Diagnostic Development on NSTX

Diagnostics are described which are currently installed or under active development for the newly commissioned NSTX device. The low aspect ratio (R/a less than or equal to 1.3) and low toroidal field (0.1-0.3T) used in this device dictate adaptations in many standard diagnostic techniques. Technical summaries of each diagnostic are given, and adaptations, where significant, are highlighted

Measurement of 14 MeV neutrons at TFTR with Si-diode detectors

A detector system based on partially depleted silicon surface barrier detectors and fast front-end electronics has been built and cross calibrated to a set of absolutely calibrated He recoil detectors. The cross-calibration factor for the channel with the widest dynamic range is 2.5×10 counts per 14 MeV source neutron. These data agree well with the independent neutron activation data. The new detector system covers a large dynamic range (corresponding to 10 -10 neutrons/s). The response is linear, except at the highest count rates where the detector dead time (∼200 ns) causes departure from linearity. The noise discrimination against 2.5 MeV neutrons and γ pileup is excellent. Measurements of D-T neutrons from a tritium gas puff experiment as well as from a high-power D-T discharge in the TFTR tokamak are presented. © 1995 American Institute of Physics. 4 -13 13 1

Compact solid-state neutral particle analyzer in current mode.

Solid state neutral particle analyzer (ssNPA) arrays are operated in current mode on the DIII-D tokamak and the National Spherical Torus Experiment (NSTX). Compared with conventional pulse-counting NPAs, current-mode operation sacrifices energy resolution to obtain economical, high-bandwidth, pitch-angle resolved measurements. With the success from a new three-channel near-vertical-view current mode ssNPA on DIII-D, the apertures on an existing array on NSTX were expanded to increase the particle influx. The sightlines of both arrays intersect heating beams, enabling both active and passive charge exchange measurements. The spatial resolution at beam intersection is typically 5 cm on both devices. Directly deposited ultra-thin foils on the detector surface block stray photons below the energy of 1 keV and also set low energy threshold about 25 keV for deuterium particle detection. Oscillations in neutral flux produced by high frequency magnetohydrodynamics (MHD) instabilities are readily detected

Design of solid state neutral particle analyzer array for National Spherical Torus Experiment-Upgrade.

A new compact, multi-channel Solid State Neutral Particle Analyzer (SSNPA) diagnostic based on silicon photodiode array has been designed and is being fabricated for the National Spherical Torus Experiment-Upgrade (NSTX-U). The SSNPA system utilizes a set of vertically stacked photodiode arrays in current mode viewing the same plasma region with different filter thickness to obtain fast temporal resolution (∼120 kHz bandwidth) and coarse energy information in three bands of >25 keV, >45 keV, and >65 keV. The SSNPA system consists of 15 radial sightlines that intersect existing on-axis neutral beams at major radii between 90 and 130 cm, 15 tangential sightlines that intersect new off-axis neutral beams at major radii between 120 and 145 cm. These two subsystems aim at separating the response of passing and trapped fast ions. In addition, one photodiode array whose viewing area does not intersect any neutral beams is used to monitor passive signals produced by fast ions that charge exchange with background neutrals

The confinement of dilute populations of beam ions in the national spherical torus experiment

Short ∼3 ms pulses of 80 keV deuterium neutrals are injected at three different tangency radii into the national spherical torus experiment. The confinement is studied as a function of tangency radius, plasma current (between 0.4 and 1.0 MA), and toroidal field (between 2.5 and 5.0 kG). The jump in neutron emission during the pulse is used to infer prompt losses of beam ions. In the absence of MHD, the neutron data show the expected dependences on beam angle and plasma current; the average jump in the neutron signal is 88 ± 39% of the expected jump. The decay of the neutron and neutral particle signals following the blip are compared to the expected classical deceleration to detect losses on a 10 ms timescale. The temporal evolution of these signals are consistent with Coulomb scattering rates, implying an effective beam-ion confinement time ≳ 100 ms. The confinement is insensitive to the toroidal field despite large values of ρ ∇ B/B (≲0.25), so any effects of non-conservation of the adiabatic invariant μ are smaller than the experiment error

Cross calibration of neutron detectors for deuterium-tritium operation in TFTR

During the initial deuterium-tritium experiments on TFTR, neutron emission was measured with 235U and 238U fission chambers, silicon surface barrier diodes, spatially collimated 4He proportional counters and ZnS scintillators, and a variety of elemental activation foils. The activation foils, 4He counters, and silicon diodes can discriminate between 14 and 2.5 MeV neutrons. The other detectors respond to both DD and DT neutrons but are more sensitive to the latter. The proportional counters, scintillators, and some of the fission chambers were calibrated absolutely, using a 14 MeV neutron generator positioned at numerous locations inside the TFTR vacuum vessel. Although the directly calibrated systems were saturated during the highest-power deuterium-tritium operation, they allowed cross calibration of less sensitive fission chambers and silicon diodes. The estimated absolute accuracy of the uncertainty-weighted mean of these cross calibrations, combined with an independent calibration derived from activation foil determinations of total neutron yield, is ±7%. © 1995 American Institute of Physics

The diffusion of fast ions in Ohmic TFTR discharges

Short duration (20 msec) neutral deuterium beams are injected into the TFTR tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. I, p. 51]. The subsequent confinement, thermalization, and diffusion of the beam ions are studied with multichannel neutron and charge exchange diagnostics. The central fast-ion diffusion (<0.05 m /sec) is an order of magnitude smaller than typical thermal transport coefficients. © 1991 American Institute of Physics.

Charge exchange and fusion reaction measurements during compression experiments with neutral beam heating in the tokamak fusion test reactor

Adiabatic toroidal compression experiments were performed in conjunction with high power neutral beam injection in the Tokamak Fusion Test Reactor (TFTR). Acceleration of beam ions to energies nearly twice the injection energy was measured with a charge exchange neutral particle analyser. Measurements were also made of 2.5 MeV neutrons and 15 MeV protons produced in fusion reactions between the deuterium beam ions and the thermal deuterium and3He ions, respectively. When the plasma was compressed, the d(d, n)3He fusion reaction rate increased by a factor of five, and the3He(d, p) α rate by a factor of twenty. These data were simulated with a bounce averaged Fokker-Planck program, which assumed conservation of angular momentum and magnetic moment during compression. The results indicate that the beam ion acceleration was consistent with adiabatic scaling. © 1986 IOP Publishing Ltd