Numerical Simulation of the RF Plasma Discharge in the Linac4 H- Ion Source

This paper presents a Particle-In-Cell Monte Carlo Collision simulation of the Radio-Frequency (RF) plasma heating in the Linac4 H- ion source at CERN. The model self-consistently takes into account the electromagnetic field generated by the RF coil, the external static magnetic fields and the resulting plasma response, including a kinetic description of the charged species ((e)-, H+, H-2(+), H-3(+), H-), as well as the atomic and molecular (vibrationally resolved) populations. The simulation is performed for the nominal operational condition of 40 kW RF power and 3 Pa H-2 pressure. Results show that the plasma spatial distribution is non-uniform in the plasma chamber, with a density peak of n(e) = 5 . 10(19) m(-3) in the RF coil region. In the filter field region the electron density drops by two orders of magnitude, with a substantial reduction of the electron energy as well. This results in a ratio e/H- approximate to 1 in the extraction region. The vibrational population is characterized by a two temperature distribution, with the high vibrational states showing a factor 2 higher termperature. A very good agreement is found between the simulation results and optical emission spectroscopy measurement performed on a dedicated test stand at CERN.

H- ion source for CERN's Linac4 accelerator

Stefano Mattei

Linac4 is the new negative hydrogen ion (H-) linear accelerator of the European Organization for Nuclear Research (CERN). Its ion source operates on the principle of Radio-Frequency Inductively Coupled Plasma (RF-ICP) and it is required to provide 50 mA of H- beam in pulses of 600 us with a repetition rate up to 2 Hz and within an RMS emittance of 0.25 pi mm mrad in order to fullfil the requirements of the accelerator. This thesis is dedicated to the characterization of the hydrogen plasma in the Linac4 H- ion source. We have developed a Particle-In-Cell Monte Carlo Collision (PIC-MCC) code to simulate the RF-ICP heating mechanism and performed measurements to benchmark the fraction of the simulation outputs that can be experimentally accessed. The code solves self-consistently the interaction between the electromagnetic field generated by the RF coil and the resulting plasma response, including a kinetic description of charged and neutral species. A fully-implicit implementation allowed to simulate the high density regime of the Linac4 H- ion source, ensuring the energy conservation while maintaining the computational resources tractable. We studied the capacitive to inductive transition characteristic of the initial phase of the pulsed discharge. The simulation results were confirmed by time-resolved photometry measurements and allowed quantifying the effect of the hydrogen pressure and of the external magnetic cusp field on the transition dynamics. This provided insights into possible modifications to the magnetic cusp field configuration to maximize the power deposited to the plasma. The optimal ion source configuration maximizes the density of volume produced H-, the flux of H0 atoms onto the cesiated molybdenum plasma electrode surface at the origin of H- emission, and minimizes the electron density and energy in the beam formation region. We simulated the high-density regime (10^19 m-3) representative of the nominal operation of the Linac4 ion source during beam extraction. We performed a parametric study of the RF current, hydrogen pressure and magnetic configuration (cusp and filter) to assess their impact on the plasma parameters. The simulation results allowed assessment of these parameters and provided guidelines for the optimization of the ion source operational and design parameters. The simulation results of electron density, electron energy and hydrogen dissociation degree showed excellent agreement with optical emission spectroscopy measurements both as a function of RF coil current and magnetic configuration. The outputs of these simulations provide crucial inputs to beam formation and extraction physics models. Dedicated PIC software packages are being developed that will eventually shed insight into essential beam parameters such as the intensity and emittance of the H° beam and the intensity of the co-extracted electrons.

EPFL2017

A fully-implicit Particle-In-Cell Monte Carlo Collision code for the simulation of inductively coupled plasmas

Stefano Mattei, Minh Quang Tran

We present a fully-implicit electromagnetic Particle-In-Cell Monte Carlo collision code, called NINJA, written for the simulation of inductively coupled plasmas. NINJA employs a kinetic enslaved Jacobian-Free Newton Krylov method to solve self-consistently the interaction between the electromagnetic field generated by the radio-frequency coil and the plasma response. The simulated plasma includes a kinetic description of charged and neutral species as well as the collision processes between them. The algorithm allows simulations with cell sizes much larger than the Debye length and time steps in excess of the Courant-Friedrichs-Lewy condition whilst preserving the conservation of the total energy. The code is applied to the simulation of the plasma discharge of the Linac4 H- ion source at CERN. Simulation results of plasma density, temperature and EEDF are discussed and compared with optical emission spectroscopy measurements. A systematic study of the energy conservation as a function of the numerical parameters is presented. (C) 2017 The Authors. Published by Elsevier Inc.

2017

Computer Modeling of YBCO Fault Current Limiter Strips Lines in Over-Critical Regime With Temperature Dependent Parameters

Joseph Duron, Bertrand Dutoit, Francesco Grilli

We present the results of an advanced numerical model for fault current limiter (FCL) based on HTS thin films in which both thermal and electromagnetic aspects are taken into account. This model allows simulating the behavior of FCL in the over-critical current regime and we used it for studying strip lines of a YBCO/Au FCL on sapphire substrate. The electromagnetic and thermal equations have been implemented in finite-element method (FEM) software in order to obtain a model for investigating the comportment of the superconductor when the current exceeds

I_{c}

. In particular, materials equations have been implemented in order to simulate the electrical behavior of superconducting devices with strong over-critical currents. We report results of simulations in voltage source mode where currents largely exceed

I_{c}

. The global behavior of the FCL is compared with measurements, showing a good agreement. The use of FEM simulations offers the advantage to give access to local variables such as current density or temperature. Studies with this model can replace expensive experiments where very high current density might damage or destroy the FCL device.

2007