Plasma parameter control for efficient operation of volume produced RF H- ion source

Abstract

In spite of its better stability for long-time operation than the cesiated negative ion source that suffers from difficulty of cesium injection control, the volume-produced RF (radio-frequency) negative ion source still requires further enhancement of current density with lower input RF power and lower operating pressure to satisfy requirements of various applications. In order to increase the volume production of H− ions, it is essential to obtain high electron temperature at the driver region while keeping both the electron temperature low below 1 eV and the electron density as high as possible at the extraction region. Although high electron density with low temperature at the extraction region has been achieved by increasing RF power and adjusting the strength of magnetic filter field, attempts to increase the electron temperature at the driver region with increasing input RF power or changing driving frequency have not yet been proven as efficient. In this study, a new approach based on a particle balance model for hydrogen plasma is suggested to increase the electron temperature effectively in the driver region by reducing the effective plasma size which is defined as the ratio of plasma volume (Vp) to effective area (Aeff). Measurements with an rf-compensated Langmuir probe show that the electron temperature in the driver region is significantly increased by reducing the length of the discharge chamber due to the reduced effective plasma size. Accordingly, H− ion density measurement with laser photo-detachment in the short chamber shows a few times increase compared to the longer one at the same heating power depending on gas pressure. However, the increase drops significantly as operating gas pressure decreases, indicating increased electron temperatures in the extraction region degrade H− ion production especially in the low operating pressure regime. Thus, characteristics of the electron temperature in the extraction region for various filter magnet strengths is investigated, and higher H− ion density is obtained as decease of electron temperature with higher magnetic filter field strength in the low pressure regime. Consequently, it is demonstrated that increasing the electron temperature in driver region by adjusting the discharge chamber geometry is efficient to increase H− ion production, provided that low electron temperature is maintained with higher filter magnetic strength in the extraction region. Based on the effects of the effective plasma size and magnetic filter field on plasma parameters, increase of extracted H− ion beam current up to 1.45 mA at RF power of only 0.9 kW (3.2 mA/cm2/kW) is definitely achieved with the short chamber length and higher magnetic filter field strength, which is comparable to the highest H− current density per unit RF power of the established major volume-produced H− ion sources. In addition, it is worth noting that even though measured H− ion density decrease in the low operating pressure regime with the short chamber due to higher electron temperature in the extraction region, higher H− ion beam current is obtained in the low operating pressure regime from the beam extraction result. This can be explained by further decrease of electron temperature due to the increased ∫B*dl value between the extraction hole and the measurement position considering both filter magnetic field strength and length. This result indicates that high ∫B*dl value is more suitable to suppress electron temperature operating at low pressure regime and with small effective plasma size configuration, since both increasing generation of highly vibrationally molecules in the driver region and lower electron temperature in the extraction region can be satisfied to increase of volume-production of H- ions effectively. On the contrary, the electron temperature at the extraction region can be low enough and high electron density can be achieved at high pressure regime. In this operating regime, the main destruction process in transport of the H- ions transport in the source region is the mutual neutralization which is increased by higher electron density. Therefore, relatively low ∫B*dl value is more effective to enhance H− beam current in considering transport loss of the volume produced H− ions in the magnetic filter field region and . Consequently, it is concluded that H− ion beam current in the volume-produced H- ion source can be optimized depending on the operating pressure regime by not only reducing electron temperature in the extraction region with higher magnetic filter field but also adjusting ∫B*dl value in considering plasma parameters to increase H- ion production. In this dissertation, the discharge chamber and magnetic filter field configuration as the ∫B*dl value are proposed as critical design parameters from the effects on the RF plasma parameters to increase volume-produced H− ion density. Moreover, it has been confirmed that the performance of the volume produced H− ion source at a given operating pressure and RF power condition can be optimized by controlling plasma parameters with changing discharge chamber and the ∫B*dl value depending on the operating pressure. The results shown here are expected to be useful to improve the performance of the established volume produced H− ion source and extend application fields of H− ion source requiring long-time operation.