Application of Thomson scattering to helicon plasma sources

Abstract

The possibility of performing electron density and temperature measurements in a high power helicon plasma is a crucial issue in the framework of the AWAKE (Advanced WAKefield Experiment) project, which demonstrates acceleration of particles using $\text{GeV}~\text{m}^{-1}$ electric fields in plasmas. For AWAKE, a helicon is currently envisaged as a candidate plasma source due to its capability for low electron and ion temperature, high electron density and production of an elongated plasma column. A plasma diagnostic to accurately determine the electron density in AWAKE regimes would be a valuable supporting tool. A demonstration Thomson scattering (TS) diagnostic was installed and successfully tested on the resonant antenna ion device (RAID) at the Swiss Plasma Center of Ecole Polytechnique Fédérale de Lausanne. RAID produces a helicon plasma column with characteristics similar to those of the AWAKE helicon source, and is therefore an optimal testbed for application to the AWAKE device. The spectrometer employed in RAID is based on polychromators which collect the light scattered by plasma electrons in spectrally filtered wavelength regions. Results from TS on RAID demonstrate conditions of electron density and temperature respectively of $n_{e}=1.10\,(\pm 0.19)\times 10^{19}~\text{m}^{-3}$ and $T_{e}=2.3\,(\pm 0.6)~\text{eV}$ in a steady-state discharge in an Ar plasma with 5 kW of RF power. If the same polychromator system is used for AWAKE, where the electron density attained is $2\times 10^{20}~\text{m}^{-3}$ , the contribution to measurement error due to coherent scattering is ${\sim}2.5\,\%$ . Presented here are details of the TS diagnostic and the first tests in RAID, and the expectations for the system when employed on the AWAKE device.

Effect of neutral pressure on the blue core in Ar helicon plasma under an inhomogeneous magnetic field

“…The on-axis T e at 300 W drops from about 11 eV at 0.08 Pa to about 9 eV at 0.68 Pa, which was caused by the increase in collision frequency due to the increase of density [9]. The T e profiles measured at 0.08 Pa and 0.36 Pa exhibited similar features in that the temperature peak was in the center at 300 W. But the case of 0.68 Pa was the opposite and became hollow at 300 W [32,44]. The hollow structure was formed under all three pressures when the power was 1000 W. In conjunction with figures 7 and 8, it is deduced that the helicon wave is dominant to produce plasma in the center and high collision frequency leads to low T e due to high n i in the case of 0.08 Pa [9,66].…”

“…The plasma density and ionization rate in the blue core region are considerably higher. The phenomenon had been studied by researchers under certain conditions in many helicon plasma apparatuses [17,30,[41][42][43][44][45]. The blue core was also regarded as a new transition mode after the traditional wave mode (W mode) [37,46].…”

Numerical and experimental investigations of a microwave interferometer for the negative ion source SPIDER

“…The aim of this work is to perform a preliminary study of the propagation of a 100 GHz beam provided these design constraints, to assess the feasibility degree of this technique in SPIDER and, possibly, provide an overview of the issues that might be expected when applying this technique to SPIDER-like plasma sources. The choice of the 100 GHz frequency is motivated by the fact of being a compromise between the minimization of beam divergence, the expected range of plasma density and the availability of a 75-110 GHz microwave diagnostic system tested for the first time in the helicon plasma device RAID [9][10][11][12].…”

The AWAKE Run 2 programme and beyond

“…This demonstration requires highly accurate and localised plasma density measurements. Specific diagnostics such as Thomson scattering [63] and optical emission spectroscopy are being developed.…”