Gamma Ray Diagnostics of Energetic Ions
Table of Contents:
One of the most interesting diagnostics of energetic ions with significant progress in recent years is the diagnostic of energetic ions via nuclear gamma rays [3.16]. A strong gamma ray emission in the MeV energy range comes from fusion-grade tokamak plasmas. This emission has both a continuum spectrum in energy and a discrete spectrum. The continuum spectrum is caused by the Bremsstrahlung, supra-thermal electrons, and a background noise, whereas the discrete spectrum consists of spectral lines emitted in nuclear reactions between energetic and thermal ions and Be and C impurities. Each type of energetic ions colliding emits a well-determined gamma ray with energy unique to this specific ion colliding with a specific impurity (as fingertips are unique to a certain person).
For ITER, a simultaneous diagnosis of two types of fast ions, the fusion-generated alpha particles (at 3.52 MeV) and NBI-produced deuterium ions (born at about 1 MeV), will be of major importance as these will be the fast ion populations with the largest energy contents. In the presence of Be impurity, energetic 4He ion with £>1.7 MeV could be detected from gamma rays with an energy above 4 MeV arising from the nuclear reactions
while energetic deuterium ions with £>0.5 MeV could be detected from gamma rays generated during the reaction
Figure 3.5 shows the spectrum of gamma rays as a function of gamma ray energy measured in one of JET discharges. In this figure, the double peak around 4 MeV results from nuclear reaction (3.14),
FIGURE 3.5 Gamma ray spectra measured by the Nal(TI) detector on JET: solid line shows the spectrum recorded in JET discharge with 70 and 110 keV He NBI; dashed line shows the spectrum recorded in a discharge with two 70 keV He NBI.
whereas the double peak around 3 MeV results from nuclear reaction (3.15). The significant energy separation between these two peaks was used in measuring gamma emission with a 19-channel two-dimensional (2D) camera on JET, as shown in Figure 3.6. Namely, each channel of this 2D camera was prescribed to measure simultaneously gamma rays in two different energy bands, one of which corresponds to the gamma rays coming from the reaction (3.14) of fast 4He and the other to the gamma rays (3.15) from fast D. Hence, the spatial profiles of gamma rays from two ITER- relevant fast ion species were measured simultaneously for the first time, as show'n in Figures 3.7 and 3.8, with the integration time of 1 s.
In these alpha particle simulation experiments [3.7] with third harmonic ICRH of 4He-beam in 4He-plasmas, gamma-radiation due to the nuclear reaction of 9Be(4He, ny)l2C was detected [3.17], showing the successful ICRH acceleration of 4He-beam ions. The first energy level 4.44 MeV of the final nucleus, l2C, is excited by alpha particles resulting in a peak at 4.44 MeV. The gamma ray emission from the reaction l2C(D, py)l3C was observed as well. A peak at 3.09 MeV (transition 3.09 —> 0), which is identified as a gamma emission from the l2C(D, py)l3C reaction, reflects the presence in the plasma of fast D ions in the MeV range. This indicates that the residual deuterium minority in helium plasmas also absorbs some ICRH power at the third harmonic D resonance that coincides with the third harmonic 4He resonance.
This gamma ray diagnostic technique is especially important for burning plasmas where several groups of different energetic ions co-exist, in addition to the alpha particles, to control the burn. Further development of gamma ray diagnostics will inevitably result in the compatibility of gamma ray diagnostics with DT operation. This is possible with the use of LiH neutron filters transparent to gamma rays, but must be tested in DT plasma first.
FIGURE 3.6 Schematic of the two-dimensional gamma ray camera on JET.
FIGURE 3.7 Two-dimensional tomographic reconstruction of spatial profile of the gamma rays from fast D ions.
Information on Highly Energetic D Ions in the Neutron Emission Spectra
The effect of D beam acceleration with third harmonic ICRH in D plasma [3.9] gives in JET a spectacular increase in D-D fusion yield and a significant increase in the energy of the generated D-D neutrons. The effect of the D-D neutron energy increase was studied in-depth on JET with the time-of-flight TOFOR spectrometer [3.18]. The TOFOR device is placed well above the JET machine, with the line-of-sight passing through the magnetic axis perpendicular to the magnetic field. Because ICRH mostly increases the beam ion velocity perpendicular to the magnetic field, the interpretation of the D-D neutron spectrum does not involve the beam velocity parallel to the magnetic field, and is relatively straightforward. In particular, it is possible to establish a correspondence between the maximum energy of the projectile D ions and the maximum energy of D-D neutrons resulting from the fusion of these projectile ions and thermal D plasma [3.19]. Figure 3.9 shows an example of JET discharge (pulse # 86459 with Br= 2.26 T, I,,=2.16 MA), with the combined synergetic NBI and third beam harmonic ICRH, with the TOFOR measurements of D-D neutrons made at different times.
As shown in Figure 3.9, by adding -3 MW of ICRH power at third harmonic to -4.5 MW of D NBI in D plasma, it was possible to increase the yield of D-D neutrons by a factor of-10. Indeed, the TOFOR measurements show much lower yield of D-D neutrons during the NBI-only heating (see Figure 3.3a and b) phase than during the NBI+ICRH phase (see Figure 3.3c and d). Furthermore,
FIGURE 3.8 Two-dimensional reconstruction of spatial profile of the gamma rays from fast JHe ions.
the fastest time-of-flight decreases significantly, in line with the expected broadening of the energy spectrum of the D-D neutrons. Note that the time-of-flight of D-D neutrons measured with TOFOR depends on the neutron energy as tTOf “ £-<'/" giving a value of ~65 ns for 2.5 MeV neutrons, and much shorter time of ~45 ns for 5 MeV neutrons.
Measurements of neutron emission spectrum were also used in D-T plasmas on JET in 1997. These provided information on the fusion reactivity in D and D-T plasmas, including its dependence on the velocity distribution of fuel ions. Thermal Maxwellian ions produce a Gaussian spectra, the width of which is determined by the Doppler broadening reflecting ion temperature. A deviation from the Gaussian shape indicates the presence of supra-thermal velocity components which appear in conjunction with ICRH and/or NBI, or with the high energy tail due to the knock-on effect [3.14, 3.19]. The spectrum of neutrons born in reactions with D and T knock-on ions extends in energy well beyond that of D-T neutron emission even with ICRH and NBI. The knock-on effect on the spectrum of D-T neutrons was observed experimentally for the first time during the DTE1 campaign on JET (1997) and was also theoretically modelled [3.20]. The knock-on tail was identified in the highest energy of 20% in the measured spectrum corresponding to the neutron energy range E, = 15.7-16.8 MeV. The observation is well described by a calculation with respect to the knock-on neutron flux/thermal neutron flux ratio assuming that the alpha particle confinement and slowing down is classical.
FIGURE 3.9 ICRH and NBI power w'ave-forms and time-of-flight of D-D neutrons measured in JET discharge # 86459. The early phase with NBI-only marked in grey in (a) generates D-D neutron yield at -15 counts/bin with the fastest neutron time-of-flight of -60 ns as (b) show's. Adding 3 MW of ICRH at the third harmonic of D beam marked in grey in (c) increases the yield of D-D neutrons to -150 counts/bin with much faster neutron time-of-flight at -45 ns as (d) show's.
DIAGNOSTICS OF LOST ENERGETIC IONS
Lost energetic ions are detected on many machines with a series of Faraday cups and/or scintillators.
The Faraday cups represent a set of several parallel foils placed inside the torus where the flux of lost energetic ions is investigated. In the presence of lost energetic ions, each of the foils provides counts of the ions hitting it, and as the foil number in the set hit by the lost ion depends on the ion- penetrating ability, the Faraday cups provide some resolution in the energy of the lost energetic ions. On JET, the Faraday cups [3.21] consist of an array of 13 individual detectors, each with a minimum of four foils. Each detector allows a modest degree of energy resolution (between 20% and 50% depending on the detailed foil and aperture geometry), a time resolution of about 1 ms, and a minimum detectable signal of about 0.5 nA. The detectors are distributed between three radial locations (equally spaced between 25 and 85 mm behind the adjacent poloidal limiter) at five poloidal locations between mid-plane and 80cm below mid-plane. The detectors have an energy resolution between 30% and 50% and a bandwidth of 1 kHz. The Faraday cups diagnostic is compatible with the harsh D-T conditions, robust, and suitable for burning plasmas.
FIGURE 3.10 Scintillator probe data (grid of gyro-radius, cm, and pitch-angle, deg) showing losses of alpha particles born in D-'He fusion reactions in JET discharge #86775 (Br=2.24 T, /,,=2 MA). Larmor radii corresponding to alpha particles with energies of 0.53, 1.5, and 2.6 MeV are shown. The white broken line shows the separatrix at the trapped/passing boundary of alpha particles.
The Faraday cups on JET are complemented by a scintillator probe [3.22] located just below the equatorial plane. The scintillator probe consists of a thin layer of P56 scintillator (0.63 lm/W). The image of the scintillator surface is transmitted through a series of lenses and fibre-optic bundles to a CCD and a photomultiplier tube outside the vessel with respective time resolution of 20 and 3 ms. The device provides a pitch angle resolution of 5%, and a best achievable resolution of the gyro- radius of 15%. To maintain a scintillator temperature below approximately 400°C, an externally supplied water cooling system is incorporated in the probe design.
Figure 3.10 shows an example of alpha particle measurements with the scintillator probe on JET. The third harmonic ICRH acceleration of D beams in D-3FIe plasmas was employed in this experiment with amounts of 1He increasing discharge-by-discharge of up to nHe/ ne~ 30% [3.9]. The scintillator probe was employed for measuring ICRH-accelerated D ions and charged fusion products of both D-3He and D-D reactions. Alpha particles generated via D-1He fusion were seen in the scintillator probe as the losses with Larmor radii in excess of-I0cm, just after both NBI and ICRFI were switched off. Figure 3.10 shows the scintillator probe signal at the time of interest.