Parametric study and design improvements for the target of NOVA ERA

2.1.Target material

For NOVA ERA the first choice for the target material is beryllium. It is one of the most employed materials in CANS today and a lot of experience has been gained with it e.g. at the RANS source in Riken, Japan [8] and at the LENS-facility at Indiana University, USA [6].

Beryllium is known to have a high neutron yield at low ion energies. Some of the beryllium alloys also show good mechanical and thermal properties, e.g. the alloy S-65C VHP, which is refereed to in the structural design guidelines for the future nuclear fusion reactor ITER [3]. Its high thermal conductivity of 200 W m⁻¹ K⁻¹, which is the half of pure copper (401 W m⁻¹ K⁻¹) at 20°C [3] and its high melting point near 1300°C make it suitable for high beam power depositions. Drawbacks of beryllium are its availability and the need of special equipment for machining due to toxic dusts and fumes. Its mechanical strength is acceptable but is quite sensible to the production process and the purchased material grade.

2.2.Target thickness

After choosing the material, the geometry of the target needs to be defined. From LENS [6] we learned, that blistering of the target is a severe problem and already occurs after short operation times. It is mainly caused by primary ions accumulated inside the solid target material. Since a large fraction of the ions which impinge on the target do not react during their path and most of them are stopped in a region which is well known as Bragg-Peak, an effective way to strongly reduce blistering by primary ions is to make the target thinner than the ion range. In Table 1(a) the ranges calculated with the SRIM software [9] for protons and deuterons of different initial energies impinging on beryllium are given. It can be observed that the range is increasing with increasing energy and that deuterons have a much shorter range than protons. In Table 1(b) suitable ion energies for target thicknesses of 0.7 mm, 1.0 mm and 2.0 mm are proposed. These exemplary thicknesses refer to the thicknesses which will be discussed later in the frame of a parametric study.

2.3.Target cooling

An effective cooling is needed to dissipate the heat generated mainly by the ionization processes in the target. From RANS [8] and LENS [6] two possible solutions are known. While at RANS, a thin layer of beryllium is bonded to a heat-sink, made from vanadium, at LENS a beryllium disc is directly cooled by water. In both cases, the beryllium layer is adjusted to the ion range to avoid blistering, but at RANS, the ions are dumped in the vanadium, which has a higher diffusion coefficient for hydrogen than beryllium. At the LENS facility, the proton beam is dumped into the cooling water.

For NOVA ERA, a simple beryllium disc directly cooled by an impinging water jet from the backside was chosen. In Fig. 1, the basic design of the target assembly is shown. It consists of an elastomer sealed assembly, where the beryllium disc is held in between two flanges. In the picture, the protons from the accelerator are coming from the left side and are impinging on the beryllium target. In the target some of the protons undergo nuclear reactions and neutrons are produced, while the majority of the beam energy is transformed into heat. This requires cooling of the target disc, which is realized by a water jet hitting the target from the right side with a velocity of 3 m/s and is subsequently forced to spread over the target surface through a 1 mm gap. The heated water leaves the target area through a number of concentrically aligned holes. The beryllium disc thickness is chosen in dependence of the ion range and selected according to Table 1(b).

The ion stopping can be investigated in more detail with the help of the Monte Carlo simulation software TRIM [9]. In Fig. 2 the range of the particles and their energy loss dE/dx by ionization for 10 MeV protons impinging on a 0.7 mm beryllium layer and a subsequent water layer are plotted, respectively. From Fig. 2(a) we can see that most ions (>99.95%) are stopped in the cooling water. In Fig. 2(b) the energy loss of the ions in eV/Å is plotted. Since the energy loss of the ionization is the dominating heat source in the target, by integrating the interval corresponding to the beryllium thickness and the interval corresponding to the water layer and building the ratio, we can conclude, that 69% of the energy is deposited in the beryllium while 31% are deposited in the cooling water.

Fig. 1.

Explosion view of the CAD model of the NOVA ERA Design.

Fig. 2.

TRIM plots of the 10 MeV protons slowed down in 0.7 mm beryllium and stopped in water. 69% of the particle energy is deposited in the beryllium and 31% of the energy are deposited in the cooling water.

2.4.NOVA ERA – concept

With the assumptions made above, a target concept was presented in the conceptual design report for NOVA ERA [4]. The target was conceived for an accelerator system with 400 W beam power and 10 MeV protons. A detailed listing of the accelerator parameters for NOVA ERA can be found in Table 2. A target disc thickness of 0.7 mm and a disc diameter of 80 mm was selected. The flange system requires a peripheral area of the disc in order for it to be held in place. This means that only a smaller concentric area is subjected to the pressure difference of the cooling water and the accelerator vacuum. This unsupported area has a diameter of 40 mm, which is at the same time the diameter of the proton beam tube. The safety margin of the beam spot to the beam tube wall is 5 mm, so that the beam spot has a diameter of 30 mm. These dimensions were selected employing similarly the pressure vessel guidelines AD-2000 for flat ends [1].

ANSYS CFX calculation performed for the above mentioned dimensions resulted in maximum temperatures below 60°C on the vacuum side of the target. The pressure drop and the kinetic pressure on the beryllium were less then 10 kPa for 3 m/s feed water velocity at 22°C.

The thermo-mechanical stress was then calculated using ANSYS mechanical with simplified but conservative input parameters in a stationary case. The 400 W average power were assumed to be homogeneously deposited in the beam spot area and to 100% inside the beryllium layer. The pressure gradient between accelerator vacuum and cooling water was assumed to be 200 kPa. The maximum equivalent stress calculated for the above mentioned dimensions was 197 MPa [4], which is below the yield strength of beryllium S-65C VHP (>236 MPa bellow 100°C) [3].

3.1.Results

Different load cases were calculated, in order to study the different contributions to the mechanical stress. When subjecting the target to a differential pressure without any heat deposition (Fig. 4(a)), one can observe a rise of the stress for increasing support radius diameters. This is due to the subjected area increasing quadratically with the support radius. For increasing thicknesses the strength of the target increases, allowing larger radii.

In the case shown in Fig. 4(b) the target is subjected to the beam. A heat source of 400 W is introduced, but pressure differences are neglected. Here the stress is decreasing with increasing radius because for constant beam power, the heat density is reduced. This leads to lower temperature gradients and thereby to lower stress.

Figure 4(c) shows the case when the target is subjected to the pressure difference as well as the heat source by heat deposition from a 400 W ion beam. In this case, the resulting stress is a combination of the previously mentioned effects. For a specific parameter set, consisting of contributions from beam power deposition, target layer thickness defined by the ion range and pressure difference, an optimum support radius exists. This minimum is shifted to larger radii for increasing thicknesses. This behaviour can be explained by the influence of the thickness on the pressure-induced stress as well as the stress induced by the temperate gradient. For the shown thicknesses and beam power, the stress induced by the pressure gradient has a stronger impact than the stress induced by the temperature gradient. This can be explained as the maximum temperature occurring in the target disc (Fig. 4(d)) is quickly decreasing with the support radius, while the thickness only has little influence on the maximum temperature.

The results show that for the thin disc shaped target design, an optimum diameter for a given particle energy (target thickness) can be found, and that the stress can be reduced if higher energies are used. With increasing beam power and particle energy, the contribution of the temperature gradient will get more important. Thereby the optimized dimensions are shifted to larger radii, allowing smaller heat deposition densities but causing a larger area subjected to the pressure gradient, increasing its contribution to the stress.

Fig. 4.

Maximum equivalent stress on the target for (a) 0 W beam power and 110 kPa water pressure, (b) 400 W beam power and 110 kPa water pressure, (c) 400 W beam power and 0 Pa water pressure and (d) the temperature for 400 W beam power, for disc thicknesses of 0.7 mm, 1.0 mm and 2.0 mm, respectively.