Naoji Yamamoto

Kilotesla Magnetic Field due to a Capacitor-Coil Target Driven by High Power Laser

Laboratory generation of strong magnetic fields opens new frontiers in plasma and beam physics, astro- and solar-physics, materials science, and atomic and molecular physics. Although kilotesla magnetic fields have already been produced by magnetic flux compression using an imploding metal tube or plasma shell, accessibility at multiple points and better controlled shapes of the field are desirable. Here we have generated kilotesla magnetic fields using a capacitor-coil target, in which two nickel disks are connected by a U-turn coil. A magnetic flux density of 1.5 kT was measured using the Faraday effect 650 μm away from the coil, when the capacitor was driven by two beams from the GEKKO-XII laser (at 1 kJ (total), 1.3 ns, 0.53 or 1 μm, and 5 × 1016 W/cm2).

Discharge Current Oscillation in Hall Thrusters

The discharge current oscillation at a frequency range of 10-100 kHz in Hall thrusters was investigated with the objective of extending their stable operational range. The amplitude of oscillation was measured using two types of Hall thrusters-the anode layer type and the magnetic layer type. The oscillation amplitude was found to be sensitive to the applied magnetic flux density, and this result indicated that the oscillation was affected by electron mobility. An oscillation model was proposed based on the experimental results, and the predicted frequency and stable operational range were found to agree qualitatively agreed with the experimental results. This model shows that the momentum transfer corresponding to plasma fluctuation, that is, the viscosity effect, is crucial to achieving stability. Thus, the oscillation amplitude for various acceleration channel configurations-divergent, parallel, and convergent-was measured because the momentum transfer could be affected by the channel configuration. The stable operational range was successfully extended by the adoption of the convergent configuration in each type of Hall thruster, as shown by this model.

Effects of magnetic field configuration on thrust performance in a miniature microwave discharge ion thruster

The effects of magnetic field configuration on thrust performance in a miniature microwave discharge ion thruster were investigated in order to improve thrust performance. First, the extracted ion beam current was measured for various levels of strength of the magnetic field. It was found that there is an optimum magnitude of the magnetic field. That this is due to the tradeoff between magnetic mirror confinement and microwave-plasma coupling was confirmed by measurement of the ion saturation current into the antenna of the ion thruster. The ion saturation current was found to decrease with an increase in magnetic field strength, due to the improvement in magnetic mirror confinement. The estimated electron temperature also decreases with an increase in magnetic field strength. This result shows that the increase in magnetic field strength leads to a decrease in microwave-plasma coupling. Next, the ion beam current for three magnetic field shapes was measured by changing the length of the central yoke. The...

Antenna Configuration Effects on Thrust Performance of Miniature Microwave Discharge Ion Engine

e = electronic charge F = thrust g = acceleration of gravity Ib = extracted ion beam current Isp = specific impulse mi = ion mass ṁi = mass flow rate for ion source ṁn = mass flow rate for neutralizer Pi = incident microwave power Pn = input power for neutralizer Pr = reflected microwave power Vb = beam voltage α = doubly charged ion current to singly charged ion current ratio γT = thrust coefficient ec = ion beam production cost ηt = thrust efficiency ηu = propellant utilization θb = beam divergence angle

Single-mode delivery of 250 nm light using a large mode area photonic crystal fiber

We demonstrate that large mode area (LMA) photonic crystal fibers (PCFs) can be used as single-mode patch-cords for 250 nm laser light. We have studied the transmission of the 250 nm output beam of a frequency-quadrupled diode laser through a triangular structure LMA PCF with 10 microm core. We have achieved single-mode output with coupling loss of 1.8 +/- 0.6 dB and transmission loss of 1.5 +/- 0.2 dB/m. The critical bend loss radius is approximately 6 cm. The transmission loss is compared with published bulk silica measurements. Effects of optically induced damage were observed after prolonged operation and have been studied as function of laser power and time. The optical damage occurs primarily at the fiber input and can be partly ameliorated by cleaving the fiber input. For input power levels of < approximately 0.3 mW stable operation can be achieved for periods of >40 hours which is sufficient for many laboratory based applications. The results show the utility of these fibers for single-mode beam delivery in a spectral region where step-index single-mode fibers are not readily available.

Cavity ring-down spectroscopy sensor for ion beam etch monitoring and end-point detection of multilayer structures

This contribution reports on the development of in situ sputter monitoring and end-point detection for ion beam etch systems using continuous-wave cavity ring-down spectroscopy (cw-CRDS). The demonstrated system is based on the detection of sputtered manganese atoms using a tunable external cavity diode laser in the vicinity of 403.07 nm. The cw-CRDS system is described and measurements from a manganese-iron target are presented. End-point detection is demonstrated by monitoring the time dependence of manganese concentration for a multilayer target comprised of alternating layers of manganese/iron and titanium. Detection limits are shown to be adequate for todays commercial ion beam sputter systems.

Measurements of electron density and temperature in a miniature microwave discharge ion thruster using laser Thomson scattering technique

In order to improve the thrust performance of a miniature microwave discharge ion thruster, the relationship between electron number density/temperature and operational conditions, mass flow rate, incident microwave power and magnetic field strength were measured by means of laser Thomson scattering. A photon counting method and a triple grating spectrometer were used against a small Thomson scattering signal and a strong stray laser light. Electron number density increased with incident microwave power and was saturated at critical incident microwave power; it was about 1.2 × 1018 m−3 at incident microwave power >8 W. In addition, electron number density increased with mass flow rate and became saturated; it was about 1.7 × 1018 m−3 at mass flow rate > 0.04 mg s−1. The electron number density gradually increased with an increase in the number of magnets, i.e. magnetic field strength. There was a sudden jump at thirteen magnets, although the thruster failed to ignite at fourteen magnets. This is because there is an optimum distance between the antenna and the electron cyclotron resonance layer. These results suggest that future improvement in thrust efficiency in miniature microwave discharge ion thrusters may come from the fine adjustment of the magnetic field configuration inside the discharge chamber.

Sputter erosion sensor for anode layer-type Hall thrusters using cavity ring-down spectroscopy

We report the development of a sputter erosion monitoring system to study Hall thruster lifetime and contamination. The laser-based sensor uses the continuous-wave cavity ring-down spectroscopy technique and allows for in situ measurements in near-real time. The continuous-wave cavity ring-down spectroscopy technique diagnostic allows direct probing of sputter products in their ground state, thereby providing a reliable quantitative measure of their overall number density. Combining the number density of sputtered particles with their velocity allows determination of the flux of sputtered particles and erosion rate. We perform proof of principle experiments, in which sputtered manganese atoms from the acceleration channel of an anode layer-type Hall thruster are measured. The measurement strategy is to detect the manganese atoms via an absorption line from the ground state at a wavelength of 403.076 nm (air). The measured path-integrated number density of sputtered manganese atoms is 1.7 ± 0.3 × 10 13 m -2 for an argon anode mass flow rate of 2.08 mg/s and a discharge voltage of 250 V. A finite element sputter model is used to compare the cavity ring-down spectroscopy results against validating mass loss measurements and shows good agreement.

Measurement of xenon plasma properties in an ion thruster using laser Thomson scattering technique

This paper reports on the development of a method for measuring xenon plasma properties using the laser Thomson scattering technique, for application to ion engine system design. The thresholds of photo-ionization of xenon plasma were investigated and the number density of metastable atoms, which are photo-ionized by a probe laser, was measured using laser absorption spectroscopy, for several conditions. The measured threshold energy of the probe laser using a plano-convex lens with a focal length of 200 mm was 150 mJ for a xenon mass flow rate of 20 μg/s and incident microwave power of 6 W; the probe laser energy was therefore set as 80 mJ. Electron number density was found to be (6.2 ± 0.4) × 10(17) m(-3) and electron temperature was found to be 2.2 ± 0.4 eV at a xenon mass flow rate of 20 μg/s and incident microwave power of 6 W. The threshold of the probe laser intensity against photo-ionization in a miniature xenon ion thruster is almost constant for various mass flow rates, since the ratio of population of the metastable atoms to the electron number density is little changed.

Direct measurement of the impulse in a magnetic thrust chamber system for laser fusion rocket

An experiment is conducted to measure an impulse for demonstrating a magnetic thrust chamber system for laser fusion rocket. The impulse is produced by the interaction between plasma and magnetic field. In the experiment, the system consists of plasma and neodymium permanent magnets. The plasma is created by a single-beam laser aiming at a polystyrene spherical target. The impulse is 1.5 to 2.2 μNs by means of a pendulum thrust stand, when the laser energy is 0.7 J. Without magnetic field, the measured impulse is found to be zero. These results indicate that the system for generating impulse is working.