In order to support the operation of ITER and the planned experimental programme an extensive set of plasma and first wall measurements will be required. The number and type of required measurements will be similar to those made on the present-day large tokamaks while the specification of the measurements—time and spatial resolutions, etc—will in some cases be more stringent. Many of the measurements will be used in the real time control of the plasma driving a requirement for very high reliability in the systems (diagnostics) that provide the measurements. The implementation of diagnostic systems on ITER is a substantial challenge. Because of the harsh environment (high levels of neutron and gamma fluxes, neutron heating, particle bombardment) diagnostic system selection and design has to cope with a range of phenomena not previously encountered in diagnostic design. Extensive design and R&D is needed to prepare the systems. In some cases the environmental difficulties are so severe that new diagnostic techniques are required. a Author to whom any correspondence should be addressed.

https://iopscience.iop.org/article/10.1088/0029-5515/47/6/S07/pdf

Chapter 7: Diagnostics

Fusion product diagnostics can be used to determine a fusion reaction rate, which indicates how close the plasma is to the ultimate goal of making a power plant based on nuclear fusion. However, th ...

Fusion product diagnostics

The neutral beam injection facility in the National Institute of Advanced Industrial Science and Technology was used to irradiate a polycrystalline tungsten specimen with high energy and high flux helium and hydrogen particles. The incidence energy and flux of the beam shot were 25 keV and 8.8 × 1022 particles/m2 s, respectively. The duration of each shot was approximately 30 ms, with 6 min intervals between each shot. Surface temperatures over 1800 K were attained. In the two cases of helium irradiation, total fluence of either 1.5 × 1022 He/m2 or 4.0 × 1022 He/m2 was selected. In the former case, large sized blisters with diameter of 500 nm were densely observed. While, the latter case, the blisters were disappeared and fine nanobranch structures appeared instead. Cross-sectional observations using a transmission electron microscope (TEM) with the focused ion beam (FIB) technique were performed. According to the TEM image, after irradiation with a beam shot of total fluence 4.0 × 1022 He/m2 , there were very dense fine helium bubbles in the tungsten of sizes 1-50 nm. As the helium bubbles grew the density of the tungsten matrix drastically decreased as a result of void swelling. These effects were not seen in hydrogen irradiation case.

http://www.jspf.or.jp/PFR/PDF/pfr2010_05-012.pdf

Microscopic Deformation of Tungsten Surfaces by High Energy and High Flux Helium/Hydrogen Particle Bombardment with Short Pulses

Recent JET and JT-60U results, have underlined the need to look into the measurement requirements of fast particles diagnostics in ITER. Not only alpha particles but all fast ions (He4,p,D,T,He3) present in the plasma should be diagnosed. Furthermore, the electron diagnostics, monitoring the deviations from a Maxwellian distribution function, play a central role in the electron cyclotron heating and current drive. In the present ITER measurement requirements only alpha particles and He ions are mentioned: space resolution is δr = a/10 (a is the minor radius), time resolution is δτ = 100 ms and accuracy 10–30%. The new proposed measurement requirements for fast particles are spatial resolution ~a/20 (10 cm on ITER), time resolution (minimum) ~100 τA (~100 µs for ITER), density of fast ions: the minority ions could be 4–10% of the plasma density. The candidate diagnostic techniques considered for ITER are amongst others γ-ray spectroscopy, fast ion collective Thomson scattering (CTS) and charge exchange recombination spectroscopy (CXRS). γ-ray spectroscopy detects ions in the energy range 1 < E < 5 MeV and RD the system includes a high power gyrotron operating in a region not yet tested in current applications (i.e. using a gyrotron at a frequency below the first ECE harmonic). For CXRS the spatial resolution and accuracy are not achievable for r/a < 0.3 and R&D is needed to assess the minimum figures in terms of accuracy possible for measurements close to the centre. The measurement requirements deduced from the need for detecting the effects of interaction of fast particles with MHD instabilities put forward important new objectives to the candidate diagnostic systems. In particular diagnostics for fast particles in the energy region 200 < E < 1300 keV and diagnostics for escaping fast particles need urgent development.

/pdf/requirements-for-fast-particle-measurements-on-iter-and-36fc9h328y.pdf

Requirements for fast particle measurements on ITER and candidate measurement techniques

A dual-source injector for tandem accelerators to produce negative ion currents of H, D, and He has been developed and built. The system consists of a central vacuum enclosure that accommodates two multicusp ion sources. Beam currents of several mA of H− and D− have been obtained by direct negative extraction of one of the multicusp ion sources. The yield for He− was optimized applying differentially pumped vacuum sections that support space compensation in the He+ transport section a well as low neutralization loss of He− after charge exchange. In addition, an optimized extraction geometry and beam transport system enabled in excess of 10 mA He+ within the low acceptance of the charge exchange canal. Early measurements yielded ∼50 μA for He− using Li as a charge exchange medium, but currents over 100 μA are anticipated if Na would be used.

A high-current light-ion injector for tandem accelerators.

The beam neutralization system for measurement of the spatial and velocity distributions of alpha particles of ITER plasmas was studied. As forward angle detection against the beam injection direction is required for effective neutralization, arrangement of the measurement system using possible ports in ITER configuration is proposed. The count rate of neutralized alpha particles produced by the double charge exchange interaction with energetic He0 beam particles injected is estimated. The ratios of signal to neutron-induced noise are evaluated. When a He0 beam produced by autodetachment from a 1–1.5MeV He− beam of 10mA is injected, the signal to noise ratio becomes greater than 1 at ρ<0.4, even without beam modulation. Usage of a lock-in technique at the frequency of radio-frequency quadrapole accelerator will make measurement at the outer region possible.

Confined alpha particle diagnostic system using an energetic He0 beam for ITER

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Confined alpha particle diagnostic system using an energetic He0 beam for ITER