CEM Engineers Successful ITER Electron Cyclotron Emission Diagnostic System Hot Calibration Source Design

CEM engineers are a part of a global team working on the International Thermonuclear Experimental Reactor (ITER).  Fusion requires high temperature and pressure to merge smaller atoms and release energy.  The temperature of the reaction chamber is critical to an effective process.  Precise calibration is required to obtain accurate temperature readings of the chamber plasma.  Fusion chamber temperatures must be measured using an Electron Cyclotron Emission (ECE) diagnostic system.  A microwave receiver in the ECE system is continually calibrated using a hot source of known temperature and emissivity in the fusion chamber.  CEM engineers recently completed the first prototype hot calibration source for ITER, bringing fusion one step closer to reality. 

Without proper ECE calibration, it will be impossible to accurately know the fusion chamber plasma temperature, and thus properly electromagnetically contain the plasma.  If the plasma is not properly contained, a disruption in the current that runs through the vacuum chamber occurs.  This disruption not only makes the fusion process less effective, but can also lead to large vibration and jarring to the system that could damage other parts.

The ITER-ECE diagnostic system actually includes two calibration sources permanently located within the ITER Diagnostic Shield Module (DSM). They provide calibration radiation directed by moveable mirrors towards optical paths of two complementary plasma views. They are designed to a set of requirements that assure proper calibration of the ECE instrument when ITER is not operating. The main calibration requirements include a high-emissivity surface heated to 700 0C, 5000 hours operational lifetime over 20 years, and ability to perform the calibration in the presence of a magnetic field. Long term materials stability and compatibility with ITER vacuum are also required.  Major design drivers include a heater current limit of 40 A, no direct fluid cooling to mitigate failure risks, small size to avoid compromising neutron shielding, and adequate structural support to mitigate vibration loads.

Each calibration source consists of an engineered emissive surface to generate blackbody emission, a heating element to control the temperature of the emissive surface, heat shields, housing, power cables, and temperature sensors for feedback control. Early R&D focused on the heater and started with a commercial-off-the-shelf Inconel heater. Figure 1 shows a model of the test prototype set-up for radiant heat transfer from the heater to the emitter and a picture of the prototype assembly ready for installation and testing in the vacuum chamber.

Test Prototype
Fig. 1.Model of the calibration source test prototype (left) and actual prototype (right).

After extensive testing, temperature limits, long term emissivity instability, and material vaporization issues prompted consideration of alternative heaters. Further research and testing led to a custom-designed encapsulated heater (Figure 2) using molybdenum discs with flat and v-grooved surfaces, inserted between the molybdenum wire heater and the back surface of the emitter, to bring the SiC emitter to the required temperature (700 0C).

 vacuum chamber
Fig. 2.The encapsulated molybdenum heater (left) and model of the test prototype inside the vacuum chamber (right).

Preliminary Hot Source Design

The preliminary design of the prototype calibration source uses a fully integrated approach that combines high thermal performance, long life, and shock tolerance into a compact space. A preliminary model of calibration source prototype is shown in Figure 3.

 

preliminary design

Fig. 3.Model of the preliminary design of the prototype calibration source.

In addition to numerical simulation, CEM researchers work closely with bus vendors to get field-measured data on zero-emission buses, as a means of improving model fidelity in PSAT. Key model parameters, including motor efficiency, converter efficiency, and accessory load, are tuned to improve the bus model accuracy. The proposed modeling approach is a promising method for transit bus energy efficiency studies. The developed numerical models can be used to plan fast charging stations and fleet schedules. Validated bus simulation models are a valuable resource to insure the successful transition of electric bus technology to market.  It is an essential method for of studying the grid impact of the public transportation electrification. In June 2017, the detailed study results were presented at IEEE International Transportation Electrification Conference and Expo (ITEC) at Chicago IL.

The major parts of the design are: the mounting foot [I], the top plate [P], the heater assembly [N], the emitter support fingers [D], the emitter contact spheres [C], the emitter [B], the outer housing assembly [K] with integrated thermal shields [A & L], and the thermocouple preload assemblies [M & R]. The emitter [B] is retained in the mechanical assembly with 16 fingers [D] holding silicon carbide contact spheres [C]. These balls engage corresponding spherical cavities in the emitter and provide a degree of rotation flexibility that allows the forces applied to the emitter to be purely radial along its axis of symmetry.

Mechanical analysis has evaluated the preload at room temperature, the preload at operating temperature combined with thermal expansion stresses, modal analysis, and random vibration analysis with PSD (power spectral density) acceleration inputs. Analysis results indicated preload stresses would not cause failure in the emitter.

FEA thermal analysis of the prototype calibration source design was performed to verify its main performance metric, mainly a surface temperature of the emitter equal to 700 0C. The calculated temperature distribution in the hot calibration source and the temperature distribution in the emitter, for a heater input power of 1400 W, are shown in Figure 4. The calculated emitter surface temperature is within the required 700 0C with a temperature uniformity less than 10 0C.

Temperature distribution
Fig. 4.Temperature distribution in the prototype calibration source (left) and the SiC emitter (right).

The results of this development effort were recently presented at the Symposium on Fusion Engineering (SOFE2017), held June 4-8, 2017 in Shanghai, China. A corresponding publication is in preparation.