-up coil

- Rationale:
- reduce RITES/TIEMF voltage, which is directly related to the thermal gradient in the coil, caused by nuclear heating
-  avoid vacuum spaces between conductors
-  select materials for high thermal conductivity and low TIEMF cables
-  reduce paths to thermal sink (ITER vacuum vessel)
-  reduce coil and reel mass (in order to reduce the nuclear power generated)

- Implementation concepts:
-  Fluid ceramic filling ("potting") for improved thermal conductivity between turns and to the external case
-  Flexible ceramic-coated conductors (thin inter-turn insulation)
-  thin stainless-steel or machinable ceramic reel
-  copper external Case and Endplate providing good thermal anchor to the vessel
-  ground insulation only if necessary (plasma sprayed or fibreglass tape)

## Expected advantages / disadvantages of the design based on potted flexible ceramic-coated conductors

- Expected advantages:
-  better thermal conductivity of the winding pack (conductor volume/total volume ratio ~10 times larger )
-  larger conductor cross-section, lower coil resistance and larger signal with the same sensor volume
-  expected lower RIEMF/TIEMF in comparison with MIC conductors (to be confirmed)
-  porous ceramic insulation constitutes a good surface for the adhesion of the fluid ceramic
- Insulation withstand voltage and reliability expected to improve with potting
- Expected disadvantages:
-  conductor insulation layer is very thin (< 0.1 mm) and potentially delicate
-  insulation rated voltage is < 600 V at best (lower than usually required for insulation in JET) supplementary ground insulation is necessary if this level is considered not large enough
-  conductor insulation is porous and can absorb moisture from the air or dirt during handling, bakeout in vacuum necessary
-  transition to other kind of conductor inside the pick-up coil copper Case is necessary

## Thermal analysis on ceramic potted pick-up coil

-  T can be reduced below 10 °C, i.e. below the expected background thermal gradient values
-  thermal conduction coefficients of ceramic filling contact with conductors and case still to be verified by experimental tests
-  further thermal optimization useless if presently assumed conduction coefficients are confirmed

## Choice of ceramic-coated flexible conductors

- Two types of cables have been proposed for the prototype tests (manufacturer's specifications):

- Both are worth being tested

## Choice of potting fluid ceramics

- A large number of fluid ceramic compounds is available on the market
- based on previous experience with JET magnetics, two types of fluid ceramic have been proposed for the ITER prototype tests:

- a protective & insulating ceramic paint (CorrPaint from Aremco) has also been tested, but abandoned because of large grain size

## Critical issues of the potted ceramic-coated conductors

-  Viscosity of the fluid ceramic and complete filling of the winding pack during potting process
-  Adhesion of solidified ceramic to ceramic conductors, stainless steel and copper
- Formation of cracks and cavities in ceramics during solidification (affecting equivalent thermal conductivity of winding pack)
-  Effect of potting fluid on conductor coating (affecting/increasing withstand voltage of insulation)
-  Different thermal expansion of solidified ceramics, copper and stainless steel, behavior under thermal cycling
-  Degassing rate of potted winding pack
-  RIEMF/TIEMF properties of ceramic-coated conductors
- chemical compatibility of ceramic coating with ITER in-vessel environment, behavior of ceramic insulation material under neutron radiation,
- Prototype construction and test necessary
- Tests on prototypes are in progress at Consorzio RFX  for issues 1-6 in the frame of EFDA task.

## geometry of prototype pick-up coils

*Coil case with slot to limit eddy currents (not used for first prototypes)*

*Coil winding*

*Coil reel*

*AISI304  wire cloth cover*

*Plate with openings for ceramic filling*

## fluid ceramic "impregnation" procedure

- Impregnation test rig

- slitted reel covered with stainless steel wire cloth (permeable to air but not to fluid ceramic), stainless steel might be substituted with insulating fibreglass fabric
- coil assembly "vacuum impregnated" with radial fluid ceramic flow
- fluid ceramic cured at 370 °C

## Prototype test plan

- Insulation resistance test of conductor samples (after vacuum bakeout) up to breakdown voltage level.
- Insulation resistance test of potted prototype coils up to rated voltage level.
- Thermal test with measurement of thermal conductance of winding pack
- Insulation resistance test of potted prototype coils up to breakdown voltage level (before or after thermal cycles)
- micrographic analysis of coil section for the evaluation of  bubbles and cracks (before or after thermal cycles)
- residual gas analysis during out-gassing
- X-ray tomography (if necessary)
- measurement of magnetic effective area and sensitivity to transverse fields
- total number of  prototype coils ≈ 20, depending on results, first prototypes without copper case.

## (original) workplan for in-vessel pick-up coils R&D

- perform test cycle on initial series of prototypes
- evaluate first test results
- define transition and connections to MIC or Sulzer cables
- update design and drawings on the basis of the first experimental results
- prepare technical specifications for construction of new prototypes
- proceed with second test cycle
- evaluate second test cycle results
- assess performances of optimal design

## Results of electrical tests on conductors samples

- 	Wire #1
- After curing for 45 min at 700°C, the insulation is transformed into a thin layer with the consistence of ash
- insulation resistance value is 600 MOhm at 100V.
- Insulation breakdown occurred at a voltage value of ~ 250 V, well below the rated voltage of 600 V.
- Damage of the insulation was found in several points of the wire just after delivery. Electrical tests were carried out on specimens of wire apparently undamaged.

-  	Wire #2
- insulation resistance to ground quickly diminishes during the application of voltage and its value is low (≈ 1M) even at few tens of Volt.
- Insulation breakdown occurred at a voltage level <50 V, well below the rated voltage of 200 V
- effect of humidity adsorption from the air seems to be little related to the problem, tests repeated after longer vacuum bakeout at 140 °C gave marginal improvement

- Manufacturers have been have been questioned, but have given little help so far.
- Manufacturer's specifications not confirmed by tests.

- 0.5 mm

- 0.5 mm

## The  results of the insulation tests on potted prototype coils were not better than those of the conductor samples, in all cases the coil insulation brakes down below 50 V.

- Insulation resistance tests on conductor samples and on  the prototype coils

- Ceramawire conductor insulation after vacuum bakeout

- Calfinewire Conductor insulation resistance after vacuum bakeout at 140 °C

## Scheme for measuring the thermal conductance of the winding pack

- +
- -

- vacuum oven with active
- temperature control

- Regulated
- Power supply

- Pick-up coil with
-  copper case

- Resistance meter

- +
- -

- Thermocouples

- switch

- Temperature meter

- Coil heating power controlled by driving a DC current in coil conductor
- Coil external temperature controlled by vacuum tank and measured with thermocouples
- Coil average internal temperature measured by switching off briefly the power supply and recording the resistance variation with respect to a calibration value obtained  at zero power and uniform oven temperature.

-  Measurement of heat conductance of winding pack not possible on prototypes, due to conductor insulation failure

## preliminary micrographic analysis of coil section

- Section of the coil showing that:
- the winding pack is filled with fluid ceramic
- only in a few areas the space between conductors is not filled with ceramic
-  during the cutting process a large deformation of the reel has occurred, which  caused the detachment of the winding pack from the outer ceramic layer and from the inner stainless steel fabric layer

## micrographic analysis of coil section

-  the adhesion of cured ceramic is poor when the ceramic layer is thin, the ceramic tends to be pulverized under stress.
-  the "impregnation" technique shall be improved to ensure that all volume is properly filled
-  the sectioning technique should be improved, precision cutting machine purchased for repetitive tests, x-ray tomography might be used

## Conclusions

-  Continue with present design concept, with suitable ground insulation layer
-  Alternatives presently under consideration:
**flexible conductors with braided fibreglass insulation** "P.O.Zh. conductor", produced by a Russian manufacturer. Very promising, does not require major modification of the present design.
**stack of ceramic plates with printed metallic deposit**. The technology for the metallic deposit of printed circuits on ceramic substrate is available for microwave and some automotive applications. Good electrical and thermal properties are expected, RIEMF/TIEMF properties of metallic deposit and of the brazing/welding  between adjacent layers should be verified. The design of the  pick-up coils shall be completely changed. (see also Takahashi [4])
- bare conductors with solid insulating spacers (fluid-ceramic potted or not).
- flexible ceramic-coated wires with plasma-sprayed ceramic between each layer of conductors. Even if the turn-to-turn insulation is poor, the plasma-sprayed ceramic should ensure good insulation to ground and between layers. Requires collaboration with a plasma-spray industry.

## references

- [1] G. Vayakis, T. Sugie, at al. “**Radiation-induced thermoelectric sensitivity (RITES) in ITER prototype magnetic sensors**” Review of Scientific Instruments Volume 75, Number 10 October 2004
- [2] R. Vila, E.R. Hodgson: “**RIEMF in MgO and Al****2****O****3**** insulated MI cable ITER magnetic diagnostic coils**” , Journal of Nuclear Materials 329–333 (2004)
- [3] R. Van Nieuwenhove and L. Vermeeren, “**Experimental study of radiation induced electromotive force effects on mineral insulated cables**”, Review of Scientific Instruments Volume 74, No 11 November 2003
- [4] H. Takahashi, S. Sakakibara, Y. Kubota, H. Yanada "**Magnetic probe construction using thick-film technology** " Rev. Sci. Instrum vol 2, n 8 2001
- [5] Design study of the ITER magnetics diagnostic TW4-TPDS-DIASUP, EFDA 04-1210 contract, D6.2 Final Report (Association Euratom-ENEA, Consorzio RFX, Padova, Italy) Jun 2006.
- [6] Thermo-mechanical tests for ITER in-vessel tangential pick-up coil prototypes (RFX internal report IP-NT-159) 6/11/2006
- [7] Procedure of electrical tests on ceramic coated conductors and on prototype in-vessel coils for ITER (RFX internal report SM-NT-278) 7/11/2006
- [8] Results of electrical tests on ceramic coated wires (Calfinewire) (RFX internal report SM-NT-279) 07/12/2006
- [9] Results of electrical tests on ceramic coated wires (Ceramawire) (RFX internal report SM-NT-280) 25/1/2007

## JET-EP- Halo current sensors 

- Up to 8 rogowski coils

- 8 modified CFC tiles (modified to house the rogowski coils)

- Up to 2 toroidal field pick up coils

- 1 base plate (supporting both the sensors and a conduit along the poloidal direction)

- 1 articulated conduit to Remote Handling plug & socket

- 1 toroidal conduit (22.5° long) from RH socket to the feedthrough

**JET-EP- Halo current sensors**_ _

## JET-EP Halo current Rogowski sensors

**JET-EP Halo current Rogowski sensors**

- ~ 100 mm

- Rogowski equivalent area ≈0.03 m2
- Rogowski constant ≈10-7 H

## JET-EP Halo current Rogowski sensors 

**JET-EP Halo current Rogowski sensors**_ _

## P.Sonato, EP-DIA-HCS-R-063 DVCM  Meeting, 13/02/2007 Slide 16

- typical drift ≈ 1 kA/s
- typical offset voltage ≈ 100 V

## P.Sonato, EP-DIA-HCS-R-063 DVCM  Meeting, 13/02/2007 Slide 26

**JET-EP- Halo current sensor: signal drift (detail)**

## P.Sonato, EP-DIA-HCS-R-063 DVCM  Meeting, 13/02/2007 Slide 17

**JET-EP- Halo current sensors - effect of braided cables (Sulzer cables) with no shielding**_ _

## JET-EP- Magnetic sensors: upper coils 

## Thank you for your attention