| 1. |
- Chapman, I. T., et al.
(författare)
-
Overview of MAST results
- 2015
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 55:10
-
Tidskriftsartikel (refereegranskat)abstract
- The Mega Ampere Spherical Tokamak (MAST) programme is strongly focused on addressing key physics issues in preparation for operation of ITER as well as providing solutions for DEMO design choices. In this regard, MAST has provided key results in understanding and optimizing H-mode confinement, operating with smaller edge localized modes (ELMs), predicting and handling plasma exhaust and tailoring auxiliary current drive. In all cases, the high-resolution diagnostic capability on MAST is complemented by sophisticated numerical modelling to facilitate a deeper understanding. Mitigation of ELMs with resonant magnetic perturbations (RMPs) with toroidal mode number n(RMP) = 2, 3, 4, 6 has been demonstrated: at high and low collisionality; for the first ELM following the transition to high confinement operation; during the current ramp-up; and with rotating n(RMP) = 3 RMPs. n(RMP) = 4, 6 fields cause less rotation braking whilst the power to access H-mode is less with n(RMP) = 4 than n(RMP) = 3, 6. Refuelling with gas or pellets gives plasmas with mitigated ELMs and reduced peak heat flux at the same time as achieving good confinement. A synergy exists between pellet fuelling and RMPs, since mitigated ELMs remove fewer particles. Inter-ELM instabilities observed with Doppler backscattering are consistent with gyrokinetic simulations of micro-tearing modes in the pedestal. Meanwhile, ELM precursors have been strikingly observed with beam emission spectroscopy (BES) measurements. A scan in beta at the L-H transition shows that pedestal height scales strongly with core pressure. Gyro-Bohm normalized turbulent ion heat flux (as estimated from the BES data) is observed to decrease with increasing tilt of the turbulent eddies. Fast ion redistribution by energetic particle modes depends on density, and access to a quiescent domain with 'classical' fast ion transport is found above a critical density. Highly efficient electron Bernstein wave current drive (1 A W-1) has been achieved in solenoid-free start-up. A new proton detector has characterized escaping fusion products. Langmuir probes and a high-speed camera suggest filaments play a role in particle transport in the private flux region whilst coherence imaging has measured scrape-off layer (SOL) flows. BOUT++ simulations show that fluxes due to filaments are strongly dependent on resistivity and magnetic geometry of the SOL, with higher radial fluxes at higher resistivity. Finally, MAST Upgrade is due to begin operation in 2016 to support ITER preparation and importantly to operate with a Super-X divertor to test extended leg concepts for particle and power exhaust.
|
|
| 2. |
- Meyer, H., et al.
(författare)
-
Overview of physics results from MAST towards ITER/DEMO and the MAST Upgrade
- 2013
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 53:10, s. 104008-
-
Tidskriftsartikel (refereegranskat)abstract
- New diagnostic, modelling and plant capability on the Mega Ampere Spherical Tokamak (MAST) have delivered important results in key areas for ITER/DEMO and the upcoming MAST Upgrade, a step towards future ST devices on the path to fusion currently under procurement. Micro-stability analysis of the pedestal highlights the potential roles of micro-tearing modes and kinetic ballooning modes for the pedestal formation. Mitigation of edge localized modes (ELM) using resonant magnetic perturbation has been demonstrated for toroidal mode numbers n = 3, 4, 6 with an ELM frequency increase by up to a factor of 9, compatible with pellet fuelling. The peak heat flux of mitigated and natural ELMs follows the same linear trend with ELM energy loss and the first ELM-resolved T-i measurements in the divertor region are shown. Measurements of flow shear and turbulence dynamics during L-H transitions show filaments erupting from the plasma edge whilst the full flow shear is still present. Off-axis neutral beam injection helps to strongly reduce the redistribution of fast-ions due to fishbone modes when compared to on-axis injection. Low-k ion-scale turbulence has been measured in L-mode and compared to global gyro-kinetic simulations. A statistical analysis of principal turbulence time scales shows them to be of comparable magnitude and reasonably correlated with turbulence decorrelation time. T-e inside the island of a neoclassical tearing mode allow the analysis of the island evolution without assuming specific models for the heat flux. Other results include the discrepancy of the current profile evolution during the current ramp-up with solutions of the poloidal field diffusion equation, studies of the anomalous Doppler resonance compressional Alfven eigenmodes, disruption mitigation studies and modelling of the new divertor design for MAST Upgrade. The novel 3D electron Bernstein synthetic imaging shows promising first data sensitive to the edge current profile and flows.
|
|
| 3. |
- Johnson, M. Gatu, et al.
(författare)
-
Neutron spectrometry of JET discharges with ICRH-acceleration of helium beam ions
- 2010
-
Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 81:10, s. 10D336-
-
Tidskriftsartikel (refereegranskat)abstract
- Recent experiments at JET aimed at producing He-4 ions in the MeV range through third harmonic ion cyclotron resonance heating (ICRH) acceleration of He-4 beams in a He-4 dominated plasma. MeV range D was also present through parasitic ICRH absorption on residual D. In this contribution, we analyze TOFOR neutron spectrometer data from these experiments. A consistent description of the data is obtained with d(d, n)(3) He and Be-9(alpha, n)C-12 neutron components calculated using Stix distributions for the fast D and He-4, taking finite Larmor radius effects into account and with a ICRH power partition of P-D(RF) = 0.01 X P-4He(RF), in agreement with TOMCAT simulations.
|
|
| 4. |
- Cecconello, Marco, et al.
(författare)
-
A neutron camera system for MAST
- 2010
-
Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 81:10, s. 10D315-
-
Tidskriftsartikel (refereegranskat)abstract
- A prototype neutron camera has been developed and installed at MAST as part of a feasibility study for a multichord neutron camera system with the aim to measure the spatial and time resolved 2.45 MeV neutron emissivity profile. Liquid scintillators coupled to a fast digitizer are used for neutron/gamma ray digital pulse shape discrimination. The preliminary results obtained clearly show the capability of this diagnostic to measure neutron emissivity profiles with sufficient time resolution to study the effect of fast ion loss and redistribution due to magnetohydrodynamic activity. A minimum time resolution of 2 ms has been achieved with a modest 1.5 MW of neutral beam injection heating with a measured neutron count rate of a few 100 kHz.
|
|
| 5. |
- Ericsson, G., et al.
(författare)
-
Neutron spectroscopy as a fuel ion ratio diagnostic : Lessons from JET and prospects for ITER
- 2010
-
Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 81:10, s. 10D324-
-
Tidskriftsartikel (refereegranskat)abstract
- The determination of the fuel ion ratio n(t)/n(d) in ITER is required at a precision of 20%, time resolution of 100 ms, spatial resolution of a/10, and over a range of 0.01<10. We use simplified but realistic Monte Carlo models of ITER to assess the possibility to use neutron emission spectroscopy ((NES) for such measurements. We show that NES meets the requirements for ion temperatures T-i>6 keV and for n(T)/n(D)<0.6. A crucial issue is the signal-to-background situation in the measurement of the weak 2.5 MeV emission from DD reactions in the presence of a background of scattered 14 MeV DT neutrons. Important experimental input and corroboration for this assessment are presented from the time-of-flight neutron spectrometer at JET where the presence of a strong component of backscattered neutrons is observed. Neutron emission components on ITER due to beam-thermal and tritium-tritium reactions can further enhance the prospects for NES.
|
|
| 6. |
- Eriksson, Jacob, et al.
(författare)
-
Finite Larmor radii effects in fast ion measurements as demonstrated using neutron emission spectrometry of JET plasmas heated with 3rd harmonic ICRF
- 2011
-
Ingår i: 38th EPS Conference on Plasma Physics 2011 (EPS 2011): Europhysics Conference Abstracts. - 2914771681
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This paper demonstrates how the finite Larmor radii (FLR) of fast ions can affect fast ion measurements by studying data from the neutron time-of-flight spectrometer TOFOR. Neutron spectra were calculated from a model of the fast ion velocity distribution for a JET experiment with 3rd harmonic ICRF heating of deuterium beams. It was found that FLR effects need to be considered to get a good description of the data, if the Larmor radius of the fast ions are comparable to the width of the field of view of the instrument. This applies not only to results from neutron spectrometry but also to other types of fast ion diagnostics.
|
|
| 7. |
- Sangaroon, S., et al.
(författare)
-
Neutron and gamma-ray transport calculations in support of the design of the radiation shielding for the TOFED neutron spectrometer at LHD
- 2021
-
Ingår i: Fusion engineering and design. - : Elsevier. - 0920-3796 .- 1873-7196. ; 166
-
Tidskriftsartikel (refereegranskat)abstract
- The neutron emission spectrometer (NES) has been commissioned at the Large Helical Device (LHD) with a new Time-Of-Flight-Enhanced-Diagnostics (TOFED). The aim of the TOFED is to measure the neutron energy spectrum to study the fast-ions behavior such as high-energy ions created by ion cyclotron resonance heating, beam ions, and D-D fusion born 1 MeV tritons in LHD. The instrument is assembled of the first detector S1 and the second detector S2. In 2019, TOFED was installed at Z = 13.5 m below midplane without the radiation shielding and was initially commissioned. Due to neutrons streaming through a considerable number of holes in the LHD torus hall, the ambient background radiation at the TOFED was higher than expected, which causes the large number of coincidental events on the detectors. The obtained time difference between S1 and S2 provides the strong suggestion of TOFED relocation and of TOFED shielding development. In 2020, TOFED was relocated to the basement at Z = 18.5 m below midplane. In this work, a dedicated radiation shielding design is investigated using the Monte Carlo N-Particle code (MCNP6). The shielding design fulfills the design criteria on the radiation moderation capability and engineering constraints. Due to the design criteria, borated polyethylene with thickness of 20 cm is being considered as a neutron shielding and lead with thickness of 5 cm is being considered as a gamma-ray shielding. The shielding is suggested to be a rectangular polyhedron shaped roof. With the designed shielding, neutron flux is reduced by approximately two orders of magnitude at S1 and by approximately more than two orders of magnitude at S2 while prompt gamma-ray fluence is reduced by approximately two orders of magnitude. Here, the detailed design of radiations shielding for the TOFED at LHD is presented. The appropriate radiation shielding from this work will be installed at TOFED.
|
|
| 8. |
- Turnyanskiy, M., et al.
(författare)
-
Measurement and control of the fast ion redistribution on MAST
- 2013
-
Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 53:5, s. 053016-
-
Tidskriftsartikel (refereegranskat)abstract
- Previous experiments on MAST and other tokamaks have indicated that the level of fast ion redistribution can exceed that expected from classical diffusion and that this level increases with beam power. In this paper we present a quantification of this effect in MAST plasmas using a recently commissioned scanning neutron camera. The observed fast ion diffusivity correlates with the amplitude of n = 1 energetic particle modes, indicating that they are the probable cause of the non-classical fast ion diffusion in MAST. Finally, it will be shown that broadening the fast ion pressure profile by the application of neutral beam injection at an off-axis location can mitigate the growth of these modes and result in the classical fast ion behaviour.
|
|
