Operation of Alcator C-Mod with high-Z plasma facing components and implications

Abstract

Studies of potential plasma facing component ͑PFC͒ materials for a magnetic fusion reactor generally conclude that tungsten is the best choice due to its low tritium ͑T͒ retention, capability to handle high heat fluxes with low erosion, and robustness to nuclear damage and activation. ITER ͓F. Perkins et al., Nucl. Fusion 39, 2137 ͑1999͔͒ may operate with all tungsten PFCs to provide the necessary operational experience for a reactor. Alcator C-Mod ͓I. Hutchinson et al., Phys. Plasmas 1, 1511 ͑1994͔͒ operates with molybdenum ͑Mo͒ high-Z PFCs, which have very similar properties to tungsten. The experiments described herein have provided a unique comparison of operation with or without in situ boron coatings applied to the molybdenum PFCs; the latter are likely most relevant to ITER and beyond. ICRF-heated H-modes were readily achieved without boron coatings although the resultant enhancement in energy confinement was typically small ͑H ITER,89 ϳ 1͒. Molybdenum concentrations, n Mo / n e , rise rapidly after the H-mode transition up to 0.1%, cooling the plasma by line radiation, reducing energy confinement, and/or causing a back H / L transition. Surprisingly, the primarily molybdenum PFC surfaces retain 3.5− 5.0ϫ 10 20 of injected D 2 molecules per discharge, corresponding to 50% of the injected gas. Plasma current disruptions, both randomly occurring over the course of a day, or planned, reduce the retained D long term. After applying boron coatings, n Mo / n e was reduced by a factor of 10-20 with H ITER,89 approaching 2. A world-record volume-average plasma pressure of 1.8 atm at 5.4 T was achieved at the ITER normalized ␤. The effects of each boronization are found to be limited in time, correlated to time-integrated input energy. Intra-and inter-discharge boronization techniques have been developed with the latter being the most successful. This initial study indicates that a low-Z coating over at least a fraction of the Mo PFCs in C-Mod is needed to reduce core molybdenum levels and achieve the best energy confinement. This, together with the larger than expected D retention, raises concerns for the performance of uncoated tungsten surfaces in ITER and beyond.

An in situ accelerator-based diagnostic for plasma-material interactions science on magnetic fusion devices

“…Multiple PFC materials, such as beryllium and tungsten, can be used at different regions in order to exploit the unique PMI and thermomechanical properties of each material. Thin film coatings of PFCs, such as boron on molybdenum (Alcator C-Mod) [9] and lithium on CFC (NSTX) [21], provide an additional source of non-PFC materials. The steady-state cycle of erosion, migration, and redeposition in a miultiple-material environment can lead to the formation of solid materials with complex -and often undesirable -compositions, typically referred to as "tokamakiuni" [22].…”

“…These reactions include 12 C(n,a) 9 Be (Ethresohld = 6.18 MeV), 12 C(n,p) 12 B (13.65 MeV), and 12C(n,d)"B (14.89 MeV) as well as the more complex breakup reactions such as 1 2 C(n,n') 12 C* =-3a (7.89 MeV), in which the residual excited nucleus fragments into lighter nuclei. Because the reaction products are almost exclusively light ions and recoil nuclei, they contribute significantly to the low energy region of the detector response function.…”

“…To evaluate PSD capabilities of the EJ301-PMT detector, the detector was exposed to a 24 1Anmericium-Berylliumn (AmBe) source. The unstable 24 1 Am nucleus spontaneously decays via the emission of an alpha particle, which generates neutrons and gammas via the nuclear reaction 9 Be(a,n) 2 C* => 12 C + -y(4.44 MeV) (4.…”

“…9 functional form was chosen and fit to the scattering background between 0.5 and 1.8 MeVee. There is nothing special nor physically significant about the x-1 9 function; it simply provides a good fit to the background and, when extrapolated to low energy, provides a clear delineation between neutrons from the deuteron-boron background (below the fit) and neutrons from the 2 H(d, n) 3 He reaction (above the fit).…”

Fuel retention measurements in Alcator C-Mod using accelerator-based in situ materials surveillance

“…Analysis of these plasma-deposited B layers from Alcator C-Mod [14], TEXTOR [15], and laboratory experiments [16] have all found consistent D/B atomic ratios, typically saturated up to 0.4. Thus, the areal D deposition rate can be estimated as …”