R. Fetzer

The quantification of the key physics parameters for the DEMO fusion power reactor and analysis of the reactor relevant physics issues (KIT Scientific Reports ; 7661)

-The transient events could pose a severe tread causing melting and erosion of plasma facing components in the fusion power plant DEMO. Here we analyze the impact of edge localized modes (ELMs) on the divertor target and the first wall surface. The expected ELMs characteristics in DEMO are derived by extrapolating predictions made for ITER and by using the scaling arguments found from existing experiments. The tungsten armor damage and effect of melt layer motion due to the repetitive ELM loads is numerically investigated by using the MEMOS code. It is shown that due to unmitigated repetitive ELM impact, the divertor plate melts whereas the first wall does not. The divertor surface of monoblock W divertor module with water coolant tolerates the mitigated ELMs with ~33 time higher frequency, as in ITER, and does not melt even in the case of advanced version of DEMO loads. 1.2 Introduction High pedestal pressure, required for good core confinement in DEMO plasma, may lead to disadvantages of the increased edge localized modes (ELMs) energy loss to the plasma facing components (PFC) [1]. We consider here the PPCS model C DEMO design with the major radius R=7.5m, the aspect ratio A=3, the toroidal magnetic field B=6T and the safety factor qa=4.5 [2]. We also do some estimation for recently suggested two versions of DEMO, which are based on a near future technology DEMO1 and on steady-state technologically advanced DEMO2 [3] A sandwich type module made of W-clad EUROFER steel (Fig. 1a) and a pure tungsten divertor module (Fig. 1b) are examined here against heat loads impact due to the ELMs as the first wall (FW) and divertor target for DEMO. The modules consists of a water coolant tube of rectangular cross-section within the EUROFER (a) or W (b) matrix that are used as a heat diffuser [4,5]. Although W/EUROFER bound is of “low-activation” type, it has relatively low creep temperature (823°K) and EUROFER has limited heat diffusivity, which could be the drawback of EUROFER as a heat diffuser. Water is used as a coolant both in the FW blanket module and in the W divertor. In this paper we first derive the characteristics of ELMs in DEMO based on scaling arguments. Then the effect of ELMs on the tungsten armor melting and roughness formation. The magnitude of roughness after many ELMs is simulated applying the quasi-one-dimensional fluid dynamics model, which describes the motion of melted material along the surface in the ‘shallow water’ approximation of the Navier-Stokes equations with the surface tension, viscosity of molten metal and the radiative losses from the hot surface taken into account. Details of this model used in MEMOS code are presented. Then, the effect of mitigated ELMs is calculated. Finally we discuss the viability of W/EUROFER sandwich type module and W mono-block module under DEMO ELM conditions. 1 Modelling of DEMO PFC Erosion due to ELM impact 2 Fig. 1 (a) Mock-up of W/EUROFER sandwich type castellated module (two segments are shown) w =3mm, EUROFER=4mm with cooling channel imbedded into EUROFER, (b) W monoblock divertor module with embedded cooling channel;w=8mm. PFC. Models are used for the ENDEP computation of the ELMs impact [6]. 1.3 Specification of Type I ELMs in DEMO In ITER the thermal energy ΔWELM released during unmitigated ELM is expected to be~20MJ. The deposition time on the divertor plates is about 0.25ms (rising phase) and 0.5ms (decay phase). The peak energy on inboard diverter is~0.5-4MJ/m [3,7,8]. In DEMO the ELM characteristics can be derived based on scaling arguments by extrapolating data envisaged for ITER and the data from the large-scale tokamaks. The ELM energy deposition to the divertor. The ELM plasma energy loss ΔWELM to the divertor is proportional to pedestal energy Wped and correlated with the pedestal collisionality  (Fig.2) and with the ion losses time // along the magnetic field lines to the divertor target (Fig.3) [9]. For DEMO1/DEMO2 design parameters (R~8.5/9m, q95~3/4.5, nped~0.8/1.25 10 m, Tped~7.0/7.8keV and plasma volume Vped~1527/2275m 3 [3]) the pedestal energy can be estimated as Wped≈3npedTpedVped~410/610 MJ. The normalized pedestal collisionality ped•q95R/Cs ≈ 0.46•q95R(m)/T (keV) ~0.015/0.021 is smaller than for ITER (~0.036), mainly because of higher DEMO pedestal temperature. As it seen from Fig.2 at that collisionalityW ELM is about 25-30% of the edge pedestal energy Wped. The correction due to the finite ion loss time [7] ~(1ELM) -1 is estimated as ≈ 0.7 (see below). Therefore, in DEMO1/2 the thermal energy ΔWELM for unmitigated ELM must be expected in the range of ~80/160MJ. Maximum of the ELM energy loads due to in/out asymmetry [10] should be in inboard divertor about 50/110MJ and in outboard divertor ~40/80MJ. We further assume that the shape of ELM power loading at the mid-plane is the same as in ITER and the ELM decay phase is twice of that of rising phase. The full width at half maximum varies and for the case of poloidaly tilted plate on angle ~20o the ELMs deposited area on the plate is assumed to be about ~2Rcos20o≈(2.53)m. Here depends on the magnetic connection length in the SOL and can be estimated based on the ELM model [11] as~0.05-0.07m (similar to ITER). Therefore, one can expect that for DEMO1 energy density load to the out-inboard divertor plates could vary in the range of 15-20MJ/m for with energy 1.3 Specification of Type I ELMs in DEMO 3 deposition time (see below) about 0.6ms (rising phase) and 1.2ms (decay phase). Maximum energy density parallel to B on inboard diveror is estimated as Q//~20MJ/m•q95R /a≈400MJ/m . Fig. 2 Normalized ELM energy loss fraction as a function of collisionality ν* at the pedestal for various machines [8] and for DEMO versions (intersections between dashed lines with the fit curve); Wped is the energy stored in pedestal before an ELM crash. The ELM energy deposition time can be assessed as ELM(s) = 0.29•s))1.38 ≈ 580s [12]. This time corresponds to the ELM rising phase and according to experiments (see Fig.4) is well correlated with the characteristic time for ion transport from the pedestal to the divertor, Rq95•(1+(3/2)0.5•))/Cs,ped ~250s, where Cs,ped~7.10 m/s is the ion sonic speed calculated from plasma pedestal temperature~7.8kev, R=8.5m and q95~4.5. The good correlation of ELM suggests that convective transport is important for ELM heat deposition to the divertor at low collisionality in the SOL, expected for DEMO [13] (see Fig.2). If conduction dominates the ELM energy transport in DEMO then (12)ms will likely be the timescale range. The fraction of ELM energy transported as particles would likely arrive at the target with a time duration ~0.2ms. The Type 1ELM frequency. For extrapolation to DEMO one can use the fact that the Type I ELM frequency scales as fELM~(E) [13], where τE is the energy confinement time. The confinement time for the H-mode ELMy discharges in DEMO is IPB98(y,2)~6.47sec [2], which exceeds ~1.8 times the confinement time for ITER. Consequently, the ELM frequency in DEMO is about 0.8 Hz which is slightly lower than in ITER (~1-2Hz) [7]. 1 Modelling of DEMO PFC Erosion due to ELM impact 4 Fig 3. Normalized ELM energy loss fraction as a function of characteristic time for ion flux to divertor for various machines and ITER [7,8]; the ELM loss fraction for DEMO is indicated; Wped is the energy stored in pedestal before an ELM crash. Fig.4 The ELM energy deposition time (rising phase) as a function of characteristic time for ion flux to divertor [12]. The DEMO point is indicated. ELM deposition on the DEMO first wall. The ELM deposition on the DEMO first wall (FW) is assumed in agreement with experiments to be 5-20% of the ELM energy lost from the main plasma [14] and is derived for unmitigated ELMs as Q≈ Q//•q95R~0.1÷0.2MJ/m , As it seen from experiments [15] the ELM deposition time on to the FW is about half of that on outer divertor and is 0.6ms. The mitigated ELMs in DEMO. The controlled ELMs in DEMO can be assumed as a pace making by pellet injection or by control coils. Similar to ITER we suggest that the amplitude can be reduced ~ 33 times, so that the energy loss over 1.2ms is reduced down to 0.6MJ/m and, since the product of amplitude and I.4 The model for melt-motion simulation 5 frequency remains unchanged for the Type I ELMs, the frequency increases up to~26Hz. Table I summarized the characteristics of DEMO1 ELMs for the inboard divertor case discussed above.

Effect of thermal loads on different modules of DEMO PFCs

Abstract The thermal performance of different modules of plasma-facing components (PFCs) is analyzed for the DEMO reactor conditions in steady-state operation with the inclusion of the transient edge-localized modes (ELMs) for mitigated and unmitigated cases. As an example, the effect of these loads is considered for the tungsten (W) alloy mono-block design with a Cu OFHC/EUROFER water coolant tube first proposed in the framework of the Power Plant Physics and Technology (PPP&T) divertor study. A variant of this design with a EUROFER tube connected to the W block with a diamond/copper composite (DCC) used in the diagnostic windows is also analyzed. A design goal is to find the optimal thicknesses of material layers that allow one to keep the maximum temperatures within the allowable design limits under ITER water cooling conditions. Heat transfer and armor erosion due to the plasma impact has been modeled by using the MEMOS code.

Surface Layer Dynamics During E-Beam Treatment

The interaction of a pulsed intense electron beam with a metal target leads to rapid heating and subsequent cooling of the surface layer, accompanied by a series of phase transitions among the solid, liquid, vapor, and plasma phase. As a consequence of the treatment, depending on the beam parameters, the metal target is eroded and a topographical pattern (waviness, craters, etc.,) evolves on its surface. Surface roughening, a major drawback of pulsed intense electron beam treatment, is well known but lacks comprehensive understanding. In this paper, the process of pulsed intense electron beam interaction with metal targets is studied with special attention to the dynamics of the target surface layer and the development of surface roughness. The pulsed electron beam facility GESA generates electron beams with power density 0.5-2 MW/cm2, electron energy up to 120 keV, and pulse duration up to 200 μs. Different fast in situ diagnostics are applied to study the various processes occurring at the target surface: 1) melting and resolidification are visualized by time and space resolved imaging of the surface specular reflectivity; 2) spectroscopy is used to characterize the plasma phase adjacent to the target surface; 3) the evolution of irregularities and bubbles at the surface is studied by high-resolution microscopy; and 4) a stroboscopic imaging technique is applied to catch the evolution of the surface topography. The experimental data are compared with numerical simulations of heat transfer. All results and processes involved in pulsed intense electron beam treatment are discussed with respect to the target surface layer dynamics.

Design Strategy for the PFC in DEMO Reactor (KIT Scientific Reports ; 7637)

The performance of the plasma facing components (PFC) and materials in fusion reactor DEMO are fundamental issues affecting the ultimate technological and economic feasibility of fusion power. Many factors influence the choice of a functional and structural material in a fusion reactor. Component lifetime is mainly limited by radiation damage, disruptions, and sputtering erosion. Our design strategy is to determine the structure and coating thicknesses, which maximize component lifetime against all life limitations.

Numerical investigations of radially converging electron beam generated in cylindrical GESA IV facility

The cylindrical triode-type electron accelerator GESA IV was developed for treatment of metallic rods, specifically cladding tubes for nuclear reactors. The target (anode) diameter is therefore fixed at about 10 mm by the application, which leads to problems of homogeneity and stability of the radially converging beam. Due to the large difference between cathode diameter (about 150 mm) and anode diameter, a virtual cathode may form between grid and anode, electrons may miss the target and start to circulate around the anode, and the self-induced magnetic field may lead to large distortion of the electron trajectories. In this study, we investigate the influence of various crucial effects on the beam performance by PIC code simulations using the software package MAGIC. In particular, we consider monopolar and bipolar flow (i.e., the influence of ions generated at the target and moving towards the cathode), the effects of scattering at the grid and of backscattering at the target, the angular velocity spread of the electrons at emission, and the influence of the grid potential. The numerical results are compared with experiments performed at the GESA IV facility, where the influence of the target material and of the self-induced magnetic field on the beam performance are investigated.

Cathode plasma as electron source in long pulse accelerator gesa

Summary form only given. The use of plasma as an emission source has many advantages such as a rather simple implementation and very high emissivity and reproducibility. Therefore, plasma emission sources have established themselves as an essential part of modern electron accelerators. One drawback is that each application requires a specific control of the emission source, which represents a challenge. Of special concern are accelerators with pulse duration similar to or longer than the characteristic timescale of the plasma dynamics. In such cases, plasma motion into the accelerating gap crucially influences the beam quality and operation stability. At Karlsruhe Institute of Technology, a new pulsed electron beam facility was designed and constructed, which is specially equipped to investigate the cathode plasma performance. Proven diagnostic tools such as fast framing imaging, X-ray diagnostics, and Langmuir probes were installed to investigate the plasma dynamics and its influence on the beam characteristics. The new results on the axial and azimuthal plasma motion and its correlation with the beam performance provide new material for discussion of well-known but hitherto unresolved phenomena such as the impedance collapse and the precession of the electron beam.

Optical investigations of cathode plasma dynamics of long pulse electron accelerator gesa

Summary form only given. A specific problem of long pulse (several tens of μs) electron accelerators is the uncontrolled decrease of the machine impedance, caused by filling of the cathode-grid space with emission plasma. Because the intensity of cathode plasma generation and the decrease of the impedance are related by a positive feedback, soon an unstable operation of the accelerator is reached, resulting in beam breakdown. Our study concerns the search for an optimum operation regime of the accelerator. For this task, comprehensive optical diagnostics of the plasma dynamics was performed, combined with X-ray analysis of the beam profile. The characteristics of the plasma generation process was observed, as function of the electron flow backscattered at the target, of the magnetic field configuration, and of the potential distribution between the electrodes. As possible solutions, a change of the electrode geometry and limitation of the total current were tested.Optimization of the accelerator geometry by means of MAGIC3D PIC code simulations resulted in a new machine called GESA-SOPHIE.

Thermo-mechanical analysis of the DEMO FW module

Thermomechanical performance of the first wall (FW) W/EUROFER sandwich type module is analyzed under DEMO reactor conditions. Engineering heat loads to the FW panels are estimated for steady state operation with the edge localized modes (ELMs). Calculations carried out by MEMOS code show the inhomogeneity of the material temperature due to discrete location of the water cooling tubes embedded into EUROFER. The hot spots are formed in the W armor and EUROFER between the cooling sectors and depend on the distance of their mutual locations. The bending stress due to vertical temperature gradients in W and EUROFER layers is calculated and remains smaller than the ultimate tensile stress for expected temperatures. Calculations show that under the Type I ELMs expected in DEMO the W surface melts at the ELMs peak positions and solidifies between ELMs. There is no temperature difference found between hot and cool spots during ELMs.

Investigation of radially converging electron beams generated by GESA IV

Summary form only given. The pulsed electron beam facility GESA IV generates radially converging beams by means of a cylindrically shaped explosive emission cathode. Along the cylinder axis a rod-shaped target serves as anode of the triode system. In this study, the generation, homogeneity, dynamics, and stability of electron beams in GESA IV are investigated numerically using MAGIC-3D. Thereby, the reflection of electrons at the target is taken into account as well as the magnetic field induced by the current along the target. Various geometrical details of the GESA IV facility are modified to find the optimum parameters. An apparent anode is introduced with diameter exceeding the dimensions of the target. Also, the effect of lamellae placed radially around the target is studied. Although the target current could be enhanced considerably by the presence of the lamellae, the divergence of the beam along the symmetry axis could not be avoided. The numerical findings are discussed and compared with experimental measurements conducted at the GESA IV facility.

Target surface layer dynamics during application of intense electron beams

Summary form only given. Intense pulsed electron beams are commonly used to improve mechanical properties of metal targets in near-surface regions or for surface alloying. In some cases, however, the intended property changes are accompanied by the development of surface roughness. The exact origin of this phenomenon is still under debate. In this work, the dynamics of the target surface layer in its melted stage is investigated experimentally and theoretically. The pulsed electron beam facility GESA at KIT is used to generate electron beams with power density 0.5-2 MW/cm2, electron energy 120 keV, and pulse duration up to 200 μs. Various fast in-situ optical diagnostic tools have been set up and successfully tested during treatment of stainless steel, copper, and aluminum targets. After this preceding work, a systematic investigation of the influence of various materials and of specific beam parameters on the surface layer dynamics is now performed. The experimental studies are accompanied by numerical simulations of heat transfer and melt motion and by theoretical considerations concerning the relevance of possible hydrodynamic instabilities.