A. V. Krasilnikov

Chapter 7: Diagnostics

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.

Status of ITER neutron diagnostic development

Due to the high neutron yield and the large plasma size many ITER plasma parameters such as fusion power, power density, ion temperature, fast ion energy and their spatial distributions in the plasma core can be measured well by various neutron diagnostics. Neutron diagnostic systems under consideration and development for ITER include radial and vertical neutron cameras (RNC and VNC), internal and external neutron flux monitors (NFMs), neutron activation systems and neutron spectrometers. The two-dimensional neutron source strength and spectral measurements can be provided by the combined RNC and VNC. The NFMs need to meet the ITER requirement of time-resolved measurements of the neutron source strength and can provide the signals necessary for real-time control of the ITER fusion power. Compact and high throughput neutron spectrometers are under development. A concept for the absolute calibration of neutron diagnostic systems is proposed. The development, testing in existing experiments and the engineering integration of all neutron diagnostic systems into ITER are in progress and the main results are presented.

TFTR natural diamond detectors based D–T neutron spectrometry system

Three natural diamond detectors (NDD) based spectrometry systems have been developed and used on TFTR to perform D–T neutron spectrum and flux measurements. DT neutrons interact with the NDD through the 12C(n, alpha)9Be reaction to produce a narrow peak in the pulse height distribution which has 2%–3% energy resolution and is well isolated from other reactions by ∼2 MeV in energy. The NDD detector is also highly radiation resistant (5×1014 n/cm2) and compact in size (diameter ∼4 mm, thickness ∼0.2 mm). Three detectors have been installed near TFTR. One, installed in a central sight line of the neutron collimator views perpendicularly from below. Two other detectors placed inside radiation shields in the TFTR test cell have tangential and perpendicular cones of view. These three spectrometers provide spectrometry of D–T neutrons escaping the tokamak with angles with respect to plasma current in the ranges close to 90°, 110°–180°, and 60°–120° correspondingly. The fastest standard electronics have been used...

Overview of neutron and confined/escaping alpha diagnostics planned for ITER

Fusion product measurements planned for ITER are reviewed from the viewpoint of alpha particle-related physics studies. Recent advances in fusion plasma physics have extended the desirable measurement requirements to the megahertz region for neutron emission rate, better resolution of neutron profiles for the study of internal transport barriers (ITBs), etc. Employing threshold counters and/or scintillation detectors confers megahertz capability on neutron emission rate measurement. The changes in the neutron/alpha particle birth profile due to the formation of ITB and its deviation from uniformity on the magnetic flux surface can be measured by addition of eight viewing chords in an equatorial port plug and seven viewing chords from the divertor to the original radial neutron camera. On the other hand, it is still difficult to measure the distributions of confined and escaping alpha particles. Several proposals to resolve these difficulties are currently under investigation.

Charge exchange neutral particle analysis with natural diamond detectors on LHD heliotron

Semiconductor detectors based on natural diamonds have been installed on the Large Helical Device (LHD) heliotron to measure the energy distribution of charge exchange fast neutral particles from different viewing angles. Advantages of a natural diamond detector (NDD) are (1) very compact size, (2) relatively easy handling, and (3) high energy resolution. Although NDDs are sensitive to visible light, vacuum ultraviolet, and soft x rays, unfavorable pulses produced by such radiation were greatly reduced by choosing an appropriate stainless steel shield in this experiment. In LHD, the time-resolved energy distribution of counter-going beam ions and ion cyclotron range of frequency-produced energetic ions have been successfully obtained by means of an NDD. The performance of NDDs as a neutral particle analyzer and its good suitability to LHD plasmas were demonstrated throughout this work.

Natural diamond detector as a high energy particle spectrometer

Natural Diamond Detectors (NDD) were developed in Russia and have been used for deuterium-tritium (DT) neutron, and energetic particles spectrometry during high power tokamak experiments. The NDD itself has a metal-insulator-metal structure, where metal is Au, Ti/Mo/Au or Ti/Pt/Au and insulator is selected very pure natural diamond. A high band gap, resistivity and break down resistance, large saturation velocity and mean free drift time (10-15 ns) are very important properties for materials for radiation detectors. Spectrometry application of NDD based on big (up to some mm) charge collection distance of selected natural diamonds. The energy resolution of our best detectors was in the range 1.5-3%. Such high resolution coupled with the high radiation resistance of diamonds (5*10/sup 14/ n/cm/sup 2/) provided their successful application for 14 MeV neutron spectrometry and flux monitoring in experiments on SNEG-13, FNG, NS neutron generators, and tokamaks TFTR and JT-60U. The possibility of its operation at extremely high temperatures makes the NDD ideal for neutron and high energy particles spectrometry in future burning plasma and other high energy physics experiments.

Study of d–t neutron energy spectra at JET using natural diamond detectors

Abstract Four natural diamond detectors (NDDs) have been used for deuterium–tritium neutron spectrometry and flux monitoring during the 1997 tritium experiments (DTE1) carried out in the Joint European Torus (JET). Neutron energy spectra have been measured with three NDDs for discharge scenarios that included (a) hot ion H-mode studies using combined neutral-beam (NB) and ion cyclotron resonance frequency (ICRF) heating, (b) optimized shear experiments using combined NB and ICRF heating, (c) alpha-particles heating experiments with NB heating only and (d) ICRF heating studies without NB heating. Within the statistical accuracy of the data, the spectra can be adequately represented by Gaussian distributions, whose fwhm values provide effective ion temperatures that characterize the energy distributions of the ions taking part in fusion reactions.

Application of Natural Diamond Detector to Energetic Neutral Particle Measurements on NSTX

Two natural diamond detectors have been installed on the National Spherical Torus Experiment (NSTX) to look at escaping neutrals at or near the neutral beam injection energy. Time resolved measurements have been obtained from these detectors at various tangency radii. The close proximity of the detector to the vessel required the development of a very fast low-noise preamplifier, which has been shown to be superior to similar commercial units. With this amplifier arrangement, electromagnetic pick-up noise was reduced to acceptable levels. However, radiation shielding was required to reduce the background levels from neutron-induced pulses in the detector. Calibration data along with the measured energy resolution is presented in the useful energy range for NSTX. Example data from plasma discharges is also presented.

Conceptual design of the charge exchange recombination spectroscopy diagnostic for ITER

Active charge exchange recombination spectroscopy (CXRS) is used in most of the present fusion experiments as a proven tool for local measurements of the main ions in the plasma [R. Isler, Plasma Phys. Controlled Fusion 36, 171 (1994)]. A comprehensive diagnostic coverage of intrinsic and injected impurities is essential for any self-consistent plasma simulation and prediction of plasma performance. In particular, for the assessment of local helium ash densities [M. von Hellermann et al., Plasma Phys. Controlled Fusion 35, 799 (1993)], CXRS will play a key role for future fusion devices such as ITER. However, it should be emphasized that any helium ash analysis is only viable in a fully diagnosed plasma, that is, all other ions need to be measured as well. Two fundamental limitations are considered in the following assessment. First is the detectability of a weak CX signal against a strong background of plasma continuum radiation. A second, equally important requirement is the accuracy with which local ne...

Fundamental ion cyclotron resonance heating of JET deuterium plasmas

Radio frequency heating of majority ions is of prime importance for understanding the basic role of auxiliary heating in the activated D-T phase of ITER. Majority deuterium ion cyclotron resonance heating (ICRH) experiments at the fundamental cyclotron frequency were performed in JET. In spite of the poor antenna coupling at 25 MHz, this heating scheme proved promising when adopted in combination with D neutral beam injection (NBI). The effect of fundamental ICRH of a D population was clearly demonstrated in these experiments: by adding similar to 25% of heating power the fusion power was increased up to 30-50%, depending on the type of NBI adopted. At this power level, the ion and electron temperatures increased from T-i similar to 4.0 keV and T-e similar to 4.5 keV (NBI-only phase) to T-i similar to 5.5 keV and T-e similar to 5.2 keV (ICRH + NBI phase), respectively. The increase in the neutron yield was stronger when 80 keV rather than 130 keV deuterons were injected in the plasma. It is shown that the neutron rate, the diamagnetic energy and the electron as well as the ion temperature scale roughly linearly with the applied RF power. A synergistic effect of the combined use of ICRF and NBI heating was observed: (i) the number of neutron counts measured by the neutron camera during the combined ICRF + NBI phases of the discharges exceeded the sum of the individual counts of the NBI-only and ICRF-only phases; (ii) a substantial increase in the number of slowing-down beam ions was detected by the time of flight neutron spectrometer when ICRF power was switched on; (iii) a small D subpopulation with energies slightly above the NBI launch energy was detected by the neutral particle analyzer and gamma-ray spectroscopy.