Open Access

Online and FREE access to plasma physics experiments

Pedro A. Mendes Rossa
Pedro A. Mendes Rossa
, 
Pavel Kuriščák
Pavel Kuriščák
, 
João N. Silva
João N. Silva
, 
José Veiga
José Veiga
, 
João P. S. Loureiro
João P. S. Loureiro
, 
João Oliveira
João Oliveira
, 
Daniel Hachmeister
Daniel Hachmeister
 and 
Horácio Fernandes
Horácio Fernandes
   | Apr 03, 2023
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Article Category: ORIGINAL PAPER
Published Online: Apr 03, 2023
Page range: 37 - 46
Received: Aug 31, 2022
Accepted: Nov 16, 2022
DOI: https://doi.org/10.2478/nuka-2023-0006
Keywords
© 2023 Pedro A. Mendes Rossa et al., published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

Fig. 1.

Depicted FREE architecture where the central server acts as a broker between several users and the experiments with dedicated hardware, usually comprising a Linux proxy running in a Raspberry Pi and connected to the embedded system, custom made board with a PIC (peripheral interface controller), running a native C code.
Depicted FREE architecture where the central server acts as a broker between several users and the experiments with dedicated hardware, usually comprising a Linux proxy running in a Raspberry Pi and connected to the embedded system, custom made board with a PIC (peripheral interface controller), running a native C code.

Fig. 2.

Screen capture from the main list of selected protocols, showing the available experimental protocols that can be executed in a particular apparatus.
Screen capture from the main list of selected protocols, showing the available experimental protocols that can be executed in a particular apparatus.

Fig. 3.

The UI of the experiment EM cavity: (a) the configuration parameter to set your experiment, (b) the live stream of the experiment, and (c) framed by a brief description of the experiment.
The UI of the experiment EM cavity: (a) the configuration parameter to set your experiment, (b) the live stream of the experiment, and (c) framed by a brief description of the experiment.

Fig. 4.

Main components inside the chamber where the Langmuir probe is immersed and where the plasma is generated by a high AC voltage applied between the glow discharge molybdenum electrodes.
Main components inside the chamber where the Langmuir probe is immersed and where the plasma is generated by a high AC voltage applied between the glow discharge molybdenum electrodes.

Fig. 5.

Common rail vacuum system serves both experiments. On the left is the Langmuir probe and on the right is the EM cavity each with its own injection system but sharing the same gas rail. Only the EM cavity is equipped with magnetic field coils.
Common rail vacuum system serves both experiments. On the left is the Langmuir probe and on the right is the EM cavity each with its own injection system but sharing the same gas rail. Only the EM cavity is equipped with magnetic field coils.

Fig. 6.

The gas injection system was homemade with 200 μm clinical needles glued (epoxy) in the gas rail couplings.
The gas injection system was homemade with 200 μm clinical needles glued (epoxy) in the gas rail couplings.

Fig. 7.

Raw data from Langmuir probe displaying the initial electrical transient of the plasma. During the time period of the experiment, the pressure reading shows zero to identify the acquisition window.
Raw data from Langmuir probe displaying the initial electrical transient of the plasma. During the time period of the experiment, the pressure reading shows zero to identify the acquisition window.

Fig. 8.

Raw data from several sweeps displays a clear hysteresis when using a symmetrical triangular wave.
Raw data from several sweeps displays a clear hysteresis when using a symmetrical triangular wave.

Fig. 9.

Fitting of the ion saturation region, using Eq. (2), for correction of the data set.
Fitting of the ion saturation region, using Eq. (2), for correction of the data set.

Fig. 10.

By doing a semi-log graph, it is evident the exact location of the exponential part of the characteristic, and despite the lack of symmetry, the slopes are very similar in both situations.
By doing a semi-log graph, it is evident the exact location of the exponential part of the characteristic, and despite the lack of symmetry, the slopes are very similar in both situations.

Fig. 11.

Relative intensities of the electric field (a) and magnetic field (b) from an EM wave propagating in the TM010 mode. The electric field has a maximum in the middle of the cavity where the antenna must be inserted, jeopardizing the plasma. Conversely, in (b) the maximum is close to the walls where a magnetic antenna can be positioned to avoid disturbing the plasma and leading to the adopted solution.
Relative intensities of the electric field (a) and magnetic field (b) from an EM wave propagating in the TM010 mode. The electric field has a maximum in the middle of the cavity where the antenna must be inserted, jeopardizing the plasma. Conversely, in (b) the maximum is close to the walls where a magnetic antenna can be positioned to avoid disturbing the plasma and leading to the adopted solution.

Fig. 12.

Hardware parts from the EM cavity: (a) spectrum analyzer used, (b) the main cavity block, (c) assembly with the plexiglass windows on the top, and (d) ionizing HV electrode mesh with partial view of the loop antennas.
Hardware parts from the EM cavity: (a) spectrum analyzer used, (b) the main cavity block, (c) assembly with the plexiglass windows on the top, and (d) ionizing HV electrode mesh with partial view of the loop antennas.

Fig. 13.

Broad spectrum scan to identify the first resonant modes excited by the loop antenna.
Broad spectrum scan to identify the first resonant modes excited by the loop antenna.

Fig. 14.

The existence of plasma (Ar at 28 Pa) originates a drift on the resonant peak and a drop in the transmitted signal due to plasma losses and a consequent reduction in the quality factor.
The existence of plasma (Ar at 28 Pa) originates a drift on the resonant peak and a drop in the transmitted signal due to plasma losses and a consequent reduction in the quality factor.

Fig. 15.

Electron density in the function of the mean free path of the particles inside the chamber, as shown by data points in Table 3.
Electron density in the function of the mean free path of the particles inside the chamber, as shown by data points in Table 3.

Results of multiple executions with diffrent neutral gas (Ar) pressures and the resultants frequency shift measured

Pressure injection (Pa) f (MHz) Δf (MHz) ne (×1014 m−3)
    8 3563.6 – –
  10 3573.6 10.0   8.8
  12 3580.8 17.2 15.2
  13 3584.8 21.2 18.7
  16 3586.4 22.8 20.2
  18 3584.4 20.8 18.3
  20 3582.5 18.9 16.7
  21 3580.0 16.4 14.4
  25 3578.4 14.8 13.1
  28 3574.5 10.9   9.6
  34 3572.8   9.2   8.1
  44 3570.0   6.4   5.7
  48 3568.8   5.2   4.6
  60 3567.2   3.6   3.2
  98 3566.8   3.2   2.8
120 3566.8   3.2   2.8

Results of multiple executions with different neutral gas pressure (Ar) and results for the ion saturation current, and the respective calculation of the speed of sound and the density

Pressure (Pa) (μgA)isat+$$\mathop {\left( {{\rm{\mu gA}}} \right)}\limits^{i_{{\rm{sat}}}^ + } $$ Te (eV) Cs (m/s) n (×1015 m−3)
  64 −15.2 4.8 3405 8.9
100 −14.2 6.3 3901 7.2
150 −12.7 5.7 3711 6.8

Parameters obtained after the fitting shown in Fig. 9, the ion saturation current

Slope α(1/V) Vf (V) isat+ (μA)$i_{sat}^ + {\rm{ }}\left( {{\rm{\mu A}}} \right)$
1 0.96 12.38 −17.2
−1 0.49 35.33 −17.6
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