DESIGN STUDY OF INDUCTION COIL FOR GENERATING MAGNETIC FIELD FOR CANCER HYPERTHERMIA RESEARCH V. Nemkov, R. Ruffini, R. Goldstein, J. Jackowski

– AMF Life Systems, LLC, Michigan, USA

T. L. DeWeese, R. Ivkov

- Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine

Overview • Coil Design for Low Volume In Vitro and Small Animals Research • Coil Design for Large Volume In Vitro Research • Magnetic Field Distributions for 2D and 3D models • Parameter Comparison for Different Coils • Temperature Distribution in Magnetic Core • Conclusions

Induction Coil for Low Volume In Vitro and Small Animals Research 5 4 3 -2 -3 -4 -5 0 -1 2 1 0 200 400

Field Strength (Oe)

600 Coil features: - Planar turns with gap variation Fluxtrol magnetic “caps” on the coil ends Magnetic field distribution along the center line

Magnetic Field Mapping Power supply 3 kW Induction coil with Field Probe on the stand

Large Volume Cell Culture Coil • Goal: Design an inductor with even flux density for heating of culture specimens • The region of concern is a specimen holding dish (24 or 96-well dish) • Frequency must be 140-160 kHz • Max flux density B m =400 Gs • Thermal influence of the coil on the cell dish must be minimal

Concept of New Induction Coil Cooling plate Coil tubes Magnetic controller Coil “opening” dimensions: A x B x H = 110 x 175 x 40 mm

Inductor with Magnetic Core

Challenges:

- 3D System - Intensive heating of magnetic core due to strong field, high frequency and long cycle time

Core temperature control:

1. Material selection with account for orientation 2. Intensive heat transfer to copper through a layer of thermo-conductive epoxy compound 3. Use of additional cooling plate 4. Coil copper design with reduced 3D effects

Flux Density Map of Rectangular Coil with Magnetic Core Flux 2D program

Temperature Maps in 2D Approach Tmax = 240 C Tmax = 140 C a – Core of Fluxtrol 50; b – Core of oriented Fluxtrol 75 Flux density 400 Gs

Induction Coil with Extended Cross Legs Cooling plate Extended Cross Legs Slot for thermal protection screen

Temperature Prediction for the Core Made of Oriented Fluxtrol 75 a Uniform coil winding b Winding with widened cross-over leg

Electrical Parameters for Helmholtz Coil and Rectangular Coils Coil Type Helmholtz Rectangular Rectangular

Widened Cross-Leg

Core None Fluxtrol 50 Program Flux 2D Flux 2D Fluxtrol 50 Flux 3D

Fluxtrol 75 Flux 3D

Bm (Gs) 400 400 U (V) 1750 650 I (kA) 8.4

3.8

400

400

720

660

3.3

3.5

S (MVA) 14.7

2.5

2.4

2.3

P (kW) 74 24 26

25

Laboratory Tests Power supply 25 kW Frequency 150 kHz Used power 18 kW Coil head voltage 480 V Magnetic field density 280 Gs Maximum core temperature 100 0 C

Magnetic Flux Density Distribution Plot of magnetic flux density through the center of the inductor

Induction Equipment at JHU Inductor and capacitor battery Power Supply 80 kW 15

Summary • Induction coils for small volume tests require careful manufacturing to provide uniform magnetic field in test area; power supply may be small – 3 -12.5 kW for field density 500 - 1000 Gs • Design of induction system for large cell-well plates is a challenging task • Helmholtz coils require much higher reactive power (6x), active power (3x), voltage and current than a special coil with magnetic concentrator • 2D simulation resulted in overvaluation of coil current (24%) & undervaluation of voltage (10%) vs. 3D

Summary • 3D effects lead to significant increase of the magnetic core temperature especially in the corners • Extension of cross leg copper significantly reduces 3D effects and diminishes local flux density and core temperature • Special attention must be paid to magnetic material selection, orientation and application technique • Fluxtrol 75 with optimal orientation and thermally conductive glue provides the best results • Results of the coil tests were in good agreement with predicted values

Acknowledgement This work was funded by a grant from the Prostate Cancer Foundation