Determination of Radiation Damage Limits to High-Temperature Superconductors in Reactor-Relevant

Conditions to Inform Compact Fusion Reactor Design

B. N. Sorbom, Z. S. Hartwig, L.A. Kesler, S. Jepeal, D. G. Whyte, J. Minervini, M. Short, G. Wright, M. Takayasu, M. Ames, G. Kohse, and ARC Design Team

New superconductors open the path to compact, high field fusion reactors

• Recent commercialization of high-temperature superconductors (HTS) enables possibility of high field reactor design

• Crucial advantage of high field is B4 dependence of fusion power

• ARC reactor [1] exploits high field to achieve the same fusion power as ITER (500 MW) at 1/7th the volume

• Although ARC’s magnetic field is higher, structural stress due to JxB forces is similar to ITER due to ARC’s smaller magnetic volume

• Use of ion beams to emulate neutron damage is an established technique for fission reactor material studies [3]

• Calculations performed with SRIM [4] on HTS sample geometry show approximately uniform damage in superconducting region and no H ion deposition in HTS layer

• Accelerator irradiation/analysis of HTS samples is ~100x faster than fission reactor irradiation, allowing use of ion beams as a screening tool for neutron experiments

Nuclear damage to superconductors limits reactor size reduction

Fission reactor irradiations can be used to test HTS – but are lengthy

and expensive

• All superconductors experience critical current degradation due to neutron radiation damage

• Lack of shielding space in compact designs makes this a critical issue for small reactors (long pulse or steady state)

• Nb3Sn radiation response extensively characterized for ITER [2]• Few REBCO damage studies performed—none in relevant compact

reactor conditions (cryogenic temperature, high fluence, strain)

Ions will be used to emulate neutron damage and screen

neutron irradiation experiments

Beam target stage controls irradiation temperature and ion

profile on HTS sample

This work was supported by the Plasma Science and Fusion Center at MIT. The author is grateful to the PSFC technical and engineering staff for help assembling the ion irradiation chamber and critical current analysis station. TEM work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under award number DMR-1419807. In particular, the author is grateful to Akihiro Kushima for help with TEM sample preparation.

• HTS samples will be mounted on special cryogenically-cooled target stage and irradiated while cold to investigate temperature dependent effects of irradiation defect formation

• 3.5 Tesla HTS magnet designed with field uniformity of >99% at sample location for use in upcoming in-situ accelerator critical current measurements

• Critical current (at cryogenic temp and applied field) will be measured without having to warm and remove sample from accelerator target chamber

• Fission reactors produce a neutron spectrum similar to that seen by a fusion magnet but experiments are expensive and require long cooldown times due to sample activation

• MIT Reactor Lab has been used to irradiate HTS samples to ~1018 n/cm2 for comparison with critical current degradation due to ion irradiation

• First fission reactor irradiations were performed in pneumatic tube insert in reactor pool, secondary irradiations planned in reactor core where neutron energy spectrum more closely matches spectrum seen by fusion magnet

• Heated target stage maintained within +/- 5⁰ C of desired temperature with PID controller

• Uniform irradiation important, as total fluence to tape is calculated by average beam current on target

• Beam profile measured with CCD intensity analysis of beam on gold-coated quartz scintillating window and optimized to ensure as uniform irradiation as possible

• Gaussian beam peak centered on 4x4 mm HTS sample using four symmetric beam current pickups on target collimator

• 77 K, self-field results show clear degradation of critical current with proton fluences on the order of 1015

p/cm2 (degradation corresponds to previously observed neutron results for ~1018 n/cm2 fluence [6])

• Higher irradiation temperature slightly reduces damage with fluence, but more data required to establish clear trend

• Proton irradiation technique has been developed to allow experiments on the order of 1 hour per sample, dramatically reducing experimental turn-around time and allowing a wide range of experimental conditions to be investigated

Proton irradiation degrades critical current

Simplified, axisymmetric MCNP5 simulation of ARC neutron fluence to TF

1014

n/cm2-s

109

n/cm2-s

Schematic of tape irradiation location with respect to reactor core (upper left) and typical neutron flux spectrum observed at the pneumatic tube, reactor core, and MCNP simulation of a fusion magnet.

ITER and ARC to scale

CCD image (top) and 1D slide of intensity through a point on the Y axis shows Gaussian beam profile with intensity falling ≤25% from center within 4mm target width

Comparison of neutron and proton irradiations at different temperatures to create similar damage size distributions in stainless steel (figure from [5]). In this example, the higher temperature of the proton irradiation leads to a lower sink strength in the material, offsetting the fact that proton irradiation by itself would produce fewer interstitial clusters than neutron radiation.

COMSOL magnetic field uniformity simulation for in-situ HTS magnet design. Note second field scale for tape region in center of magnet.

First accelerator irradiation results show degradation of HTS critical current with moderate proton fluences (note suppressed y axis). Irradiation temperatures (denoted by different sets of points) kept constant within +/- 5 degrees C during entire irradiation. Trend line drawn through data to guide the eye.

[1] SORBOM, B. et al, Fusion Engineering and Design 100 (2015) 378.[2] WEBER, H, International Journal of Modern Physics E 20.06 (2011): 1325-1378.[3] WAS, G. S., et al, Scripta Materialia 88 (2014): 33-36.[4] ZIEGLER, J. F., et al, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 268.11 (2010): 1818-1823.[5] GAN, J., et al, Journal of Nuclear Materials 297.2 (2001): 161-175.[6] PROKOPEC, R., et al, Superconductor Science and Technology28.1 (2014): 014005.

Cryogenically-cooled target mount on accelerator beamline. Cold-head can exhaust 6 W at 20 K, absorbing irradiating beam energy without significant target warming.

CCD image analysis of beamspot

15

Fusion Power

Plasma Volume

NormalizedBeta

On-axis Field

Future experiments will damage HTS at prototypical reactor conditions