Z.S. Hartwig

Detailed Report of the MuLan Measurement of the Positive Muon Lifetime and Determination of the Fermi Constant

We report results from the MuLan measurement of the positive muon lifetime. The experiment was conducted at the Paul Scherrer Institute using a time-structured surface muon beam and a segmented plastic scintillator array. Two different in-vacuum muon stopping targets were used: a ferromagnetic foil with a large internal magnetic field and a quartz crystal in a moderate external magnetic field. From a total of 1.6 x 10^{12} decays, we obtained the muon lifetime tau_mu = 2196980.3(2.2) ps (1.0 ppm) and Fermi constant G_F = 1.1663787(6) x 10^{-5} GeV^{-2} (0.5 ppm).

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

This paper presents a novel particle accelerator-based diagnostic that nondestructively measures the evolution of material surface compositions inside magnetic fusion devices. The diagnostics purpose is to contribute to an integrated understanding of plasma-material interactions in magnetic fusion, which is severely hindered by a dearth of in situ material surface diagnosis. The diagnostic aims to remotely generate isotopic concentration maps on a plasma shot-to-shot timescale that cover a large fraction of the plasma-facing surface inside of a magnetic fusion device without the need for vacuum breaks or physical access to the material surfaces. Our instrument uses a compact (~1 m), high-current (~1 milliamp) radio-frequency quadrupole accelerator to inject 0.9 MeV deuterons into the Alcator C-Mod tokamak at MIT. We control the tokamak magnetic fields--in between plasma shots--to steer the deuterons to material surfaces where the deuterons cause high-Q nuclear reactions with low-Z isotopes ~5 μm into the material. The induced neutrons and gamma rays are measured with scintillation detectors; energy spectra analysis provides quantitative reconstruction of surface compositions. An overview of the diagnostic technique, known as accelerator-based in situ materials surveillance (AIMS), and the first AIMS diagnostic on the Alcator C-Mod tokamak is given. Experimental validation is shown to demonstrate that an optimized deuteron beam is injected into the tokamak, that low-Z isotopes such as deuterium and boron can be quantified on the material surfaces, and that magnetic steering provides access to different measurement locations. The first AIMS analysis, which measures the relative change in deuterium at a single surface location at the end of the Alcator C-Mod FY2012 plasma campaign, is also presented.

Simulated plasma facing component measurements for an in situ surface diagnostic on Alcator C-Mod

The ideal in situ plasma facing component (PFC) diagnostic for magnetic fusion devices would perform surface element and isotope composition measurements on a shot-to-shot (∼10 min) time scale with ∼1 μm depth and ∼1 cm spatial resolution over large areas of PFCs. To this end, the experimental adaptation of the customary laboratory surface diagnostic--nuclear scattering of MeV ions--to the Alcator C-Mod tokamak is being guided by ACRONYM, a Geant4 synthetic diagnostic. The diagnostic technique and ACRONYM are described, and synthetic measurements of film thickness for boron-coated PFCs are presented.

Neutronics Studies for a Compact, High-Field Tokamak Neutron Source

Abstract A critical aspect of the design of a tokamak-based neutron source is to ensure that radiation limits of the structural and magnet-insulating materials are not approached during the lifetime of the tokamak. To this end, we present an exploratory neutronics study of a materials testing facility that is based on Ignitor, a high-field tokamak. It shown that sufficient radiation damage to test materials located in the Ignitor first wall can be obtained by sustaining a reaction rate of 3.33×1019 neutrons per second for 7 operational months. Solutions to mitigate terminal damage to the toroidal field coil insulators, including its substitution for modern radiation-resistant insulators and the use of advanced radiation shield materials, are explored, and their implication for the design of the facility is discussed.

Glass-panel 6 Li neutron detector

We report on the development of a neutron detector utilizing solid enriched lithium, which has substantial neutron detection efficiency. The detector employs large, thin sheets of lithium in a gas-filled multi-wire proportional chamber (MWPC). Using low-cost design methods, readout electronics, and a small fraction of the already available enriched lithium available from Y-12/Oak Ridge National Laboratory, the amount of 3He equivalent detection capability for nuclear non-proliferation activities can be greatly increased.

Engineering upgrades to the accelerator-based in-situ materials surveillance diagnostic on Alcator C-Mod

This paper presents an overview of the engineering upgrades being made to optimize the AIMS diagnostic on the Alcator C-Mod tokamak, a novel, particle accelerator-based diagnostic that can nondestructively measure the evolution of material surface compositions inside magnetic fusion devices. Three major AIMS subsystems are presented: the RFQ deuteron accelerator; the particle detectors; and the Alcator C-Mod tokamak. The combined results of the upgrades will enable AIMS to routinely map critical plasma-material interaction quantities, such as net erosion/redeposition and fusion fuel retention, over large areas of PFC surfaces between plasma shots and after the run day.

Compact tokamak neutron sources as a first step towards hybrid fission-fusion reactors

Abstract An Ignitor-like tokamak that is compact, high field, and high density device, could make full use of the its intense neutron flux, without reaching ignition as a source of neutrons for materials testing in support of a fission-fusion hybrid device. The main features of this High Field Neutron Source Facility, which would have about 50% more plasma volume than Ignitor, are illustrated and the R&D required in order to achieve relevant dpa quantities in test materials are discussed. Several full-power months of operation are sufficient to obtain relevant radiation damage values in terms of dpa, and a scoping study of shielding the magnetic insulators to reduce radiation damage has been performed.

A High Field Tokamak Neutron Source Facility

Fusion creates more neutrons per energy released than fission or spallation, therefore DT fusion facilities have the potential to become the most intense sources of neutrons for material testing. An Ignitor-like device, that is a compact, high field, high density machine could be envisaged for this purpose making full use of the intense neutron flux that it can generate, without reaching ignition. The main features of this High Field Neutron Source Facility, which would have about 50% more volume than Ignitor, are illustrated and the R&D required in order to achieve relevant dpa quantities in the tested materials are discussed, in particular the adoption of superconducting magnet coils. Radiation damage evaluations have been performed by means of the ACAB code, showing the potential of high field, neutron-rich devices for fusion material testing. Few full-power months of operation are sufficient to obtain significant radiation damage values (in terms of dpa) of large size samples (~m3). The setup of a duty cycle for the device in order to obtain such operation times is discussed. The problem of radiation damage to the insulator of the Toroidal Field Coils has been explored. Two strategies for mitigating damage to the TF coil insulators have been demonstrated, and it is likely that both will need to be implemented to ensure the survival of the insulating material for the lifetime of the tokamak.