Fusion Divertor

Fusion Divertor Engineering

The divertor concept is widely used to achieve better confinement in many fusion devices. In the divertor configuration, an open magnetic field intersects with the divertor target and therefore, high heat and particle fluxes come to the divertor targets along the magnetic field. In the ITER, the divertor target is expected to get 10-20 MW/m2 heat flux and 1e24 /m2/s particle flux. Thus, it is required to develop robust divertor system that withstands such high heat and particle fluxes. This is so-called “divertor problem” and is one of the most challenging issues in fusion research.

In order to study the divertor problem, the divertor simulator (a.k.a. linear devices) is widely used in the fusion community. Instead of using conventional linear devices, we built a divertor simulator using applied-field magnetoplasmadynamic (AF-MPD) thrusters at KAERI. The AF-MPD thruster was chosen because it can produce a high-density plasma in cw mode which is necessary for achieving both high heat and particle fluxes.

Our MPD thrusters consist of copper anode, thoriated tungsten cathode, and ceramic insulators. An NdFeB permanent magnet is placed around the thruster body to provide the axial magnetic field. Our magnet provides 0.17 T B-field at the center of the magnet. We successfully ignited plasmas with H2, D2, He, Ar, and Xe gases. Plasmas are typically ignited at 600 V. Our MPD plasmas initially show features of abnormal glow discharge and as we increase the plasma current, thermionic electron emission starts and the transition from abnormal glow discharge to arc plasma occurs.

In order to measure the heat flux produced by our facility, we built a calorimeter consisting of a copper block and a copper cooling tube. The heat flux is obtained from the heat conduction equation or the calorimetric equation. The maximum heat flux was measured to be 10 MW/m2 when the plasma current is 200 A at 30 cm distance.

To measure the ion flux, a Langmuir probe is placed at the target position. Then, the ion saturation (I) current is measured by applying -200 V to the probe tip. The ion flux (Γ) is then calculated from the relation, Γ = I/eA where e is the elementary charge and A is the probe tip area. The measured hydrogen and deuterium ion fluxes are as high as 1e23 /m2/s (see figures a and b shown below).

After the divertor simulator was developed, we have studied the blister formation on the tungsten after deuterium ions are irradiated. A tungsten sample holder was built and installed at the 30-40 cm apart from the plasma source. In order to control the sample temperature, the water cooling system is prepared. A thermocouple is installed on the backside of tungsten sample and a pyrometer is used to monitor the surface temperature of tungsten target. The incident ion energy is controlled by the bias power supply and the ion flux and total fluence are measured in-situ using the Langmuir probe circuit.

A sketch and a photo of our beam irradiation experiment are shown in figures c and d, respectively. SEM image obtained after the deuterium beam irradiation is also displayed in figure e. As seen in figure e, many blisters ranging between 200 nm and 1 μm are observed after the sample was exposed to deuterium ions. In this case, the incident ion energy was –100 V, ion flux 7.5×1e22 /m2/s, and the total ion fluence was 2.5×1e25 /m2. The temperature of the tungsten sample measured by thermocouple was 960 degree Celsius.

After the divertor simulator was developed, we have studied the blister formation on the tungsten after deuterium ions are irradiated in collaboration with Korea Institute of Fusion Energy (KFE). A tungsten sample holder was built and installed at the 30-40 cm apart from the plasma source. In order to control the sample temperature, the water cooling system is prepared. A thermocouple is installed on the backside of tungsten sample and a pyrometer is used to monitor the surface temperature of tungsten target. The incident ion energy is controlled by the bias power supply and the ion flux and total fluence are measured in-situ using the Langmuir probe circuit.

A sketch and a photo of our beam irradiation experiment are shown in figures c and d, respectively. SEM image obtained after the deuterium beam irradiation is also displayed in figure e. As seen in figure e, many blisters ranging between 200 nm and 1 μm are observed after the sample was exposed to deuterium ions. In this case, the incident ion energy was –100 V, ion flux 7.5×1e22 /m2/s, and the total ion fluence was 2.5×1e25 /m2. The temperature of the tungsten sample measured by thermocouple was 960 degree Celsius.