ALBERT

Notes: Charge exchange diagnostics in a hostile neutron and gamma radiation environment require substantial shielding against radiation-induced noise signal. Minimizing the dimensions of the apparatus significantly reduces the shielding complexity. Conceptual designs for compact analyzers are presented. The reduced size is achieved primarily by employing high-field-strength (≤8 kG) permanent rare earth magnets to simplify magnetic field generation. In addition, a carbon foil is used in place of a gas cell to eliminate the customary analyzer vacuum pumping systems. Alternative designs employing electric and magnetic fields for mass and energy analysis will be presented that afford a reduction in volume by an order of magnitude compared with the E(parallel)B analyzers currently in use on the tokamak fusion test reactor (TFTR).

Notes: The resonant nuclear reactions D(α,γ) 6Li, 6Li(α,γ) 10B, and 7Li(α,γ) 11B are examined as diagnostics of the energy distribution of confined fast alpha particles in tokamak plasmas. Reaction rates for Q=1 D-T plasmas are estimated. The design of and preliminary results from the prototype fusion gamma ray detector on the tokamak fusion test reactor (TFTR) will be presented. The activation reactions 10B(α,n) 13N, 14N(α,γ) 18F, 25Mg(α,p) 28Al, and 27Al(α,p) 30P are similarly examined as diagnostics of fast escaping alpha particles. Count rate estimates for Q=1 D-T plasmas will be presented.

Notes: A case for substantial loss of fast ions degrading the performance of tokamak fusion test reactor plasmas [Phys. Plasmas 2, 2176 (1995)] with reversed magnetic shear (RS) is presented. The principal evidence is obtained from an experiment with short (40–70 ms) tritium beam pulses injected into deuterium beam heated RS plasmas [Phys. Rev. Lett. 82, 924 (1999)]. Modeling of this experiment indicates that up to 40% beam power is lost on a time scale much shorter than the beam–ion slowing down time. Critical parameters which connect modeling and experiment are: The total 14 MeV neutron emission, its radial profile, and the transverse stored energy. The fusion performance of some plasmas with internal transport barriers is further deteriorated by impurity accumulation in the plasma core. © 1999 American Institute of Physics.

Notes: Neoclassical simulations of alpha particle density profiles in high fusion power plasmas on the Tokamak Fusion Test Reactor [Phys. Plasmas 5, 1577 (1998)] are found to be in good agreement with measurements of the alpha distribution function made with a sensitive active neutral particle diagnostic. The calculations are carried out in Hamiltonian magnetic coordinates with a fast, particle-following Monte Carlo code which includes the neoclassical transport processes, a recent first-principles model for stochastic ripple loss and collisional effects. New calculations show that monotonic shear alpha particles are virtually unaffected by toroidal field ripple. The calculations show that in reversed shear the confinement domain is not empty for trapped alphas at birth and allow an estimate of the actual alpha particle densities measured with the pellet charge exchange diagnostic. © 1999 American Institute of Physics.

Notes: Data from an E(parallel)B charge exchange neutral analyzer (CENA), which views down the axis of a neutral beamline through an aperture in the target chamber calorimeter of the TFTR neutral beam test facility, exhibit two curious effects. First, there is a turn-on transient lasting tens of milliseconds having a magnitude up to three times that of the steady state level. Second, there is a 720 Hz, up to 20% peak-to-peak fluctuation persisting the entire pulse duration. The turn-on transient occurs as the neutralizer/ion source system reaches a new pressure equilibrium following the effective ion source gas throughput reduction by particle removal as ion beam. Widths of the transient are a function of the gas throughput into the ion source, decreasing as the gas supply rate is reduced. Heating of the neutralizer gas by the beam is assumed responsible, with gas temperature increasing as gas supply rate is decreased. At low gas supply rates, the transient is primarily due to dynamic changes in the neutralizer line density and/or beam species composition. Light emission from the drift duct corroborate the CENA data. At high gas supply rates, dynamic changes in component divergence and/or spatial profiles of the source plasma are necessary to explain the observations. The 720 Hz fluctuation is attributed to a 3% peak-to-peak ripple of 720 Hz on the arc power supply amplified by the quadratic relationship between beam divergence and beam current. Tight collimation by CENA apertures cause it to accept a very small part of the ion source's velocity space, producing a signal linearly proportional to beam divergence. Estimated fluctuations in the peak power density delivered to the plasma under these conditions are a modest 3%–8% peak to peak. The effects of both phenomena on the injected neutral beam can be ameliorated by careful operation of the ion sources.

Notes: Results are given from the first comprehensive and complementary measurements using the final production U.S. Common Long Pulse Ion Sources mounted on both the TFTR neutral beam test beamline and the TFTR neutral beam injection system, with actual tokamak experimental conditions, power systems, controls, and operating methods. The set of diagnostics included water calorimetry, thermocouples, vacuum ionization gauges, photodiodes, neutron, gamma-ray, and charged particle spectroscopy, optical multichannel analysis, charge exchange spectroscopy, Rutherford backscatter spectroscopy, and implantation/secondary ion mass spectroscopy. These systems were used to perform complementary measurements of neutral beam species, impurities, spatial divergence, energy dispersion, pressure, and reionization. The measurements were performed either in the neutralizer region, where the beam contained both ions and neutrals, or in the region of the output neutral beam. The average of the neutral particle ratios in the range from 80 to 114 keV is D0[E]:D0[E/2]:D0[E/3]=53(5):27(4):20(4), where the quantities in parentheses are the average experimental uncertainties.The corresponding neutral power ratio is P0[E]:P0[E/2]:P0[E/3]=72(9):19(3):9(2). The half widths (1/e) in the horizontal plane for the full-, half-, and third-energy components were 0.26°, 0.34°, and 0.42°, respectively. The dispersions of the full-, half-, and third-energy components were 1.20 keV, 2.35 keV, and 2.26 keV, respectively. The carbon impurity concentration in a 80 keV D0 beam was not greater than 2×10−4 per D0 beam particle, and exhibited an apparent acceleration state of C+. The oxygen impurity concentration was less than 5×10−4 per D0 beam particle, and exhibited an apparent acceleration state of O+. A variety of vacuum conditions were observed depending on the operating conditions. Typically, pressures in the transition ducts were in the range from 0.3 to 0.7×10−5 Torr at the beginning of injection pulses, and reionized power losses were in the range from 0.75% to 1.5% of incident power. At the end of injection pulses, pressures in the transition ducts were in the range from 0.6 to 2×10−5 Torr and reionized power losses were in the range from 2% to 6% of incident power. This work describes generic results, new apparatus, and advances in measurement techniques for the optimization of tokamak neutral beam heating operations and the analysis of neutral beam heated plasmas.

Notes: The utility of charge exchange neutral particle analyzers for studying energetic ion distributions in high-temperature plasmas has been demonstrated in a variety of tokamak experiments. Power deposition profiles have been estimated in the Princeton large torus (PLT) from particle measurements as a function of energy and angle during heating in the ion cyclotron range of frequencies (ICRF) and extensive studies of this heating mode are planned for the upcoming operational period in the tokamak fusion test reactor (TFTR). Unlike the horizontally scanning analyzer on PLT, the TFTR system consists of vertical sightlines intersecting a poloidal cross section of the plasma. A bounce-averaged Fokker–Planck program, which includes a quasilinear operator to calculate ICRF-generated energetic ions, is used to simulate the charge exchange flux expected during fundamental hydrogen heating. These sightlines also cross the trajectory of a diagnostic neutral beam (DNB), and it may be possible to observe the fast ion tail during 3He minority heating, if the DNB is operated in helium for double charge exchange neutralization.

Notes: The high source strength of 2.45-MeV neutrons (∼1.2×1016 n/s) and neutron-induced gamma rays produced by the Tokamak Fusion Test Reactor (TFTR) have substantially degraded the performance of many diagnostics. In particular, data obtained by the charge exchange neutral analyzer (CENA) were seen to degrade at modest production levels of ∼1015 n/s. To reduce the radiation-induced noise to acceptable levels, massive shielding was required. A design and cost-effectiveness study was performed for the TFTR CENA which showed that a double-layered enclosure consisting of a 10-cm-thick inner housing of lead surrounded by a 23-cm-thick layer of 1.0% borated polyethylene was required to reduce the neutron and gamma noise level by two orders of magnitude. This design was recently implemented on TFTR and was shown to perform as predicted. The details of the enclosure will be presented and the contributing factors that led to the choice of this particular design will be reviewed.

Notes: The primary mission of the Compact Ignition Tokamak (CIT) is to study the physics of alpha-particle heating in an ignited D–T plasma. A burn time of about 10 τE is projected in a divertor configuration with base-line machine design parameters of R=2.10 m, a=0.65 m, b=1.30 m, Ip=11 MA, BT=10 T, and 10–20 MW of auxiliary rf heating. Plasma temperatures and density are expected to reach Te(0)∼20 keV, Ti(0)∼30 keV, and ne(0)∼1×1021 m−3. The combined effects of restricted port access to the plasma, the presence of severe neutron and gamma radiation backgrounds, and the necessity for remote handling of in-cell components create challenging design problems for all of the conventional diagnostics associated with tokamak operations. In addition, new techniques must be developed to diagnose the evolution in space, time, and energy of the confined alpha distribution as well as potential plasma instabilities driven by collective alpha-particle effects. The design effort for CIT diagnostics is presently in the conceptual phase, with activity being focused on the selection of a viable diagnostic set and the identification of essential research and development projects to support this process. A review of these design issues and other aspects impacting the selection of diagnostic techniques for the CIT experiment will be presented.

Notes: Co- and counter-viewing bolometers aimed along a common tangency chord are being used to study power losses due to charge exchange (CX) of fast ions in neutral beam injection (NBI) heated TFTR plasmas. For unidirectional injection, tangential bolometers oriented to view CX loss of circulating fast ions detect losses from the thermal target plasma (impurity radiation and CX) plus power due to the fast ion CX loss, whereas bolometers oppositely directed measure only the target plasma contribution. The difference between the two signals is a measure of the fast ion CX loss. Additional information is obtained by comparing the tangential bolometer signals with those of perpendicularly viewing bolometer monitors and arrays. The measurements are compared to results of the TRANSP code analysis.