F. E. Cecil

Fusion gamma diagnostics

Nuclear reactions of interest in fusion research often possess a branch yielding prompt emission of gamma radiation in excess of 15 MeV which can be exploited to provide a new fusion reaction diagnostic having applications similar to conventional neutron emission measurements. Conceptual aspects of fusion gamma diagnostics are discussed with emphasis on application to the Tokamak Fusion Test Reactor (TFTR) during deuterium neutral beam heating of D–T and D–3He plasmas. Recent measurements of the D (T,u2009γ)5He, D(3He,u2009γ)5Li, and D(D,u2009γ)4He branching ratios at low center‐of‐mass energy (30–100 keV) and of the response of a large volume Ne226 detector for gamma detection in high neutron backgrounds are presented. Using a well‐shielded Ne226 detector during 20 MW–120 kV deuterium beam heating of a tritium plasma in TFTR, the D(T,u2009γ)5He gamma signal level is estimated to be 3.5×105 cps.

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 experimental error. PACS numbers: 52.55.Fa, 52.50.Gj

Development and operation of a thin foil Faraday collector as a lost ion diagnostic for high yield d-t tokamak fusion plasmas

We are continuing our development of a radiation-hard, charged-particle detector consisting of a series of thin parallel conducting foils as a lost ion diagnostic for high yield d-t tokamak fusion plasmas. Advantages of this detector concept include economy, ability to operate in relatively intense neutron/gamma ray radiation backgrounds and at moderately high temperatures, and a modest degree of energy resolution. A detector consisting of four parallel foils of Ni, each of thickness 2.5 μm, was operated in the Joint European Torus during the recent DTE-1 experiment. During the highest yield pulses of this campaign, (16 MW), the flux of energetic alpha particles at the detector was measured to be less than about 2u2009nA/cm2. This upper limit is significantly greater than the expected flux assuming classical losses and given the geometry of the detector. During most of the nearly 2500 pulses of the DTE-1 experiment for which the detector response has been inspected, a relatively intense (up to 200u2009nA/cm2) flu...

Gamma-diagnostics of alpha-particles in 4He and D–T plasmas

This article reports on γ-ray diagnosis of ICRF-accelerated 4He ions and fusion α particles used in the JET tokamak. This diagnostic based on the analysis of γ rays emitted in nuclear reaction 9Be(α,nγ)12C. The results of the first observation of the 4.44-MeV γ-radiation due to the reaction 9Be(α,nγ)12C in JET deuterium–tritium experiment (DTE1) are presented. Gamma-ray spectra have been recorded with collimated spectrometer, and the γ-ray emission radial profiles measured with the JET neutron/gamma profile monitor. Doppler broadening effects which could be used for the ICRF-accelerated 4He-ion diagnosis with high energy resolution γ-ray detectors are discussed. Capabilities of the γ-ray measurements in next step D–T devices are discussed.

A thin film device as a low energy, high flux charged particle spectrometer

We are continuing our investigation of the use of stacks of electrically isolated thin metal foils as spectrometers for lost ions from tokamak fusion plasmas. Devices of this type in which the foil thicknesses were a few micrometers were installed on the Joint European Torus during the recent first deuterium–tritium experiment in an effort to observe lost energetic alpha particles. While there was no convincing evidence of lost alpha particles in this experiment, we did observe significant fluxes of low energy (<500 keV) charged particles. In an effort to provide an instrument for the investigation of this phenomenon and of escaping relatively low energy (<100 keV) ions from other fusion plasma devices, we have developed alternative devices with very thin (few hundred nanometers) alternating layers of conductor and insulator. Four such devices have been fabricated and tested for protons with energies between 20 and 160 keV and demonstrated good energy resolution (typically about 10%) for proton bombarding...

Fast ion loss diagnostic plans for the National Spherical Torus Experiment

The prompt loss of neutral beam ions from the National Spherical Torus Experiment is expected to be between 12% and 42% of the total 5 MW of beam power. There may, in addition, be losses of fast ions arising from high harmonic fast wave (HHFW) heating. Most of the lost ions will strike the HHFW antenna or the neutral beam dump. To measure these losses in the 2000 experimental campaign, thermocouples in the antenna, several infrared camera views, and a Faraday cup lost ion probe will be employed. The probe will measure loss of fast ions with E>1 keV at three radial locations, giving the scrape-off length of the fast ions.

Measurement and application of DDγ, DTγ and D3Heγ reactions at low energy

Abstract The gamma ray branching ratios for the reactions T(d, γ)5He, D(d, γ)4He, 3He(d, γ)5 Li have been measured between center of mass energies of 25 to 100 keV. The branching ratios are observed to be roughly constant over the energy range of the measurements with best values Γdtγ/ Γdtα = (5.4±1.3)× 10−5Γddγ/Γddp= (1.1 ±0.3) × 10−7 and Γd3Heγ/ Γd3Heα = (5.0±1.2) × 10−5. These branching ratios are used to calculate the reactivities for the production of the gamma rays by high temperature plasmas. Specific applications to TFTR plasmas will be presented.

Thin foil Faraday collectors as a radiation hard fast lost-ion diagnostic

We are investigating thin foil Faraday collectors as a diagnostic for lost fast ions from tokamak fusion plasmas. Prototype devices have been recently installed in the National Spherical Torus Experiment and DIII-D. Initial results from these devices indicate a loss of energetic ions from a variety of plasma conditions. Results from a device installed immediately outside a thin Be window on ALCATOR C-mod, as a test on the response to moderately intense fluxes of soft x rays indicate an upper limit of about 2×10−22u2009A/photon/cm2 at a plasma electron temperature of 1.8 keV. An important property of the diagnostic is the expected ability to operate under fairly high neutron/gamma radiation backgrounds. We have tested this expectation by measuring the current from a thin (2.5 μm) Ni foil placed in the core of a TRIGA fission reactor. At a maximum steady-state power of 950 kW (1013u2009n/cm2/s), a current of 1.2 nA/cm2 was measured.

The low energy high current particle accelerator at the Colorado School of Mines

The General Ionex model 1545 particle accelerator recently acquired by the Colorado School of Mines Physics Department is described. Operating conditions such as beam energy, currents, ionic species, energy resolution, and vacuum are described. Measurements of radiative capture reactions of protons on D, 6Li, 7Li, and 11B between bombarding energies of 10 and 180 keV are reported. Application of these measurements to the diagnostics of advanced fuel controlled fusion reactors is suggested.

Evaluation of an ITER compatible, thin foil Faraday collector as a lost alpha particle diagnostic for high yield D–T fusion plasmas

We have examined the concept of a thin foil Faraday collector as a lost alpha particle detector capable of operating under ITER-like conditions. A prototype detector consisting of a single set of four 2.5 μm Ni foils was installed on the JET first wall and operated during a variety of deuterium plasma conditions during the 1995 JET run period. Although there was no significant production of alpha particles during these plasmas, the prototype demonstrated the expected resistance to the high temperature and x-ray backgrounds, as well as moderate neutron and gamma ray backgrounds characteristic of these plasmas. In addition, this prototype showed no significant response to neutral beam, rf, or lower hybrid plasma heating. The device did pick up a low level signal when neutral beams were injected simultaneously with heavy gas puffing. Strong intermittent correlations were seen with excursions in the Hα edge brightness signal. In addition, the detector produced a significant signal in response to a roughly 250...