H- ion source for CERN's Linac4 accelerator

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.

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

Stefano Mattei, Minh Quang Tran

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.

Amer Inst Physics2017

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

Cyclotron Designs for Ion Beam Therapy with Cyclinacs

Adriano Garonna

This thesis presents new superconducting compact (as opposed to separated-sector ) cyclotron designs for injection in CABOTO, a linac developed by the TERA Foundation delivering C6+/H2+ beams up to 400 MeV/u for ion beam therapy. This association of a variable energy linac injected by a fixed energy cyclotron is called cyclinac. Two superconducting cyclotron designs are compared under the same design constraints and methods: a synchrocyclotron and an isochronous cyclotron, both at the highest possible magnetic field and with an output energy of 230 MeV/u. This energy allows to use the cyclotron as a stand-alone accelerator for protontherapy. Once the optimal cyclotron is determined, lower energy cyclotrons can easily be designed. The short pulse length (1.5 µs), fast repetition rate (100-300 Hz) and small beam transmission of the cyclinac (0.2%) require intense pulsed ion sources. To deliver the desired clinical dose rate, the average pulse current of 60 eµA of C6+ at 300 Hz can be produced by three commercial EBIS (EBIS-SC by Dreebit Gmbh) operating at 100 Hz and connected to the beamline in alternating mode. A multicusp ion source is sufficient to produce compatible H2+ beams. The synchrocyclotron design features a central magnetic field of 5 T, an axisymmetric pole and a constant field index of 0.02. The beam is injected axially with a spiral inflector (K = 1.4). A static magnetic perturbation of 0.1 T and 5° width boosts the beam radial gain per turn (with no emittance degradation) by exciting the first radial integer resonance and thus allows beam ejection with moderate beam losses (30%). The RF system operates in first harmonic (Q = 2500). The 180° Dee provides 28 kV peak voltage and the RF is modulated (30-38 MHz) by a rotating capacitor (90-900 pF). The synchrocyclotron's best features are the simple and compact magnet (300 tons) and the low RF power requirements (30 kW power supply). The isochronous cyclotron design features a 3.2 T central magnetic field, four sectors and a pole characterized by elliptical gaps in the hills (3-30 mm) and in the valleys (11-50 cm). Spiraling is minimized (80° total hill axis rotation) and beam ejection is achieved with a single electrostatic deflector placed inside an empty valley. The two RF cavities operate in fourth harmonic at 98 MHz (Q = 7100). The RF system provides peak voltages of 70-120 kV and is powered by a single 100 kW unit. The synchrocyclotron reliability is brought into question by the need of a rotating capacitor and by the complexity of the injection and ejection systems. However, the isochronous cyclotron requires a much more complex magnet. Overall, the isochronous cyclotron is a better solution compared to the synchrocyclotron, because it is as compact but more reliable. To quantitatively determine the industrial and clinical optimum for the CABOTO injection energy, three complementary isochronous cyclotrons of 70, 120 and 170 MeV/u are studied, based on the 230 MeV/u design. The optimal cyclotron energy strongly depends on the clinical aim of the facility. For a dual proton and carbon ion centre, the best compromise between clinical flexibility, accelerator size and power consumption is to accelerate particles up to 150 MeV/u in the cyclotron. In this configuration, the 150 MeV/u isochronous cyclotron has similar weight and spiraling as the most widely used cyclotron for protontherapy (C235 by IBA S.A.), CABOTO is 24 m long and the overall power consumption of the cyclinac is 650 kW. Adding to these characteristics, the property of fast energy variation of the linac makes the cyclinac presented in this thesis a strongly competitive accelerator for dual proton and carbon ion therapy.

EPFL2011

Cyclotron Designs for Ion Beam Therapy with Cyclinacs

Adriano Garonna

EPFL2011