S. Briefi

Linac4 H⁻ ion sources.

CERNs 160 MeV H(-) linear accelerator (Linac4) is a key constituent of the injector chain upgrade of the Large Hadron Collider that is being installed and commissioned. A cesiated surface ion source prototype is being tested and has delivered a beam intensity of 45 mA within an emittance of 0.3 π ⋅ mm ⋅ mrad. The optimum ratio of the co-extracted electron- to ion-current is below 1 and the best production efficiency, defined as the ratio of the beam current to the 2 MHz RF-power transmitted to the plasma, reached 1.1 mA/kW. The H(-) source prototype and the first tests of the new ion source optics, electron-dump, and front end developed to minimize the beam emittance are presented. A temperature regulated magnetron H(-) source developed by the Brookhaven National Laboratory was built at CERN. The first tests of the magnetron operated at 0.8 Hz repetition rate are described.

Generation of an atmospheric plasmoid from a water discharge: An analysis of the dissipated energy

A plasmoid in air at atmospheric pressure of about 20 cm in diameter and up to 500 ms duration is generated from a water discharge which is powered for a short time period by a capacitor bank. The analysis of the electrical circuit and the comparison with experimental values show that the energy dissipated into the system is given by the conventional equation for discharging capacitors. The resistance of the system is governed by the resistances of the water reservoir, the plasma, and the plasma-water transition, which are represented as one time-averaged resistance in the equation. Thus, the dissipated energy can be influenced by the energy available (capacitance and voltage), the voltage-on time, the conductivity of the water, the electrode gap and the size of the container (plate electrode) within the experimental boundaries. An estimation of the energy channels for a discharge at standard conditions revealed that the dominant part of the energy is dissipated into the water reservoir. About 25% of the ...

Quantification of the VUV radiation in low pressure hydrogen and nitrogen plasmas

Hydrogen and nitrogen containing discharges emit intense radiation in a broad wavelength region in the VUV. The measured radiant power of individual molecular transitions and atomic lines between 117 nm and 280 nm are compared to those obtained in the visible spectral range and moreover to the RF power supplied to the ICP discharge. In hydrogen plasmas driven at 540 W of RF power up to 110 W are radiated in the VUV, whereas less than 2 W is emitted in the VIS. In nitrogen plasmas the power level of about 25 W is emitted both in the VUV and in the VIS. In hydrogen–nitrogen mixtures, the NH radiation increases the VUV amount. The analysis of molecular and atomic hydrogen emission supported by a collisional radiative model allowed determining plasma parameters and particle densities and thus particle fluxes. A comparison of the fluxes showed that the photon fluxes determined from the measured emission are similar to the ion fluxes, whereas the atomic hydrogen fluxes are by far dominant. Photon fluxes up to 5 × 1020 m−2 s−1 are obtained, demonstrating that the VUV radiation should not be neglected in surface modifications processes, whereas the radiant power converted to VUV photons is to be considered in power balances. Varying the admixture of nitrogen to hydrogen offers a possibility to tune photon fluxes in the respective wavelength intervals.

Simulation of the A–X and B–X transition emission spectra of the InBr molecule for diagnostics in low-pressure plasmas

Inductively coupled low-pressure discharges containing InBr have been investigated spectroscopically. In order to obtain plasma parameters such as the vibrational and rotational temperature of the InBr molecule, the emission spectra of the and the transitions have been simulated. The program is based on the molecular constants and takes into account vibrational states up to v = 24. The required Franck–Condon factors and vibrationally resolved transition probabilities have been computed solving the Schrodinger equation using the Born–Oppenheimer approximation. The ground state density of the InBr molecule in the plasma has been determined from absorption spectra using effective transition probabilities for the A–X and B–X transition according to the vibrational population. The obtained densities agree well with densities derived from an Arrhenius type vapour pressure equation.

Diagnostics of low-pressure discharges containing InBr studied for lighting applications

The utilization of InBr in low-pressure rare-gas plasmas for lighting applications may serve as an efficient alternative to hazardous mercury, which is used in common fluorescent lamps as a radiator. In order to perform systematic investigations of these discharges, diagnostic methods are required to gain insight into the relevant plasma parameters. This goal can be achieved by using white light absorption and optical emission spectroscopy supported by an extended corona model of the indium atom and a simulation of the relative intensity of the InBr emission. The set of diagnostic methods is exemplarily applied to measurements on an inductively coupled argon discharge at 100 W power with varying InBr content. The plasma parameters are derived and the processes determining their changes with varying InBr density are identified. Increasing the InBr density results in a decrease in Te but an increase in ne, which can be explained by considering the ionization and power balance. The relevant population processes for the rovibrational states of InBr are inelastic collisions with heavy particles with an increasing importance of electron impact excitation at a higher InBr density. The radiated power is maximal at a cold spot temperature between 210 and 220 °C as reabsorption occurs at a high InBr density.

Correction factors for saturation effects in white light and laser absorption spectroscopy for application to low pressure plasmas

In white light absorption spectroscopy, the broadening of the absorption signal due to the apparatus profile of the spectrometer may lead to an underestimation of the determined density as one measures an apparent optical depth. This is in particular true for high optical depth where saturation effects of the transmitted intensity occur. Provided that the line profile of the absorption line is known, the apparent optical depth effect can be accounted for by introducing a correction factor. The impact of the saturation and the approach of considering the effect are demonstrated for argon and indium lines in low pressure plasmas where correction factors of one order of magnitude or even higher are reached very easily. For the indium line, the hyperfine splitting has been taken into account. In laser absorption, the line profile is resolved. However, the weak but rather broad background emission of the laser diode can cause a saturation signal at the photo diode resulting also in an underestimation of the de...

Experimental investigation of plasma impedance in Linac4 H- source

CERN ’s new particle accelerator Linac4 is part of the upgrade of the LHC accelerator chain. Linac4 is required to deliver 160 MeV H− beam to improve the beam brightness and luminosity in the Large Hadron Collider (LHC). The Linac4 H− source must deliver 40-50 mA, 45 keV H− beam in the RFQ acceptance. Since the RF power coupled to the H− source plasma is one of the important parameters that determines the quality of the H− beam, the experimental investigation of the dependence of the load impedance on the operational parameters is mandatory. In this study, we have measured the impedance of the H− source plasma varying the RF power coupled to the plasma and the condition of the hydrogen gas. Also, optical emission spectroscopy (OES) measurements have been carried out simultaneously with the impedance measurement in order to determine the plasma parameters. The determination of the plasma parameters allows us to compare the experimental results with the analytic model of the plasma parameters, which is usef...

Comparison of the B field dependency of plasma parameters of a weakly magnetized inductive and Helicon hydrogen discharge

The discharge properties of a weakly magnetized inductively coupled hydrogen discharge (operating pressure 1 Pa) are evaluated by using optical emission spectroscopy. The behaviour of the electron density n e, temperature T e and the density ratio of atomic to molecular hydrogen n H/ with varying magnetic field strength (up to 12 mT) is investigated. The results obtained from the OES measurements performed with a line of sight directed along the central axis of the cylindrical discharge vessel are compared to the case when the ICP antenna is replaced by a Nagoya-type-III Helicon antenna. In the ICP case, the electron temperature and density at the axis of the cylindrical discharge vessel decrease with increasing magnetic field due to the hindered radial electron diffusion. This results in a gradual transition from a homogeneous radial emission profile to a hollow profile with minimal emission in the discharge centre. Concerning the density ratio of atomic to molecular hydrogen, one obtains very high values of up to 0.32 at low B field and a decreasing behaviour with higher magnetic fields. For the Helicon case, the obtained values of n e and T e are virtually unaffected by the external magnetic field. Furthermore, a hollow radial emission profile is observed already at low B field strengths. In the Helicon setup one obtains an increasing trend for n H/ with a maximum of about 0.2 at 12 mT.

Workshop on performance variations in H− ion sources 2012: PV H−12

This paper briefly summarizes a workshop held in Jyvaskyla the day after NIBS’12. The half-day workshop aimed at globally capturing the issue of performance variations in H− sources. There was a focus on production facilities and facilities that work under production-like conditions, because there are often high expectations to be met.

Note: implementation of a cold spot setup for controlled variation of vapor pressures and its application to an InBr containing discharge lamp.

In order to allow for a systematic investigation of the plasma properties of discharges containing indium halides, which are proposed as an efficient alternative for mercury based low pressure discharge lamps, a controlled variation of the indium halide density is mandatory. This can be achieved by applying a newly designed setup in which a well-defined cold spot location is implemented and the cold spot temperature can be adjusted between 50 and 350 °C without influencing the gas temperature. The performance of the setup has been proved by comparing the calculated evaporated InBr density (using the vapor pressure curve) with the one measured via white light absorption spectroscopy.