Initial operation of perpendicular line-of-sight compact neutron emission spectrometer in the large helical device

The perpendicular line-of-sight compact neutron emission spectrometer (perpendicular CNES) was newly installed to understand the helically trapped fast-ion behavior through deuterium–deuterium (D–D) neutron energy spectrum measurement in the Large Helical Device (LHD). The energy calibration of the EJ-301 liquid scintillation detector system for perpendicular CNES was performed on an accelerator-based D–D neutron source. We installed two EJ-301 liquid scintillation detectors, which view the LHD plasma vertically from the lower side through the multichannel collimator. The D–D neutron energy spectrum was measured in a deuterium perpendicular-neutral-beam-heated deuterium plasma. By the derivative unfolding technique, it was found that the D–D neutron energy spectrum had a double-humped shape with peaks at ∼2.33 and ∼2.65 MeV. D–D neutron energy spectrum was calculated based on the fast ion distribution function using guiding center orbit-following models considering the detector’s energy resolution. The calculated peak energies in the D–D neutron energy spectrum almost match the experiment. In addition, a feasibility study toward the measurement of the energy distribution of ion-cyclotron-range-of-frequency-wave-accelerated beam ions was performed

The large helical device vertical neutron camera operating in the MHz counting rate range

In the currently performed neutral beam (NB) -heated deuterium plasma experiments, neutrons are mainly produced by a beam-plasma reaction. Therefore, time-resolved measurement of the neutron emission profile can enhance the understanding of the classical and/or anomalous transport of beam ions. To measure radial neutron emission profiles as a function of time, the vertical neutron camera (VNC) capable of operation with a counting rate in the MHz range was newly installed on the Large Helical Device (LHD). This is the world’s first neutron camera for stellarator/heliotron devices. The VNC consists of a multichannel collimator, eleven fast-neutron detectors, and the digital-signal-processing-based data acquisition system (DAQ). The multichannel collimator having little cross talk was made from hematite-doped heavy concrete, which has a high shielding performance against both neutrons and gamma-rays. A stilbene crystal coupled with a photomultiplier having high-gain-stability in the high-count rate regime was utilized as a fast-neutron scintillation detector because it has a high neutron-gamma discrimination capability at high count rates. The DAQ system equipped with a field programmable logic controller was developed to obtain the waveform acquired with a 1 GHz sampling rate and the shaping parameter of each pulse simultaneously at up to 106 cps (counts per second). Neutron emission profiles were successfully obtained in the first deuterium campaign of LHD in 2017. The neutron emission profile was measured in tangentially co-injected NB-heated plasma with different magnetic axes (Rax). The neutron counts became larger in the inward-shifted configuration, which was consistent with the total neutron rate measured by the neutron flux monitor. The radial peak position of the line-integrated neutron profile which changed according to Rax showed that the VNC worked successfully as designed. The VNC demonstrated the expected performance conducive to extending energetic-particle physics studies in LHD

Sensitivity of Gaussian energy broadening function of MCNP pulse height spectra on CLYC7 scintillation detector

The Cs2LiYCl6:Ce crystal (CLYC) is an inorganic scintillator which has been developed for the γ-ray and neutron measurement with the high detection efficiency, high resolution, and no need unfolding technique. To enhance the measurement of the fast neutron, the CLYC with 7Li-enrichment (CLYC7) scintillator is developed. In this work, the response of the CLYC7 detector to γ-ray is obtained using 137Cs γ-ray calibration source and calculated using Monte Carlo N-Particle transport code (MCNP). A comparison of measured and calculated γ-rays spectra is complicated by the fact that physical radiation detectors have finite energy resolution. In this study, we treated detector energy resolution effect by Gaussian energy broadening (GEB) in MCNP pulse height spectra calculation. We observe the parameters in the GEB function which provides simulation spectrum matches the experiment spectrum, especially on the photopeak region. The detail sensitivity of GEB function on CLYC7 scintillation detector is presented in this work

On the Characteristic Difference of Neoclassical Bootstrap Current and Its Effects on MHD Equilibria between CHS Heliotron/Torsatron and CHS-qa Quasi-Axisymmetric Stellarator

The characteristic difference of neoclassical bootstrap current and its effects on MHD equilibria are described for the CHS heliotron/torsatron and the CHS-qa quasi-axisymmetric stellarator. The direction of bootstrap current strongly depends on collisionality in CHS, whereas it does not in CHS-qa because of quasi-axisymmetry. In the CHS configuration, it appears that enhanced bumpy (Bs1) and sideband components of helical ripple (By1) play an important role in reducing the magnetic geometrical factor, which is a key factor in evaluating the value of bootstrap cuffent, and determining its polarity. The bootstrap current in CHS-qa is theoretically predicted to be larger than that in CHS and produces significant effects on the resulting rotational transform and magnetic shear. In the finite B plasmas, the magnetic well becomes deeper in both CHS and CHS-qa and its region is expanded in CHS. The existence of co-flowing bootstrap current makes the magnetic well shallow in comparison with that in currentless equilibrium

Orbit Topology and Confinement of Energetic Ions in the CHS-qa Quasi-Axisymmetric Stellarator

The orbit topology and confinement of neutral beam-injected energetic ions are investigated for the current target configuration of the CHS-qa quasi-axisymmetric stellarator. It was shown that tangentially co-injected neutral beam (NB) heating is efficient even at a low magnetic field strength Bt of 0.5 T, whereas the heating efficiency of the counter-injected NB becomes significantly lower as Bt decreases because of the increase of first orbit loss. The energy loss rate increases as the beam injection angle becomes perpendicular, suggesting that the residual non-axisymmetric ripple in the peripheral domain plays a role in enhancing the transport of trapped ions. An interesting observation involves the appearance of the island structure in both the gyro motion following orbit and the guiding center collisionless orbit of counter-moving transit beam ions. It appears under a particular, narrow range of parameters, i.e., energy, pitch angle v///v, normalized minor radius r/a at the launching point and Bt

Evaluation of tritium production rate in a blanket mock-up using a compact fusion neutron source

We report a neutronics study of a blanket mock-up using a discharge-type compact fusion neutron source. Deuterium–deuterium fusion neutrons were irradiated to the mock-ups composed of tritium breeder and neutron reflector/moderator. The tritium production rate (TPR) per source neutron was measured by a single-crystal diamond detector with a 6Li-enriched lithium fluoride film convertor after the calibration process. Despite the low neutron yield, energetic alpha and triton particles via 6Li(n, t)α neutron capture as well as 12C via elastic scattering were successfully detected by the SDD with high signal to noise ratios. The TPRs were experimentally evaluated with errors of 8.4%–8.5% at the 1σ level at the positions with high thermal neutron fluxes where the errors were dominantly introduced by uncertainties in the monitoring of the neutron production rate. The calculated to experimental (C/E) values of TPR were evaluated to be 0.91–1.27 (FENDL-2.1) and 0.94–1.28 (FENDL-3.1). As the neutron source can generate 14 MeV neutrons using a mixed gas of deuterium and tritium, this approach provides more opportunities for blanket neutronics experiments

Effects of Current Profile on Global Ideal MHD Stability in a Compact Quasi-Axisymmetric Stellarator

The global ideal magnetohydrodynamic (MHD) stability for a proposed compact quasi-axisymmetric stellarator CHS-qa has been investigated taking the effect of bootstrap current into account. Assuming experimentally achievable density and temperature profiles, the stability properties of global low-n modes have been studied by using threedimensionalnumerical codes based on fixed boundary MHD equilibria including self-consistent bootstrap current for the CHS-qa reference configuration. Consequently it has been shown that values of edge rotational transform play a crucial role in triggering external kink instability. Concerning a lot of other possibilities in experimental practice to change the total parallel current, we have also studied equilibria with increased or decreased parallel current, but fixed profile. The onset of external kink modes depends on rotational transform or current profile, and we found a stable equilibrium in spite of the edge rotational transform above 0.5. The results imply the possibility of stabilizing external kink modes through current and/or pressure profile control in high beta equilibria

Fast-Ion-Diagnostics for CHS Experiment

Fast-ion-diagnostics have played an important role in investigating issues related to fast ion orbits and fast-ion-driven MHD instabilities in CHS experiments. The fast-ion diagnostics employed in CHS are reviewed and experimentally obtained knowledge is summarized

Evaluation of scintillating-fiber detector response for 14 MeV neutron measurement

A scintillating-fiber (Sci-Fi) detector has been employed to measure 14 MeV neutrons for the triton burnup study in the first deuterium plasma campaign of the Large Helical Device (LHD). The pulse-height spectra of the Sci-Fi detector are used to choose a suitable threshold for the discrimination of 14 MeV neutrons from a mix-radiation field of low-energy neutrons and gamma-rays. The measured pulse-height spectra of the Sci-Fi detector have two components with different decay slopes from the LHD experiment. To study the pulse-height property of the Sci-Fi detector, the pulse-height spectra on different energy neutrons have been measured by using the accelerator-based neutron source with d-D, p-Li, and d-Li reactions. Meanwhile, the simulations of the detector response have been performed by using the Particle and Heavy Ion Transport code System (PHITS). In the LHD experiment, the first decay component of the pulse-height spectra in low-pulse-height region has been found to correspond to the signals induced by 2.45 MeV neutrons and gamma-rays. In addition, the high-pulse-height region has been confirmed by both the accelerator experiment and the PHITS calculation to correspond to the recoil-proton edge induced by triton burnup 14 MeV neutrons. The detection efficiency of 14 MeV neutrons for the Sci-Fi detector calculated by the PHITS code agrees well with the detection efficiency of 14 MeV neutrons for the Sci-Fi detector evaluated in the LHD experiment. The Sci-Fi detector can work as a standard detector for the 14 MeV neutron measurement with a suitable threshold