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High-enery, rep.-rated, diode-pumped solid-state laser (DPSSL) is one of leading candidates for inertial fusion energy driver (IFE) and related laser-driven high-field applications. The project for the development of IFE laser driver in Japan, HALNA (High Average-power Laser for Nuclear Fusion Application) at ILE, Osaka University, aims to demonstrate 100-J pulse energy at 10 Hz rep. rate with 5 times diffraction limited beam quality. In this article, the advanced solid-state laser technologies for one half scale of HALNA (50 J, 10 Hz) are presented including thermally managed slab amplifier of Nd:phosphate glass and zig-zag optical geometry, and uniform, large-area diode-pumping.

Properties of extreme ultraviolet (EUV) and particle emissions from laser-produced tin (Sn) plasma were experimentally investigated for use in industrial applications. Dependence of EUV emission on the initial thickness of Sn overcoat on a plastic sphere was studied to clarify the minimum mass needed to keep sufficient conversion to EUV and minimize plasma debris. EUV emission from low-density targets showed narrower spectra peaked at 13.5 nm, attributed to reduction of satellite line emissions in the spectrum foot and opacity broadening at the peak. It is shown that not only the EUV generation but also ion spectra are significantly affected by the Sn coat thicknesses.

A 10-kJ PW laser (LFEX) is under construction for the FIREX-I program. This paper reports a design overview of LFEX, the technological development of a large-aperture arrayed amplifier with modified four-pass architecture, wavefront correction, a large-aperture Faraday rotator with a superconducting magnet, a new pulse compressor arrangement, and focus control.

The typical specifications of the laser driver for a commercial IFE power plant are (1) total energy (MJ/pulse) with a tailored 20-40 ns pulse, (2) repetition operation (∼10 Hz), (3) efficiency (∼10%) with enough robustness and low cost. The key elements of the DPSSL driver technology are under development with HALNA. The HALNA 10 (High Average-power Laser for Nuclear-fusion Application) demonstrated 10 J $\times$10 Hz operation and the HALNA 100 (100 J $\times$10 Hz) is now under construction. By using the high average power and high intensity lasers, new industrial applications are being proceeded. The collaborative process for the development of high power laser with industry and for the industrial applications is effective and essential in the development of the laser driver for IFE power plant.

We have been developing an EUV light source by laser-produced plasma for the use of EUV lithography system under the Leading Project promoted by MEXT. The project aims at understanding physics of laser plasma EUV source and providing database and guidelines for the practical EUV source. Progresses in theoretical modeling and experimental results are briefly reported.

We experimentally studied relativistic laser propagation in preplasmas at the highest powers ever attempted-0.2 to 0.4 petawatt. We demonstrated a single conical-shaped plasma channel formation extending several hundred microns in length from the under dense to over dense plasmas, indicating whole-beam self-focused laser channeling into the high-density plasma. The channel cone was reproduced by a three dimensional particle-in-cell simulation. The confirmation of the relativistic laser channeling into high-density plasmas holds the promise of fast igniting a highly compressed fuel plasma core.

Our institute has been investigating laser-produced tin, lithium and xenon plasma for extreme ultraviolet (EUV) light source of EUV lithography system, and found the highest conversion efficiency of 3% at 13.5 nm in 2% bandwidth. In the present paper, we introduce fabrication methods of density-controlled tin targets to generate relatively monochromatic EUV, and higher conversion efficiency than bulk tin. The first method is the nano-template method, where liquid tin solution was immersed into a polymer film with monodispersed size nanoparticle. The density can be controlled by tin concentration of the solution. The target can be classified into mass-limited target. We have shown a tendency of monochromatic EUV emission around 13.5 nm with decreasing of tin density. The intensity was higher than tin foil with bulk density. The tendency has a merit to mitigate heat effect of the first EUV mirror. The fabrication method has another merit to control not only density but also the poresize of tin oxide to be 100 nm ∼10 μm. The second is liquid crystalline template method to obtain porous tin oxide. The precursor with tin oxide and cellulose provides mechanically stable and transparent film. The film has wavy sub-microstructure derived from microscopic liquid crystal domain structure. The method is simple and short duration for the hydrolysis reaction to solidify tin compound. This material has a merit of feasibility of fabrication, and has been applied for rotation target for 10 Hz and 5 kHz laser repetition.

With the scope of future inertial fusion energy (IFE) driver, design of a diode-end-pumped 1-kJ zig-zag slab laser with cryogenically cooled ceramic Yb:YAG is presented. Pulse repetition rate is 16 Hz, and optical-optical efficiency is 34 % at 140 K operation. Gradient of Yb3+ion density increasing toward the slab center successfully suppresses the parasitic on the slab ends.

We experimentally studied relativistic laser propagation in preplasmas at the highest powers ever attempted-0.2 to 0.4?petawatt. We demonstrated a single conical-shaped plasma channel formation extending several hundred microns in length from the under dense to over dense plasmas, indicating whole-beam self-focused laser channeling into the high-density plasma. The channel cone was reproduced by a three dimensional particle-in-cell simulation. The confirmation of the relativistic laser channeling into high-density plasmas holds the promise of fast igniting a highly compressed fuel plasma core.

Properties of extreme ultraviolet (EUV) and particle emissions from laser-produced tin (Sn) plasma were experimentally investigated for use in industrial applications. Dependence of EUV emission on the initial thickness of Sn overcoat on a plastic sphere was studied to clarify the minimum mass needed to keep sufficient conversion to EUV and minimize plasma debris. EUV emission from low-density targets showed narrower spectra peaked at 13.5?nm, attributed to reduction of satellite line emissions in the spectrum foot and opacity broadening at the peak. It is shown that not only the EUV generation but also ion spectra are significantly affected by the Sn coat thicknesses.

We have been developing an EUV light source by laser-produced plasma for the use of EUV lithography system under the Leading Project promoted by MEXT. The project aims at understanding physics of laser plasma EUV source and providing database and guidelines for the practical EUV source. Progresses in theoretical modeling and experimental results are briefly reported.

Our institute has been investigating laser-produced tin, lithium and xenon plasma for extreme ultraviolet (EUV) light source of EUV lithography system, and found the highest conversion efficiency of 3% at 13.5?nm in 2% bandwidth. In the present paper, we introduce fabrication methods of density-controlled tin targets to generate relatively monochromatic EUV, and higher conversion efficiency than bulk tin. The first method is the nano-template method, where liquid tin solution was immersed into a polymer film with monodispersed size nanoparticle. The density can be controlled by tin concentration of the solution. The target can be classified into mass-limited target. We have shown a tendency of monochromatic EUV emission around 13.5?nm with decreasing of tin density. The intensity was higher than tin foil with bulk density. The tendency has a merit to mitigate heat effect of the first EUV mirror. The fabrication method has another merit to control not only density but also the poresize of tin oxide to be 100?nm ?10??m. The second is liquid crystalline template method to obtain porous tin oxide. The precursor with tin oxide and cellulose provides mechanically stable and transparent film. The film has wavy sub-microstructure derived from microscopic liquid crystal domain structure. The method is simple and short duration for the hydrolysis reaction to solidify tin compound. This material has a merit of feasibility of fabrication, and has been applied for rotation target for 10?Hz and 5?kHz laser repetition.

The typical specifications of the laser driver for a commercial IFE power plant are (1) total energy (MJ/pulse) with a tailored 20-40 ns pulse, (2) repetition operation (?10 Hz), (3) efficiency (?10%) with enough robustness and low cost. The key elements of the DPSSL driver technology are under development with HALNA. The HALNA 10 (High Average-power Laser for Nuclear-fusion Application) demonstrated 10 J ? 10 Hz operation and the HALNA 100 (100 J ? 10 Hz) is now under construction. By using the high average power and high intensity lasers, new industrial applications are being proceeded. The collaborative process for the development of high power laser with industry and for the industrial applications is effective and essential in the development of the laser driver for IFE power plant.

A 10-kJ PW laser (LFEX) is under construction for the FIREX-I program. This paper reports a design overview of LFEX, the technological development of a large-aperture arrayed amplifier with modified four-pass architecture, wavefront correction, a large-aperture Faraday rotator with a superconducting magnet, a new pulse compressor arrangement, and focus control.

High-enery, rep.-rated, diode-pumped solid-state laser (DPSSL) is one of leading candidates for inertial fusion energy driver (IFE) and related laser-driven high-field applications. The project for the development of IFE laser driver in Japan, HALNA (High Average-power Laser for Nuclear Fusion Application) at ILE, Osaka University, aims to demonstrate 100-J pulse energy at 10?Hz rep. rate with 5 times diffraction limited beam quality. In this article, the advanced solid-state laser technologies for one half scale of HALNA (50?J, 10?Hz) are presented including thermally managed slab amplifier of Nd:phosphate glass and zig-zag optical geometry, and uniform, large-area diode-pumping.

AbstractThe current status of the fabrication, injection, and tracking of fast ignition targets is summarized including on cryogenic technologies for direct-drive, laser fusion targets with and without a reentrant guide cone for additional heating lasers. The fabrication of low-density foam and the drilling of fragile foam shells are current issues in the fabrication of fast ignition targets with reentrant cones. Fuel loading to the targets is a challenging issue in the mass-production process of the targets for a fusion power plant. Two fuel-loading methods are proposed for the fast ignition target with the cone. Preliminary experiments of injection and tracking of real-size targets have started showing promising results.

Development of foam materials for cryogenically cooled fuel targets is described in this paper. The fabrication development was initiated as a part of the fast ignition realization experiment (FIREX) project at the ILE, Osaka University under a bilateral collaboration between Osaka University and National Institute for Fusion Science (NIFS). For the first stage of FIREX (FIREX-I), a foam cryogenic target was designed in which low-density foam shells with a conical light guide will be fuelled through a narrow pipe and will be cooled down to the cryogenic temperature. Acrylic polymer, resorcinol–formaldehyde (RF) resin, poly(4-methyl-1-pentene) (PMP), and polystyrene-based crosslinking polymer have been investigated as supporting materials for cryogenic fuel. The properties of the material and the present status of the material development are summarized.

AbstractThis is the review on the laser fusion research at Institute of Laser Engineering of Osaka University. Since 1996, we have concentrated our efforts on fast ignition laser fusion research. By constructing 100 TW and 1Peta watt lasers, experiments on relativistic laser plasma interactions related to fast ignition and pellet implosion and heating have been carried out. The results indicate that imploded core plasma is heated with relatively high coupling efficiency. According to the above results, we started the FIREX (Fast Ignition Realization Experiment) project for demonstrating ignition and burn with a multi 10kJ short pulse laser. The future prospects of the project are presented in this paper.

Low-density porous tin dioxide (~0.5 g/cm³) was prepared from a SnCl4/ethanol/H2O mixture and polystyrene (PS) template. Such a low density made the as-synthesized tin dioxide act as a promising target material for laser-induced extreme ultraviolet (EUV) emission. The resulting structure of the as-synthesized tin dioxide (SnO2) was a cellular foam, which was composed of large cells (ca. 10³ nm, large macropores) interconnected by windows (ca. 10² nm, small macropores). Scanning electron microscope images showed that SnO2 particles of ~10 nm constituted the cell wall. The space among particles formed mesopores, which were about 7 nm estimated by nitrogen adsorption−desorption isotherm measurements. Therefore, the as-prepared SnO2 showed a hierarchical porous system from macropores to mesopores. The window size on a submicrometer scale was tunable in the range of ~480−200 nm by changing the molar ratio of ethanol to SnCl4 in tin source solution from 2:1 to 10:1. By consideration that the windows originated from the tight contact area among the PS spheres, some parameters affecting the contacting area among the PS spheres were investigated in order to analyze the effect of ethanol content on the widow size. The wettability of the tin source solution on the PS particle film and the viscosity of the tin source solution were found to be the main reasons responsible for the variable window size. Another observed phenomenon was the change in the SnO2 filling area from the complete filling of voids to a mere coating of the colloidal spheres as the ethanol content was increased, that is, a template transition from a volume template to a surface template was realized by changing the ethanol content. The mechanism was discussed in detail.

Recent progress in the physics of fast ignition of fusion targets is reviewed here. Fundamental studies on hot electron energy transport show that the scheme looks promising if the heating pulse can be guided close enough to a compressed core. The idea of using cone-guided compression was first demonstrated experimentally under a Japan–UK collaboration. The use of the gold cone was extremely successful and showed a 103 neutron increase out of CD target implosion with a 300 J/0.5 ps enforced heating laser pulse. The heated temperature was close to 1 keV. In order to increase the temperature to 10 keV, a 10 kJ PW−1 laser system is necessary. Osaka University has started constructing such a laser system.

The Rayleigh–Taylor (RT) instability with material ablation through an unstable interface is the key physics that determines the success or failure of inertial fusion energy (IFE) generation, as the RT instability potentially quenches ignition and burn by disintegrating the IFE target. We present two suppression schemes of the RT growth without significant degradation of the target density. The first scheme is to generate a double ablation structure in high-Z doped plastic targets. In addition to the electron ablation surface, a new ablation surface is created by x-ray radiation from the high-Z ions. Contrary to the previous thought, the electron ablation surface is almost completely stabilized by extremely high flow velocity. On the other hand, the RT instability on the radiative ablation surface is significantly moderated. The second is to enhance the nonlocal nature of the electron heat transport by illuminating the target with long wavelength laser light, whereas the high ablation pressure is generated by irradiating with short wavelength laser light. The significant suppression of the RT instability may increase the possibility of impact ignition which uses a high-velocity fuel colliding with a preformed main fuel.

The Rayleigh-Taylor (RT) instability with material ablation through an unstable interface is the key physics that determines the success or failure of inertial fusion energy (IFE) generation, as the RT instability potentially quenches ignition and burn by disintegrating the IFE target. We present two suppression schemes of the RT growth without significant degradation of the target density. The first scheme is to generate a double ablation structure in high-Zdoped plastic targets. In addition to the electron ablation surface, a new ablation surface is created by x-ray radiation from the high-Zions. Contrary to the previous thought, the electron ablation surface is almost completely stabilized by extremely high flow velocity. On the other hand, the RT instability on the radiative ablation surface is significantly moderated. The second is to enhance the nonlocal nature of the electron heat transport by illuminating the target with long wavelength laser light, whereas the high ablation pressure is generated by irradiating with short wavelength laser light. The significant suppression of the RT instability may increase the possibility of impact ignition which uses a high-velocity fuel colliding with a preformed main fuel.

Recent progress in the physics of fast ignition of fusion targets is reviewed here. Fundamental studies on hot electron energy transport show that the scheme looks promising if the heating pulse can be guided close enough to a compressed core. The idea of using cone-guided compression was first demonstrated experimentally under a Japan-UK collaboration. The use of the gold cone was extremely successful and showed a 103neutron increase out of CD target implosion with a 300?J/0.5?ps enforced heating laser pulse. The heated temperature was close to 1?keV. In order to increase the temperature to 10?keV, a 10?kJ?PW?1laser system is necessary. Osaka University has started constructing such a laser system.

We have studied fast heating of highly compressed plasmas using multi 100 TW laser light. Efficient propagation of the ultra-intense laser light and heating of the imploded plasmas were realized with a cone-attached shell target. Energy deposition rate of the ultra-intense laser pulse into high-density plasmas was evaluated from neutron measurements. Generation and propagation property of energetic electrons in the ultra-intense laser interactions were also investigated with solid density targets. About 40&percent; of the laser energy converted to mega electron volts energetic electrons in the interactions with solid targets at intensities of 10¹⁹ W cm⁻². These electrons propagated in the high-density plasmas with a divergence of 20–30° or jet-like collimation. Taking account of these experimental results, heating laser spot size is optimized for laser fusion ignition with a simple estimation.

ABSTRACTWe investigated tritium (T) release behavior from silica glass. The specimens were 8 kinds of commercially available silica glass. T was injected by the 6Li (n,α)T reaction of sintered pellets of lithium oxide (Li2O) into the silica glass with thermal neutrons in JRR-2 (VT-8) up to 5 × 1018neutrons/cm2at ambient temperature (about 350 K). After irradiation, the Li2O pellets were removed from the silica glass, and T release from the silica glass was measured in a flow of hydrogen (H2) or ammonia (NH3) sweep gas at atmospheric pressure at a constant heating rate of 2 K/min between 675 K and 1375 K with a proportional counter. In the case of H2sweep gas, a maximum tritium release rate was observed around 1023 K, while in the case of NH3sweep gas, two peaks around 1023 K and around 1123 K or a peak around 1123 K with a shoulder were obserbed. After the experiments of T release, FT-IR spectra showed a decrease of SiOH bands at 3650 cm−1. On the other hand, no changes in intensities at 2250 cm−1due to SiH were observed for both samples before and after T release.

This paper describes in terms of the mass of the solvents in the oil, water, and distillate phases the curing process of highly spheric monodispersed 4-mm-in-diameter polystyrene capsules of a uniform thickness. It was determined that the water solubility of the solvents is crucial in the solvent selection in order to prepare the capsules by this method. The curing process started from the water/oil/water emulsions, where the boiling temperature and the densities of the oil were well matched by mixing benzene and 1,2-dichloroethane. The masses of the solvents were measured for the oil phases and the distillate and then calculated for the water phase. The analysis of the solvents in the water phase indicated that the process was governed by the kinetics of their dissolving from the oil phase to the outer water phase rather than by the vapor pressure of the solvents in the early stage before the oil phase lost its fluidity. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3412–3418, 2000

Multiphoton reduction of Eu³⁺ to Eu²⁺ in methanol was induced by irradiation with the second harmonic (394 nm, 2 ps) of a Ti:sapphire laser. The conversion efficiencies were increased to 0.3 for short pulse excitation, as compared to on the order of 10^-5 for a nanosecond dye laser pulse. It appears that a three-photon process pumps a deep charge-transfer state from which Eu³⁺ efficiently dissociates to Eu²⁺. On the basis of the excitation spectra, the first intermediate state is the ⁵L6 state, which is one of the f-electronic excited states. The Eu²⁺ concentration increased shot by shot and approached a plateau. The saturation behavior will be explained in terms of the back photooxidation of Eu²⁺ to Eu³⁺.

Neural organization from the superior colliculus to motoneurons in the horizontal oculomotor system of the cat. The neural organization of the superior colliculus (SC) projection to horizontal ocular motoneurons was analyzed in anesthetized cats using intracellular recording and transneuronal labeling. Intracellular responses to SC stimulation were analyzed in lateral rectus (LR) and medial rectus (MR) motoneurons and internuclear neurons in the abducens nucleus (AINs). LR motoneurons and AINs received excitation from the contralateral SC and inhibition from the ipsilateral SC. The shortest excitation (0.9–1.9 ms) and inhibition (1.4–2.4 ms) were mainly disynaptic from the SC and were followed by tri- and polysynaptic responses evoked with increasing stimuli or intensity. All MR motoneurons received excitation from the ipsilateral SC, whereas none of them received any short-latency inhibition from the contralateral SC, but some received excitation. The latency of the ipsilateral excitation in MR motoneurons (1.7–2.8 ms) suggested that this excitation was trisynaptic via contralateral AINs, because conditioning SC stimulation spatially facilitated trisynaptic excitation from the ipsilateral vestibular nerve. To locate interneurons mediating the disynaptic SC inputs to LR motoneurons, last-order premotor neurons were labeled transneuronally after injecting wheat germ agglutinin–conjugated horseradish peroxidase into the abducens nerve, and tectoreticular axon terminals were labeled after injecting dextran-biotin into the ipsilateral or contralateral SC in the same preparations. Transneuronally labeled neurons were mainly distributed ipsilaterally in the paramedian pontine reticular formation (PPRF) rostral to retrogradely labeled LR motoneurons and the vestibular nuclei, and contralaterally in the paramedian pontomedullary reticular formation (PPMRF) caudomedial to the abducens nucleus and the vestibular nuclei. Among the last-order premotor neuron areas, orthogradely labeled tectoreticular axon terminals were observed only in the PPRF and the PPMRF contralateral to the injected SC and seemed to make direct contacts with many of the labeled last-order premotor neurons in the PPRF and the PPMRF. These morphological results confirmed that the main excitatory and inhibitory connections from the SC to LR motoneurons are disynaptic and that the PPRF neurons that receive tectoreticular axon terminals from the contralateral SC terminate on ipsilateral LR motoneurons, whereas the PPMRF neurons that receive tectoreticular axon terminals from the contralateral SC terminate on contralateral LR motoneurons.

Laser raday systems for documenting spatial and temporal variation of relative density of aerosols using Mie backscattering, and that for detecting NO2in atmosphere using the resonance fluorescence of NO2are presented. For observation of aerosols, a very flexible data processing system using both a mini-computer and microprocessor has been assembled to meet the needs of various controls and computations including data acquisition, data transfer and storage, computations of the radar equation and display on CRT. The combination of the second harmonic of a Nd: YAG laser, a synchronous single photo-electron technique and a narrow pass band optical filter employed for separation of scattered light by NO2resonance fluorescence offers a sensitive system for remote monitoring of NO2in atmosphere. Experimental results by these system are also presented.

Propagation of laser pulses in a multilevel photoionization system is studied for atomic laser isotope separation. For this purpose a simulation code called CEALIS-P has been developed. Through numerical solutions of coupled Maxwell-Bloch equations we illustrate the propagation characteristics and the influence on the photoionization efficiency. It is shown that near-resonant interactions with the nontarget isotope will significantly alter the laser-pulse shape, carrier frequency, and propagation velocity, resulting in degradation of the photon-utilization efficiency and ionization efficiency.

The most important function of carotenoid pigments, especially beta-carotene in higher plants, is to protect organisms against photooxidative damage (G. Britton, in T. W. Goodwin, ed., Plant Pigments--1988, 1988; N. I. Krinsky, in O. Isler, H. Gutmann, and U. Solms, ed., Carotenoids--1971, 1971). beta-Carotene also functions as a precursor of vitamin A in mammals (G. A. J. Pitt, in I. Osler, H. Gutmann, and U. Solms, ed., Carotenoids--1971, 1971). The enzymes and genes which mediate the biosynthesis of cyclic carotenoids such as beta-carotene are virtually unknown. We have elucidated for the first time the pathway for biosynthesis of these carotenoids at the level of enzyme-catalyzed reactions, using bacterial carotenoid biosynthesis genes. These genes were cloned from a phytopathogenic bacterium, Erwinia uredovora 20D3 (ATCC 19321), in Escherichia coli and located on a 6,918-bp fragment whose nucleotide sequence was determined. Six open reading frames were found and designated the crtE, crtX, crtY, crtI, crtB, and crtZ genes in reference to the carotenoid biosynthesis genes of a photosynthetic bacterium, Rhodobacter capsulatus; only crtZ had the opposite orientation from the others. The carotenoid biosynthetic pathway in Erwinia uredovora was clarified by analyzing carotenoids accumulated in E. coli transformants in which some of these six genes were expressed, as follows: geranylgeranyl PPiCrtB----prephytoene PPiCrtE----phytoeneCrtI---- lycopeneCrtY----beta-caroteneCrtZ----zeaxanthinCrtX--- -zeaxanthin-beta- diglucoside. The carotenoids in this pathway appear to be close to those in higher plants rather than to those in bacteria. Also significant is that only one gene product (CrtI) for the conversion of phytoene to lycopene is required, a conversion in which four sequential desaturations should occur via the intermediates phytofluene, zeta-carotene, and neurosporene.

High aspect ratio pellet targets are ablatively imploded with twelve green beams of GEKKO XII. Multiplexing of shock waves with a quasi-tailored pulse is performed successfully in DT fuel compression. The highest neutron yield achieved is 1.25 × 1012per shot. The development of new diagnostics such as neutron activation and secondary fusion reaction diagnostics is also presented.

In laser fusion experiment, fuel capsules with high sphericity and high wall-thickness uniformity are often fabricated by the emulsion method. To understand the process of forming the uniform wall by the emulsion method and to further improve the thickness uniformity, we formulated a model to simulate the centering process of a water globule in a compound emulsion. We newly added a thermodynamic force to the centering force in addition to previous hydrodynamic forces. Our result indicates that nonuniform emulsion turns to be uniform by repeating instantaneous deformations followed with continuous rotation around the horizontal axis.

1. Neck motoneurons usually receive disynaptic excitation and inhibition from individual semicircular canal nerves. However, in motoneurons of some neck muscles, trisynaptic inhibition is evoked by stimulation of the contralateral vertical canal nerves. The present study was performed to analyze this pathway and the location and properties of the last-order interneurons responsible for mediating this trisynaptic inhibition from the contralateral vertical canal nerves to neck motoneurons in anesthetized cats. 2. Bipolar stimulating electrodes were implanted on the contralateral anterior (ACN), lateral (LCN), and posterior canal nerve (PCN), and postsynaptic potentials (PSPs) evoked by electrical stimulation of individual canal nerves were intracellularly recorded from motoneurons of the obliquus capitis inferior (OCI), longus capitis (LC), and rectus capitis posterior (RCP) muscles. Stimulation of the contralateral ACN evoked trisynaptic inhibitory PSPs (IPSPs) in OCI and LC motoneurons and disynaptic excitatory PSPs (EPSPs) in RCP motoneurons. Stimulation of the contralateral PCN evoked di- and trisynaptic IPSPs in OCI and RCP motoneurons and disynaptic EPSPs in LC motoneurons. Stimulation of the contralateral LCN evoked disynaptic EPSPs in all of the motoneurons examined. 3. To determine the pathway that mediates these trisynaptic IPSPs from the vertical canal nerves to neck motoneurons, a lesion was made in the lower medulla, and the patterns of PSPs evoked by stimulation of the three contralateral canal nerves were compared before and after the lesion. Interruption of the ipsilateral medial longitudinal fascicle (MLF) abolished all disynaptic EPSPs and IPSPs from the three contralateral canal nerves in OCI, LC, and RCP motoneurons. In contrast, trisynaptic IPSPs evoked by stimulation of the contralateral ACN or PCN remained unaffected by sectioning the MLFs bilaterally. Sectioning of the contralateral lateral vestibulospinal tract (LVST) eliminated the trisynaptic IPSPs in OCI and LC motoneurons evoked by contralateral ACN stimulation and trisynaptic IPSPs in OCI and RCP motoneurons evoked by contralateral PCN stimulation but did not affect disynaptic EPSPs and IPSPs. 4. Stimulation of the contralateral LVST in the lower medulla after sectioning the bilateral MLFs evoked disynaptic IPSPs in OCI, LC, and RCP motoneurons. Because the LVST only projects ipsilaterally, this finding indicates that the last-order interneurons that mediate the trisynaptic inhibition through the LVST are most likely commissural neurons located in the spinal cord. 5. To determine the locations of last-order commissural neurons terminating on OCI motoneurons, wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was injected into the OCI muscle nerve at C1.(ABSTRACT TRUNCATED AT 400 WORDS)