Atomic Ion Species Research Articles

Effect of a magnetic cusp configuration on the ion species ratio in a NBI ion source

In arc discharge plasma, ion species ratio depends on plasma density and ion confinement time. To increase an atomic ion species ratio in an NBI (neutral beam injector) ion source, various ion sources have been designed with a higher ratio of plasma volume to effective ion loss area for increasing ion confinement time. In such designs, most NBI ion sources have a large arc chamber with strong cusp magnetic field. KSTAR NBI ion source also has a large arc chamber with a strong azimuthal line cusp configuration but its atomic ion (H+) beam fraction is small because its normal operation power levels of the arc discharges are comparatively lower than those of other NBI ion sources. In particular, the H2+ ion beam fraction is relatively large at about 40%. This means that there are many primary electrons in the beam extraction region. Through several Ele-orbit code simulations, it was found that some magnetic cusp configurations are effective in preventing primary electrons from nearing the plasma grid facing a plasma and having a high voltage potential. To reduce the molecular ion fraction in KSTAR NBI ion source and consequently increase its atomic ion fraction, experiments regarding the application of several new magnetic cusp configurations to the arc chamber were performed. Experimental results show that the new magnetic cusp configuration can increase the atomic ion fraction to approximately 80%, despite the low arc operation power at KSTAR NBI ion source.

Absolute single-ion thermometry

We describe and experimentally implement a single-ion local thermometry technique with absolute sensitivity adaptable to all laser-cooled atomic ion species. The technique is based on the velocity-dependent spectral shape of a quasi-dark resonance tailored in a J $\rightarrow$ J transition such that the two driving fields can be derived from the same laser source leading to a negligible relative phase shift. We validated the method and tested its performances in an experiment on a single 88 Sr + ion cooled in a surface radio-frequency trap. We first applied the technique to characterise the heating-rate of the surface trap. We then measured the stationary temperature of the ion as a function of cooling laser detuning in the Doppler regime. The results agree with theoretical calculations, with an absolute error smaller than 100 $\mu$K at 500 $\mu$K, in a temperature range between 0.5 and 3 mK and in the absence of adjustable parameters. This simple-to-implement and reliable method opens the way to fast absolute measurements of single-ion temperatures in future experiments dealing with heat transport in ion chains or thermodynamics at the single-ion level.

An ion species model for positive ion sources: II. Analysis of hydrogen isotope effects

A one-dimensional model of the magnetic multipole volume plasma source has been developed for application to intense ion/neutral atom beam injectors. The model uses plasma transport coefficients for particle and energy flow to create a detailed description of the plasma parameters along an axis parallel to that of the extracted beam. In this paper the isotopic modelling of positive hydrogenic ions is considered and compared with experimental data from the neutral beam injectors of the Joint European Torus. The use of the code to gain insights into the processes contributing to the ratios of the ionic species is demonstrated and the conclusion is drawn that 75% of the atomic ion species arises from ionization of dissociated molecules and 25% from dissociation of the molecular ions. However, whilst the former process is independent of the filter field, the latter is sensitive to the change in distribution of fast and thermal electrons produced by the magnetic filter field and an optimum combination of field strength and depth exists. Finally, the code is used to predict the species ratios for the JET source operating in tritium and hence the neutral beam power injected into the plasma in the JET tritium campaign planned for 2016.

Technology and Applications of Neutron Generators Developed by Adelphi Technology, Inc

A standard product line of high yield neutron generators has been developed at Adelphi Technology Inc. The generators use the D-D fusion reaction and are driven by an ion beam supplied by a microwave ion source. Yields of up to 5×109 n/s have been achieved, which are comparable to those obtained using the more efficient D-T reaction. The microwave-driven plasma uses the electron cyclotron resonance (ECR) to produce a high plasma density for high current and high atomic ion species. These generators have an actively pumped vacuum system that allows operation at reduced pressure in the target chamber, increasing the overall system reliability. Variations of these generators have been produced to increase the yield and total flux available. Several of the generators have been enclosed in radiation shielding/moderator structures designed for customer specifications. These generators have been proven to be useful for prompt gamma neutron activation analysis (PGNAA), neutron activation analysis (NAA) and fast neutron radiography. Pulsed and continuous operation has been demonstrated. Larger thermal neutron fluxes are expected to be obtained by multiple ion beams striking a central target that is filled with moderating material. Thus these generators make excellent fast, epithermal and thermal neutron sources for laboratories and industrial applications that require neutrons with safe operation, small footprint, low cost and small regulatory burden.

Plasma source ion implantation using high-power pulsed RF plasma

In plasma source ion implantation of molecular gases such as nitrogen, oxygen, etc., the ratio of atomic and molecular ionic species is very important from the standpoint of implanted ion energy. Hence, it is very beneficial to produce more atomic ion species rather than molecular ones to achieve deep implant profiles. The high RF power has been known to be an efficient way to produce atomic ion species along with high ionization rate of molecular gases. A large-scale plasma source ion implantation system has been built, in which a high-power pulsed RF amplifier and a water-cooled internal antenna were used to produce high-density inductively coupled RF plasma. The pulsed RF amplifier, aiming at the higher generation rate of atomic ion species, was constructed based on a 4CX5000A RF tetrode and turned out to give output of up to 50 kW PEP and 2 kW average at 13.56 MHz. The 4CX5000A RF tetrode tube was series-connected with an RF MOSFET to form an electron tube-MOSFET cascode circuit. The Ar plasma density at 30 kW pulsed RF power was measured to reach 7.5×10 11/cm 3 at 0.13 Pa (1 mTorr) of Ar pressure. The pulsed RF amplifier was used to produce an inductively coupled N 2 plasma and plasma source ion implantation was performed on Si using a hard-tube type HV pulse modulator at 55 kV of implantation voltage. The Auger depth profile results showed that the pulsed high-power RF plasma was more advantageous in plasma source ion implantation to achieve a deeper ion implantation, which was resulted from the increased atomic ion species compared to the continuous wave (CW) mode operation.

Can dry-etching systems be designed for low damage ab initio?

Photoluminescence intensity measurements from GaAs/AlGaAs and InGaAs/InAlAs quantum well probe structures have been used to study dry-etch damage inflicted in low power reactive ion etching environments. Selective etching has been employed to accumulate damage in the materials under these relatively low damage conditions. The measured data are consistent with calculations for channeling effects of atomic ion species, using a microscopic ion channeling theory. The results indicate that atomic as opposed to molecular ion channeling may be the main mechanism for deep dry-etch damage in these environments, which suggests that gases can be selected as likely to cause low damage.

Operational characteristic of a compact microwave ion source

A small microwave ion source has been fabricated from a quartz tube with one end enclosed by a two-grid accelerator. The source is also enclosed by a cavity operated at a frequency of 2.45 GHz. Microwave power as high as 500 W can be coupled to the source plasma. The source has been operated with different geometries and for various gases in a cw mode. For hydrogen, ion current density of 200 mA cm −2 with atomic ion species concentration as high as 80% has been extracted from the source. It has also been demonstrated that low energy oxygen ion beams (5–10 eV) can also be extracted from the source.

A Compact Microwave Ion Source

A small microwave ion source has been fabricated from a quartz tube with one end enclosed by a two grid accelerator. The source is also enclosed by a cavity operated at a frequency of 2.45 GHz. Microwave power as high as 500 W can be coupled to the source plasma. The source has been operated with and without multicusp fields for different gases. In the case of hydrogen, ion current density of 200 mA/cm-2 with atomic ion species concentration as high as 80% has been extracted from the source.

Conversion rates and ion mobilities in pure neon and argon afterglow plasmas

Measurements have been made at room temperature of the rates of decay of the atomic ion species in afterglow plasmas in pure neon and argon at various pressures within the range 0·01-2 torr. In both gases, the variation of the measured decay rates with pressure, observed over about two orders of magnitude in pressure, is consistent with the loss of the atomic inert gas ions by diffusion to the plasma boundary and by conversion to molecular inert gas ions in three-body reactions with the neutral gas. The present measurements constitute the first observations of the three-body conversion process in argon by a method providing mass identification of the participating ion species. The values obtained for the rates of the conversion reaction in neon and in argon and for the reduced mobilities of the atomic inert gas ions in their pure parent gases are νNe+ = (99±5)po2s-1, νAr+ = (313±12)po2s-1 μNe+ = 4.6±0.4 cm2 V-1s-1, μAr+ = 1.75±0.15) cm2 V-1s-1. These results are compared with the values which have been obtained by other workers and comparisons are also made with the existing theoretical estimates of the measured parameters.