William L. Barr

Method for Computing the Radial Distribution of Emitters in a Cylindrical Source

A method for computing the distribution of emitters from the observed projected intensity profile is described. The method is applicable to optically thin volume sources with cylindrical symmetry. Unlike previous methods, this method yields results which are relatively insensitive to small random errors in the data.

Venetian-blind direct energy converter for fusion reactors.

A new direct energy conversion technique that uses the angular-dependent transmission of ribbon grids, which resemble Venetian blinds, to recover the energy of particles leaking out of a fusion reactor is discussed. This converter works well, even to energies as low as 10 keV, because of its excellent space-charge handling ability. This low-energy operating range makes direct energy conversion possible on closed confinement devices; operation at about 100 keV might be useful on open confinement devices. Energy recovery efficiencies of 60 to 70% are predicted. Examples are presented in which a direct energy converter with efficiencies in this range, when viewed as a topping cycle to a thermal converter, could considerably improve the reactor prospects of both open- and closed-field-line confinement schemes.

Experimental results from a beam direct converter at 100 kV

A direct-energy converter was developed for use on neutral-beam injectors. The purpose of the converter is to raise the efficiency of the injector by recovering the portion of the ion beam not converted to neutrals. In addition to increasing the power efficiency, direct conversion reduces the requirements on power supplies and eases the beam dump problem. The converter was tested at Lawrence Berkeley Laboratory on a reduced-area version of a neutral-beam injector developed for use on the Tokamak Fusion Test Reactor at Princeton. The conversion efficiency of the total ion power was 65 ±7% at the beginning of the pulse, decaying to just over 50% by the end of the 0.6-s pulse. Once the electrode surfaces were conditioned, the decay was due to the rise in pressure of only the beam gas and not to outgassing. The direct converter was tested with 1.7 A of hydrogen ions and with 1.5 A of helium ions through the aperture with similar efficiencies. At the midplane through the beam, the line power density was 0.7 MW/m, for comparison with our calculations of slab beams and the prediction of 2–4 MW/m in some reactor studies. Over 98 kV was developed at the ion collector when the beam energy was 100 keV. When electrons were suppressed magnetically, rather than electrostatically, the efficiency dropped to 40%. However, a better designed electron catcher could improve this efficiency. New electrode material released gas (mostly H2 and CO) in amounts that exceeded the input of primary gas from the beam. The electrodes were all made of 0.51-mm-thick molybdenum cooled only by radiation. This allowed the heating by the beam to outgas the electrodes and for them to stay hot enough to avoid the reabsorption of gas between shots. By minor redesign of the electrodes, adding cryopanels near the electrodes, and grounding the ion source, these results extrapolate with high confidence to an efficiency of 70–80% at a power density of 2–4 MW/m. Higher power may be possible with magnetic electron suppression.

Investigation of a Plasma from an Occluded Gas Cold Plasma Source

Results are given for various measurements made on a plasma from a hydrogen‐loaded titanium washer mounted at a dc magnetic mirror (∼6 kG), the power being supplied to the source by a variable length pulse line (15–635 μsec). Spectroscopic studies indicated the presence of H, Ti+, Ti++, Ti+++, O+, O++, Al, Al+, Al++, and Ta+. Doppler shift measurements showed ion velocities from 5 × 105 to 2 × 106 cm/sec. Microwave cutoff measurements provided time history curves of electron density and indicated that the maximum density exceeded 3 × 1014 electrons per cm3. Profile measurements using a thermistor close to the aperture of the plasma gun revealed that 10% of the particles were neutrals. Probe measurements showed that the electron temperature decreased during the pulse from 15 to 1 eV and from 3 to <1 eV for old and new source washers, respectively.

Experimental Results on Electrostatic Stoppering

In a pulsed experiment a magnetically guided plasma stream is reflected by a double electrode structure. Space‐charge‐limited densities up to 4×1011 cm−3 are calculated and verified. An application to stopper one end of a mirror fusion reactor is discussed.

A preliminary engineering design of a “Venetian blind” direct energy converter for fusion reactors

The device described reclaims the energy lost by the reactor through leakage of charged particles. Energy selection is accomplished through the angular dependence of transmission through a system of ribbon grids resembling a venetian blind. In contrast to previously described converters in which the beam of ions from the reactor is expanded in a flat fan-like expander, the beam in this device is expanded in two directions in a conical expander. Problems of grid construction, radiation damage, grid heating, and vacuum pumping are discussed. The efficiency is estimated to be 50% for a two-stage collector and 65% for a four-stage collector. Further development of the basic concept could possibly raise the efficiency to as high as 75%. Optimized designs might increase the power handled by a factor of several without significantly increasing the unit cost, thus greatly lowering the cost/kW.

Diagnostic Measurements on a Highly Ionized, Steady‐State Plasma

Techniques and results are given for various measurements made on a highly ionized helium plasma in a steady‐state plasma system (which employs a longitudinal magnetic field of approximately 1 kilogauss). Neutral‐particle pressures ranged from about 3 × 10−4 mm Hg near the source to about 10−5 mm Hg in the downstream region. Spectroscopic measurements showed principal impurities were C, N, and O ions (up to C3+, N4+, and O4+ states). Doppler broadening measurements of He+ λ4686 revealed ion temperatures up to 10 eV. Probe measurements indicated electron temperatures up to 20 eV and maximum ion densities of a little over 1013 cm−3. Microwave transmission measurements (at λ = 4 mm and 8 mm) gave supporting evidence that the electron density exceeds 1013 cm.

Efficient High-Power High-Energy Neutral Beams for the Reference Mirror Reactor

The neutral beams for the reference mirror reactor are provided via four separate injectors using negative ions created by charge exchange in a cesium-vapor cell and neutralized by photodetachment. Each of the injectors delivers the equivalent of 1800 A of the desired mixture of 150-keV deuterium and tritium neutrals. Each injector consists of 23 ion sources of a modified Lawrence Berkeley Laboratory/Lawrence Livermore Laboratory type, with an associated cesium-vapor cell that converts 20 percent of the positive-ion output into negative ions D- and T-. The negative ions are accelerated to the desired energy and subsequently pass through a photodetachment cell that is continuously illuminated by eight columns of iteratively pulsed lasers. As much as 95 percent of the negative ions are stripped, producing fast neutrals that pass between the cryopumps and shielding into the reactor. Innovations required to attain an overall efficiency of 81.2 percent include a continuously operating cathode for the ion source, a negative-ion beam line with cooled gids, a high-voltage accelerator with insulators shielded from the neutron and gamma flux, cryopanels that cycle between pumping and outgassing modes, and recovery of the waste thermal energy and charged beam energy.

Surface requirements for electrostatic direct energy converters

Abstract There are two major electrostatic direct energy converter concepts which will be discussed from the point of view of the surfaces. One is the Venetian blind concept and the other is the periodic electrostatic focusing concept. They are both of the direct collector type. Fluxes of D + , T + , He ++ , electrons, and X-rays are given. Design consideration due to thermionic emission, secondary electron emission, and radiation cooling are discussed. A detailed discussion is devoted to breakdown physics, the voltages and electric field strengths that can be employed, and how surface deterioration may affect voltage holding due to He ++ bombardment blistering.

Cyclotron Instability in a Hot Electron Plasma with a Double‐Humped Velocity Distribution

A high‐frequency instability occurring in a “hot electron” plasma (Te ∼ 25 keV; Ne ∼1011 cm−3) contained in a quadrupole‐mirror machine (i. e., “minimum‐B” field) has been studied experimentally and correlated with theory. The effects observed in the presence of instability bursts are: (1) Intense radiation is emitted at the electron‐cyclotron frequency; (2) a short pulse of electrons escapes along the field lines; and (3) the double‐humped energy distribution (characteristic of the electrons just before the instability) changes so that the distribution of the hot electron component becomes more nearly Maxwellian simultaneous with a marked increase in the 200‐eV component. The experimental observations are compared with the theoretical calculations of Hall, Heckrotte, and Kammash as modified to apply to the measured experimental distributions. In agreement with the observations, these calculations predict the occurrence of an instability at low plasma density, such that ωpe2/ωcp2 ∼ 0.01. In addition, the ...