Snowmass Fusion Meeting - CTD Magnet Insulation Input

Snowmass Fusion Meeting 1999 - CTD Magnet Insulation Input

Large, capital intensive magnets, superconducting or resistive, are essential components of fusion devices. Magnets built for fusion devices need to be reliable, possess long mean time between failures (MTBF), and be able to be manufactured using cost-effective materials and fabrication processes. Electrical insulation is often the weak-link in the design of magnets, due to its sensitivity to high radiation doses, embrittlement at cryogenic temperatures, and limitations in fabricability.

Current magnet insulation schemes typically utilize a glass/epoxy composite. These materials provide sufficient electrical insulation, suitable mechanical properties at magnet operating temperatures, flexible processing for cost-effective coil fabrication and assembly, and reasonable cost. However, these epoxy resins degrade to unacceptable levels of performance when exposed to high levels of radiation, particularly dose levels being considered for next generation fusion research devices and future commercial devices.

The combined ‘end-of-life’ radiation exposure limits estimated for the magnet insulation for the International Thermonuclear Experimental Reactor (ITER) device was 10 MGy (1.0 x 10⁹ Rads). Irradiation studies performed during the ITER engineering design activity (EDA), which tested candidate insulation materials to approximately 23 MGy (2.3 x 10⁹ Rads), identified that epoxy based insulation systems suitable for both vacuum-pressure impregnation (VPI) and pre-impregnated glass fabrics (pre-preg) provide suitable radiation resistance to meet the ITER requirements. However the performance of these materials falls off dramatically with exposure to radiation levels exceeding the ITER limits.

Lifetime radiation fluences for commercial reactors are currently limited by the degradation of the critical properties of the superconductor. According to Sawan at U. of Wisconsin the limits on fast neutron fluence to the superconductor are 1e19 n/cm2 for Nb3Sn and 1e18 n/cm2 for NbTi. The limit for Nb3Sn corresponds to a combined does (neutron + gamma) of approximately 100 MGy(1 e10 Rads) (within a factor of 3). Potentially the insulation may be located a few centimeters in front of the conductor, resulting in a slightly higher dose. Based on this analysis, Sawan suggests that it may not be necessary to push insulation beyond ~500 MGy(5e10 Rads), unless a different conductor is developed that can withstand higher radiation fluences.

Insulation materials exist (epoxies developed and tested for ITER) for the cost-effective application for ≤23 MGy(2.3e9 Rads). Polymer materials have been tested to >100 MGy(1e10 Rads) and demonstrated little or no degradation to shear strength of glass reinforced composites. However, the polyimides (PI) and bismalimides (BMI) that survived these radiation levels are difficult and expensive to process. Alternatively, inorganic and ceramic materials have shown the capability to withstand radiation doses >500 MGy(5e10 Rads) but the mechanical performance of these materials are not comparable to those of the glass/epoxy composites and do not meet the requirements for magnet insulation.

Recent ground-breaking work performed by CTD studied the hybridization of insulation chemistry. The goals of this work was to increase radiation resistance of the insulation while maintaining similar properties obtained with glass/epoxy composites and enable similar processing techniques (particularly vacuum-pressure impregnation) utilized with glass/epoxy composites.

Hybrids of different organic polymers including epoxies, polyimides, bismalimides, and other aromatic based polymers were successfully demonstrated. Furthermore, hybridization of inorganic with organic materials were also demonstrated. The former materials are well suited for magnets manufactured from resistive conductors or NbTi; while the latter materials are well suited for Nb3Sn and potentially HTS conductors because these materials enable a wind, insulate, and react fabrication scenario rather than the wind, react, and insulate approach required with currently used glass/epoxy insulation systems.

The evaluation of the hybrid organic polymer materials have shown that they can be applied using cost effective vacuum pressure impregnation processing (VPI); provide mechanical properties similar to glass/epoxy composites; and due to the high aromatic content promise to provide better radiation resistance when compared to epoxies and approaching those that have been demonstrated with PI’s and BMI’s. Radiation studies on these hybrid chemistries are still required to confirm the estimates of radiation resistance.

Hybridization of inorganic materials with organic materials show great promise in providing resistance to high levels of radiation, improved mechanical properties when compared to glass/epoxy composites, and enable coils fabricated with conductors requiring heat treatment to be insulated prior to heat treatment (currently these coils are heat treated then insulated). The primary premise for this inorganic/organic hybrid material was that a reduced organic content of the insulation would result in resistance to higher levels of radiation. Secondary advantages that have been realized with the inorganic/organic hybrids are improved mechanical performance, and simplification of the coil fabrication due to the ability of this insulation to survive the heat treatment process required for Nb3Sn superconductor. Radiation and coil fabrication studies on this inorganic/organic hybrid insulation are on-going. Radiation results are expected anytime, while coil fabrication studies will continue through CY99. Additional information on the performance of this inorganic/organic insulation can be found on CTD’s web site at www.CTD-materials.com, under technical papers.

All inorganic or ceramic insulation systems which can potentially provide even higher levels of resistance to radiation and improved dielectric strength are still in the conceptual stage. These materials are currently limited by their mechanical performance and the very costly methods required to apply them. Furthermore, depending on the performance of the hybrid materials currently under development, and the actual ‘end-of-life’ radiation exposure anticipated for fusion magnet insulation, it may not be necessary to develop an all inorganic magnet insulation system.

Considerations for the development and selection of magnet insulation materials for future fusion devices should focus on:

1. the ability of the insulation to meet the magnet designers technical requirements; and

2. use of a particular insulation for a specific coil/conductor configuration should improve the economics of magnet fabrication by

- reducing risk, and

- not necessitating the use of costly and difficult processes for applying the insulation.

Magnet designers should work closely with developers of magnet insulation to enable the use of the most efficient coil design for which a suitable insulation material can be developed.

Most likely a single solution for magnet insulation will not be possible or cost-effective for all conductors or magnet designs. Magnets for fusion are designed for fabrication using several different conductor types, different magnetics requirements, different coil geometries, and different radiation levels. The unique combination of these magnet aspects may result in selection of different or even unique insulation materials for the different fusion magnets. Table 1 lists the various different conductors available for use in near-term and long-term fusion magnets. Included in the table is an indication of the commercial status of the conductor, whether it is being considered for use in near-term or long term fusion magnets, and estimates of what the radiation limits are for the various conductors.

Table 1: Candidate Conductor Materials for Fusion Magnets

Conductor Type	Current Commercial Status	Expected to be used for near-term Fusion Magnets	Long-term Future Commercial Fusion Reactors	Radiation Exposure Limits [n/cm2]
Resistive Magnets (Generally Copper conductor)	Yes	Yes	Not likely	economic limit based on didipated power
NbTi Conductor	Yes	Yes	No	3e18
Nb3Sn Conductor	Yes, limited production	Yes	Possibly	1e19
A15 and superconducting alloys	No	No	Not likely	Unknown
HTS Conductor	For a few select applications	No	Most likely	Unknown

A variety of insulation materials have been used for fusion magnets. New concepts are being developed, while still other approaches have only been conceptualized. Each insulation approach has its applicability with different specific mechanical and electrical performance, processing requirements, maximum allowable radiation exposure, and magnitude of radiation induced gas evolution. Table 2 lists many of these insulation types, their commercial status, suitable fabrication processes that can be used to apply them, their anticipated radiation limits, tested or anticipated ‘end-of-life’ mechanical performance, and expected range of radiation induced gas evolution.

Table 2: Insulation Types Suitable for Different Fusion Magnets

Potential Insulation Solution	Status of Insulation Materials	Insulation Installation Process	Expected Radiation Exposure Limit	Range of Anticipated ‘end-of-life’ Properties	Gas Evolution After Exposure to ~2MGy Electrons [cm³g^-1MGy^-1]
Glass/Epoxy	Existing, commercially available	VPI Pre-preg Combination	10⁹ Rads	Shear: >10 ksi⁽¹⁾ Compression: >180⁽¹⁾ kV/mm: > 60⁽¹⁾	0.2 to 1.3⁽⁵⁾
Glass/Aromatic Organic Resins (Polyimide and Bismalimides)	Existing, commercially available	Pre-preg	10¹⁰ Rads	Shear: > 16 ksi⁽²⁾ Compression: >160 ksi⁽²⁾ kV/mm: > 60⁽²⁾	0.32 (single data point)⁽⁵⁾
Glass/Hybrid Aromatic Organic Resins (Polyimide, Bismalimides, Epoxies, and others	Under Development	Pre-preg Combination	10¹⁰ Rads	Shear: >16 ksi⁽³⁾ Compression: >180 ksi⁽³⁾ kV/mm: > 60⁽³⁾	?, thought to be similar to aromatic organic resins⁽³⁾
Inorganic/Organic Hybrids (reduced organic content)	Under Development	Pre-preg VPI Combination (requires high temperature processing)	>10¹⁰ Rads	Shear: > 9 ksi⁽³⁾ Compression: > 160 ksi⁽³⁾ kV/mm: > 70⁽³⁾	?, possibly lower⁽³⁾
All inorganic Systems including ceramics	Conceptual	Pre-preg VPI Spray Dip Coat Combination	possibly ≥10¹³ Rads	Shear: N/A⁽⁴⁾ Compression: N/A⁽⁴⁾ V/mm: N/A⁽⁴⁾	?, most probably lower⁽³⁾

^{(1) U.S ITER Insulation Irradiation Program Final Report
^{(2) Data extrapolated from testing performed by Harold Weber with ITER Insulation Characterization Data
^{(3) Engineering estimate by CTD

^{(4) Values for currently available materials not suitable, the hope is to develop new materials with better properties}

^{(5) Gas Evolution from Potential ITER Insulating Materials, US ITER Insulation Irradiation Program}}}
The availability of an appropriate insulation material that provides suitable mechanical and electrical properties after ‘end of life’ doses which can also be cost-effectively applied to the magnet conductor can have significant impact on the performance, reliability, and cost of building magnets for fusion applications. Recent developments of hybrid materials have shown promise for providing insulation materials that provide good mechanical and electrical performance, utilizing low cost fabrication processes, with the promise of providing sufficient radiation resistance for near-term and potentially longer-term fusion devices. Longer-term development of all inorganic insulation systems could provide the most robust radiation resistance, but developments need to be carried out to enable these materials to provide suitable mechanical properties.}