Laboratory Measurements of the Performances of the Sweeping Langmuir Probe Instrument Aboard the PICASSO CubeSat

Ranvier, Sylvain; Lebreton, Jean-Pierre

doi:https://doi.org/10.5194/gi-2022-7

Preprints

https://doi.org/10.5194/gi-2022-7

Preprints

12 May 2022

Status: a revised version of this preprint was accepted for the journal GI and is expected to appear here in due course.

Laboratory Measurements of the Performances of the Sweeping Langmuir Probe Instrument Aboard the PICASSO CubeSat

Sylvain Ranvier¹ and Jean-Pierre Lebreton² Sylvain Ranvier and Jean-Pierre Lebreton

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¹Royal Belgian Institute for Space Aeronomy (BIRA-IASB), 1180 Brussels, Belgium
²LPC2E, CNRS/University Orleans/CNES, 45071 Orleans, France

Received: 09 May 2022 – Discussion started: 12 May 2022

Abstract. The Sweeping Langmuir Probe (SLP) is one of the instruments on board the triple unit CubeSat PICASSO, an ESA in-orbit demonstrator launched in September 2020, which is flying at about 540 km altitude. SLP comprises 4 small cylindrical probes mounted at the tip of the solar panels. It aims at performing in-situ measurements of the plasma parameters (electron density and temperature together with ion density) and of the spacecraft potential in the ionosphere. Before the launch, the instrument, accommodated on an electrically representative PICASSO mock-up, has been tested in a plasma chamber. It is shown that the traditional orbital-motion-limited collection theory used for cylindrical Langmuir probes cannot be applied directly for the interpretation of the measurements because of the limited dimensions of the probes with respect to the Debye length in the ionosphere. Nevertheless, this method can be adapted to take into account the short length of the probes. To reduce the data downlink while keeping the most important information in the current-voltage characteristics, SLP includes an on-board adaptive sweeping capability. This functionality has been validated both in the plasma chamber and in space and it is demonstrated that, with a reduced number of data points the electron retardation and electron saturation regions can be well resolved. Finally, the effect of the contamination of the probe surface, which can be a serious issue in Langmuir probe data analysis, has been investigated. If not accounted for properly, this effect could lead to substantial errors in the estimation of the electron temperature.

Sylvain Ranvier and Jean-Pierre Lebreton

Status: closed

RC1:
'Comment on gi-2022-7', Laila Andersson, 27 May 2022

Dear Sylvian Ranvier and Jean-Pierre Leberton

Great paper and I have some suggestions for improvement.

On page 1 lines 12-14 and page 15 lines 288-290: From the IV-curve of a LP-instrument the electron density and temperature is a derived quantity with some uncertainty. The paper present a selected sweep methodology to optimize the telemetry volume over downlinking a full IV-sweep. In the discussion about the approach the manuscript do quantify the impact of having a limited number of data points in the IV-curve on the resulting uncertainty. Can change in the uncertainty of electron density and temperature be quantified?

On page 3 the methodology of data sampling is discussed. In the paper the notation 'inflection point' is stated but that notation is not clearly explain. This important because the MAVEN LPW instrument used a similar approach and it would be good to highlight the similarities/differences. On MAVEN at low altitudes (sweep potential +-5 V) we use a look-up table for the sweep steps (large step sized on the start and stop and small step sizes in the center similar to the PICASSO approach). This sweep is then centered on the location where the previous measured current changed sign plus an offset. The offset allows any current offset to be taken into consideration and center the smallest voltage step sizes in the region where the electron temperature is extracted, i.e. the smallest steps is not necessarily where the measured current change sign. The centering of the look-up table is only allowed to slowly change from sweep to sweep (a time constant is set in the instrument of how 'slow' this change is selected to be). In the paper it is not clear if the PICASSO approach take into account any instrument current offsets or how fast the instrument respond to large changes from one sweep to another. This last topic is an important point when looking at Figure 13.

On page 3 the stepping speed at up to 10 kHz is discussed. There has been a resent paper analyzing the sweep speed. They analyzed 10 kHz (and higher), you might want to reflect over their numerical result with your observational result.

M. Kjølerbakken, W. J. Miloch, Ø. G. Martinsen, O. Pabst and K. Røed, "Numerical Study of Non-Linear Effects for a Swept Bias Langmuir Probe," in IEEE Transactions on Plasma Science, vol. 50, no. 5, pp. 1237-1245, May 2022, doi: 10.1109/TPS.2022.3164220.

On page 5 discussing the temperature of the probe. Is the 200 oC and measured temperature or a simulated thermal estimate? I assume the temperature is for the metal rod, not the electronics such as the preamps.

Page 8. Very nice simulations capturing the problem with high density region and small SC. I might have missed it was the simulation in flowing plasma? For MAVEN LPW we did also simulations of the local plasma, I would recommend you to read that paper and comment on similarity/differences. In that paper the potential influence from the SC was not the main issue but the wake due to the probe itself.

Ergun, R. E., Andersson, L. A., Fowler, C. M., & Thaller, S. A. (2021). Kinetic modeling of Langmuir probes in space and application to the MAVEN Langmuir probe and waves instrument. Journal of Geophysical Research: Space Physics, 126, e2020JA028956. https://doi.org/10.1029/2020JA028956

Page 13 discussing the result of Figure 14 (and Figure 15). Could the authors add a few sentences how this would change for space instrument. In space the instrument often is just continuously on so this increase might not be ever observed. The paragraphs fell a slightly short with taking the result from the laboratory and spell out how it would be observed in space.

Page 12-14 about the contamination: Gold is fairly robust material and is unlikely to be effected by atomic oxygen. But for the readers understanding, it could be good to point out that atomic oxygen could change the behavior in similar way as the contamination issues. Joseph Samaniego did some laboratory experiments and that effort could be help the discussion in this paper. This is one of the papers:

Samaniego, J. I., Wang, X., Andersson, L., Malaspina, D., Ergun, R. E., & Horányi, M. (2018). Investigation of coatings for Langmuir probes in an oxygen-rich space environment. , 123, 6054– 6064. https://doi.org/10.1029/2018JA025563

Great work,

Laila Andersson

Citation: https://doi.org/10.5194/gi-2022-7-RC1
- AC1: 'Reply on RC1', Sylvain Ranvier, 07 Oct 2022
  
  Dear Laila,
  Thank you for the very interesting comments and suggestions that will definitely improve the quality of the paper.
  Comment 1:
  
  We propose to add the following text:
  
  “The distortion of the I-V characteristics due to the adaptive sweep method depends on several parameters including the number of data points, the difference of the step sizes of the three regions and the delay between the data points. The effect of the latter parameter is clearly visible by comparing Figure 9 and Figure 12, which are sweeps performed with 300 µs and 1.2 ms delay between steps, respectively. Unfortunately, not enough measurements have been performed in the laboratory plasma chamber to statistically quantify the change in the uncertainty of electron density and temperature as a function of those parameters. This assessment will be performed in a follow-up study.
  
  Comment 2:
  We propose to add the following text:
  
  "On board, the inflection point of the I-V curve (i.e. the bias for which the first derivative of the curve is maximal) which separates the electron retardation and the electron saturation regions (the plasma potential) is determined by computing the first derivative of the curve and selecting the highest value (the maximum). This is performed after each sweep and used to compute the span of the fine step region of the next sweep, as follows: the first point of this region is set by a parameter of the command sent to the instrument, but the last point of this region is defined by adding a number (another parameter of the command) of data points after the computed inflection point, as depicted in Figure XY. The step size of the electron saturation region is then also adjusted automatically. The use of additional data points ensures the fine sampling of the full retardation region, even if the inflection point is computed with some inaccuracy (e.g. due to noise). There is no limitation on how fast the inflection point can be determined accurately when the electron temperature decreases (inflection point shifting toward the retardation region). When the temperature is increasing (inflection point shifted toward the electron saturation region), the number of iterations needed to accurately define the inflection point is determined by the number of additional data points. Increasing the number of those additional data points will decrease the number of iterations needed to accurately determine the inflection point, but, at the same time, it will increase the data volume. Therefore there is a trade-off between data reduction efficiency and time response of the system. A running average (with window size a parameter of the command) is used to improve the robustness of the algorithm against the noise. Since the determination of the inflection point is done by computing the first derivative, the instrument current offsets (if any) will not impact its accuracy. Given that the algorithm defining the bias steps is fully deterministic, only one number (the on-board computed inflection point, which is sent together with the measured current) is needed to rebuild the sweep steps on ground and to compute the complete I-V curve. To get an order of magnitude, when the fine step region is sampled with 30 steps, the step size ranges from about 10 mV to 150 mV for electron temperatures of 600 K and 10.000 K, respectively. This method is different from the one used for the LPW instrument on MAVEN (Andersson et al., 2015) where a look-up table for the sweep steps (large step sizes in electron and ion saturation regions and small step sizes in the retardation region) is used and the sweep is centered on the location where the previous measured current changed sign plus an offset. The advantage of the approach used for the LPW as compared to the one used for SLP is that the beginning of the fine step region is adjusted automatically, whereas it is fixed for SLP. On the other hand, the approach used for SLP allows sampling the retardation region with a given number of points, leading to a given relative resolution, which is valid for a very broad range of electron temperature, i.e. for retardation regions with very different width. "
  
  The number of additional data points used during the measurements presented in Fig 9 et Fig 12 will be added in the respective sections.
  
  Figure XY mentioned in this new part is attached as supplement.
  
  As suggested, we will add the following reference: Andersson, L., et al., The Langmuir Probe and Waves (LPW) Instrument for MAVEN, Space Sci. Rev. (2015), 195:173–198, DOI 10.1007/s11214-015-0194-3
  
  Comment 3:
  
  Although the paper describes very well the non-linear effects linked to the fast sweeping of Langmuir probes, those effects are studied for sweep duration from 1.5 µs up to 0.2 ms, which is at least 100 times faster than the sweeps performed by SLP (minimum sweep duration about 20 ms). The effects studied by those authors are not applicable to SLP/PICASSO.
  
  Comment 4:
  
  200 °C is a simulated temperature estimate for the metal rod itself, which is much higher than the temperature of the electronics font end (between 15°C and 45 °C along the orbit, measured).
  Comment 5:
  
  The Simulation is in flowing plasma (Vram = 7.5 km/s, perpendicular to the probe).
  
  The results presented in this paper qualitatively agree with those reported in (Ergun et al., 2021) and (Marholm et al., 2020), where it is shown that in the ionosphere, for cylindrical probes with finite length, the Beta exponent lies between the one of a perfect cylinder (0.5) and the one of a perfect sphere (1).
  
  The fact that the main issue is the influence of the spacecraft potential rather than the wake due to the probe itself comes from the fact that the probe to S/C current collection area ratio is much larger for SLP on PICASSO than for LPW on MAVEN, which leads to more severe S/C charging effects for SLP. In addition, given the short distance between the S/C body and the sensor (a few Debye lengths) and the fact that there is no guard, the S/C potential effect dominates the wake effect on the I-V characteristics for SLP.
  
  Comment 6:
  
  For a space instrument, one possibility would be to periodically stop sweeping, wait for a given time (to let the probe-spacecraft system discharge), restart the sweep sequence and use the first sweep as a reference. If the plasma environment can be assumed to be stable for a few tens of seconds, then the effects of contamination on the I-V curves can be accounted for in the calibration. Measuring with multiple probes would allow acquiring continuous measurements data even if sweeps are stopped and restarted regularly (if this is done alternatively for both - or more – probes).
  
  Onother possibility is to perform increasing and decreasing sweeps with different duration as in (Lebreton, et al., 2006) and to model the contamination layer as an R-C parallel circuit as in (Fang et al., 2018 and Piel et al., 2001). If properly modeled and combined with a database of PIC simulations of contaminated probe surfaces for different R, C and plasma parameters values, this could be used to retrieve the uncontaminated I-V curve. The study of the effect of the contamination, on a single sweep and on consecutive sweeps, is an on-going work that will be subject of a follow-up publication. Hence in this article, we acknowledge that there are uncertainties in the determination of the plasma parameters, but their quantification is out of scope of this paper.
  
  Comment 7:
  
  We propose to add the following text:
  
  Similar effects (i.e. reduced current at a given probe potential, the derived plasma potential becomes more positive, the derived electron density decreases and the derived electron temperature becomes hotter) have been reported in (Samaniego, J. I. et al., 2018) for several probe coatings (including Gold and DAG) both when the probes were exposed to oxygen-rich environments and without exposition to oxygen-rich environment. In this latter case, the sources of contamination were assumed to be the deposition of vacuum pump oil and moisture from the air, similarly to the contamination of the SLP probes reported in this Section. Then, although Gold should be insensitive to atomic oxygen, it was found that the O+ and O2+ bombardment induced a moderate but noticeable oxidation effect on the probe measurements, in addition to the effects of the contamination alone.
  
  As suggested, the following reference will be added:
  
  Samaniego, J. I., Wang, X., Andersson, L., Malaspina, D., Ergun, R. E., & Horányi, M. (2018). Investigation of coatings for Langmuir probes in an oxygen-rich space environment. Journal of Geophysical Research: Space Physics, 123, 6054– 6064. https://doi.org/10.1029/2018JA025563
  
  Citation: https://doi.org/10.5194/gi-2022-7-AC1
RC2:
'Comment on gi-2022-7', Anonymous Referee #2, 19 Jul 2022

Review Laboratory Measurements of the Performances of the Sweeping

Langmuir Probe Instrument Aboard the PICASSO CubeSat

This article reports on laboratory measurements of a LP sensor aboard a cubsat.

Such measurements are important and necessary to fully exploit the data analysis of this

instrument.

The article is worth publishing, after minor comments to be addressed by the authors:

(1) it would be useful to mention the status of the PICASSO satellite to put the article into context.

The article only mentions that this is an in-orbit demonstrator launched in September 2020.

(2) Is it possible to briefly explain what

"the traditional Orbital-Motion-Limited (OML) collection theory" is for? (page 2)

(3) Figure 1 caption: clarify in which V range exactly the current is multiplied by 3.

(4) In the text sometimes it's PI-CASSO or PICASSO. Update for coherence.

(5) Page 7: SPIS (Spacecraft Plasma Interaction System) simulations were performed. it would be useful to mention here

all assumptions for these simulations (surface properties etc..).

Citation: https://doi.org/10.5194/gi-2022-7-RC2
- AC2: 'Reply on RC2', Sylvain Ranvier, 07 Oct 2022
  
  Dear Referee,
  Thank you for your comments that will help making the paper clearer.
  
  Comment 1:
  The PICASSO mission ended in June 2022 because of issues with the platform. Although a limited amount of time has been allocated to the payload commissioning, it has been possible to validate all the measurement and diagnostic modes of SLP.
  Comment 2:
  
  The Orbital-Motion-Limited (OML) collection theory can be applied if, inter alia, the cylindrical probe radius is small compared to the Debye length and the length of the probe is much longer than the Debye length (in practice more than 10 times the Debye length). If all assumptions for the OML theory are met, the equations (1) and (2) can be used to determine the electron density and temperature from the I-V characteristics. In principle, the electron retardation region obeys the OML theory, irrespective of the probe geometry, hence it can always be retrieved, if one can properly isolate the electron current contribution to the I-V curve.
  Comment 3:
  
  Figure 1 has been replaced by a figure where the scale is kept constant throughout the bias range (no current multiplication).
  Comment 4:
  
  The text has been updated for coherence, it is PICASSO everywhere now.
  Comment 5:
  
  Surface properties:
  
  Cube: AL2K (Aluminium according to NASCAP-2k). Probe: AL2K (Aluminium according to NASCAP-2k)
  
  No photo emission
  Absolute spacecraft capacitance: fixed at 200 pF
  
  plasma density: 2e10 /m³
  
  electron and ion temperature: 0.05 eV
  
  flowing plasma (V= 7.5 km/s)
  
  Ions treated as PIC
  
  electrons treated as a Maxwell Boltzmann equilibrium distribution
  
  Citation: https://doi.org/10.5194/gi-2022-7-AC2

Status: closed

RC1:
'Comment on gi-2022-7', Laila Andersson, 27 May 2022

Dear Sylvian Ranvier and Jean-Pierre Leberton

Great paper and I have some suggestions for improvement.

On page 1 lines 12-14 and page 15 lines 288-290: From the IV-curve of a LP-instrument the electron density and temperature is a derived quantity with some uncertainty. The paper present a selected sweep methodology to optimize the telemetry volume over downlinking a full IV-sweep. In the discussion about the approach the manuscript do quantify the impact of having a limited number of data points in the IV-curve on the resulting uncertainty. Can change in the uncertainty of electron density and temperature be quantified?

On page 3 the methodology of data sampling is discussed. In the paper the notation 'inflection point' is stated but that notation is not clearly explain. This important because the MAVEN LPW instrument used a similar approach and it would be good to highlight the similarities/differences. On MAVEN at low altitudes (sweep potential +-5 V) we use a look-up table for the sweep steps (large step sized on the start and stop and small step sizes in the center similar to the PICASSO approach). This sweep is then centered on the location where the previous measured current changed sign plus an offset. The offset allows any current offset to be taken into consideration and center the smallest voltage step sizes in the region where the electron temperature is extracted, i.e. the smallest steps is not necessarily where the measured current change sign. The centering of the look-up table is only allowed to slowly change from sweep to sweep (a time constant is set in the instrument of how 'slow' this change is selected to be). In the paper it is not clear if the PICASSO approach take into account any instrument current offsets or how fast the instrument respond to large changes from one sweep to another. This last topic is an important point when looking at Figure 13.

On page 3 the stepping speed at up to 10 kHz is discussed. There has been a resent paper analyzing the sweep speed. They analyzed 10 kHz (and higher), you might want to reflect over their numerical result with your observational result.

M. Kjølerbakken, W. J. Miloch, Ø. G. Martinsen, O. Pabst and K. Røed, "Numerical Study of Non-Linear Effects for a Swept Bias Langmuir Probe," in IEEE Transactions on Plasma Science, vol. 50, no. 5, pp. 1237-1245, May 2022, doi: 10.1109/TPS.2022.3164220.

On page 5 discussing the temperature of the probe. Is the 200 oC and measured temperature or a simulated thermal estimate? I assume the temperature is for the metal rod, not the electronics such as the preamps.

Page 8. Very nice simulations capturing the problem with high density region and small SC. I might have missed it was the simulation in flowing plasma? For MAVEN LPW we did also simulations of the local plasma, I would recommend you to read that paper and comment on similarity/differences. In that paper the potential influence from the SC was not the main issue but the wake due to the probe itself.

Ergun, R. E., Andersson, L. A., Fowler, C. M., & Thaller, S. A. (2021). Kinetic modeling of Langmuir probes in space and application to the MAVEN Langmuir probe and waves instrument. Journal of Geophysical Research: Space Physics, 126, e2020JA028956. https://doi.org/10.1029/2020JA028956

Page 13 discussing the result of Figure 14 (and Figure 15). Could the authors add a few sentences how this would change for space instrument. In space the instrument often is just continuously on so this increase might not be ever observed. The paragraphs fell a slightly short with taking the result from the laboratory and spell out how it would be observed in space.

Page 12-14 about the contamination: Gold is fairly robust material and is unlikely to be effected by atomic oxygen. But for the readers understanding, it could be good to point out that atomic oxygen could change the behavior in similar way as the contamination issues. Joseph Samaniego did some laboratory experiments and that effort could be help the discussion in this paper. This is one of the papers:

Samaniego, J. I., Wang, X., Andersson, L., Malaspina, D., Ergun, R. E., & Horányi, M. (2018). Investigation of coatings for Langmuir probes in an oxygen-rich space environment. , 123, 6054– 6064. https://doi.org/10.1029/2018JA025563

Great work,

Laila Andersson

Citation: https://doi.org/10.5194/gi-2022-7-RC1
- AC1: 'Reply on RC1', Sylvain Ranvier, 07 Oct 2022
  
  Dear Laila,
  Thank you for the very interesting comments and suggestions that will definitely improve the quality of the paper.
  Comment 1:
  
  We propose to add the following text:
  
  “The distortion of the I-V characteristics due to the adaptive sweep method depends on several parameters including the number of data points, the difference of the step sizes of the three regions and the delay between the data points. The effect of the latter parameter is clearly visible by comparing Figure 9 and Figure 12, which are sweeps performed with 300 µs and 1.2 ms delay between steps, respectively. Unfortunately, not enough measurements have been performed in the laboratory plasma chamber to statistically quantify the change in the uncertainty of electron density and temperature as a function of those parameters. This assessment will be performed in a follow-up study.
  
  Comment 2:
  We propose to add the following text:
  
  "On board, the inflection point of the I-V curve (i.e. the bias for which the first derivative of the curve is maximal) which separates the electron retardation and the electron saturation regions (the plasma potential) is determined by computing the first derivative of the curve and selecting the highest value (the maximum). This is performed after each sweep and used to compute the span of the fine step region of the next sweep, as follows: the first point of this region is set by a parameter of the command sent to the instrument, but the last point of this region is defined by adding a number (another parameter of the command) of data points after the computed inflection point, as depicted in Figure XY. The step size of the electron saturation region is then also adjusted automatically. The use of additional data points ensures the fine sampling of the full retardation region, even if the inflection point is computed with some inaccuracy (e.g. due to noise). There is no limitation on how fast the inflection point can be determined accurately when the electron temperature decreases (inflection point shifting toward the retardation region). When the temperature is increasing (inflection point shifted toward the electron saturation region), the number of iterations needed to accurately define the inflection point is determined by the number of additional data points. Increasing the number of those additional data points will decrease the number of iterations needed to accurately determine the inflection point, but, at the same time, it will increase the data volume. Therefore there is a trade-off between data reduction efficiency and time response of the system. A running average (with window size a parameter of the command) is used to improve the robustness of the algorithm against the noise. Since the determination of the inflection point is done by computing the first derivative, the instrument current offsets (if any) will not impact its accuracy. Given that the algorithm defining the bias steps is fully deterministic, only one number (the on-board computed inflection point, which is sent together with the measured current) is needed to rebuild the sweep steps on ground and to compute the complete I-V curve. To get an order of magnitude, when the fine step region is sampled with 30 steps, the step size ranges from about 10 mV to 150 mV for electron temperatures of 600 K and 10.000 K, respectively. This method is different from the one used for the LPW instrument on MAVEN (Andersson et al., 2015) where a look-up table for the sweep steps (large step sizes in electron and ion saturation regions and small step sizes in the retardation region) is used and the sweep is centered on the location where the previous measured current changed sign plus an offset. The advantage of the approach used for the LPW as compared to the one used for SLP is that the beginning of the fine step region is adjusted automatically, whereas it is fixed for SLP. On the other hand, the approach used for SLP allows sampling the retardation region with a given number of points, leading to a given relative resolution, which is valid for a very broad range of electron temperature, i.e. for retardation regions with very different width. "
  
  The number of additional data points used during the measurements presented in Fig 9 et Fig 12 will be added in the respective sections.
  
  Figure XY mentioned in this new part is attached as supplement.
  
  As suggested, we will add the following reference: Andersson, L., et al., The Langmuir Probe and Waves (LPW) Instrument for MAVEN, Space Sci. Rev. (2015), 195:173–198, DOI 10.1007/s11214-015-0194-3
  
  Comment 3:
  
  Although the paper describes very well the non-linear effects linked to the fast sweeping of Langmuir probes, those effects are studied for sweep duration from 1.5 µs up to 0.2 ms, which is at least 100 times faster than the sweeps performed by SLP (minimum sweep duration about 20 ms). The effects studied by those authors are not applicable to SLP/PICASSO.
  
  Comment 4:
  
  200 °C is a simulated temperature estimate for the metal rod itself, which is much higher than the temperature of the electronics font end (between 15°C and 45 °C along the orbit, measured).
  Comment 5:
  
  The Simulation is in flowing plasma (Vram = 7.5 km/s, perpendicular to the probe).
  
  The results presented in this paper qualitatively agree with those reported in (Ergun et al., 2021) and (Marholm et al., 2020), where it is shown that in the ionosphere, for cylindrical probes with finite length, the Beta exponent lies between the one of a perfect cylinder (0.5) and the one of a perfect sphere (1).
  
  The fact that the main issue is the influence of the spacecraft potential rather than the wake due to the probe itself comes from the fact that the probe to S/C current collection area ratio is much larger for SLP on PICASSO than for LPW on MAVEN, which leads to more severe S/C charging effects for SLP. In addition, given the short distance between the S/C body and the sensor (a few Debye lengths) and the fact that there is no guard, the S/C potential effect dominates the wake effect on the I-V characteristics for SLP.
  
  Comment 6:
  
  For a space instrument, one possibility would be to periodically stop sweeping, wait for a given time (to let the probe-spacecraft system discharge), restart the sweep sequence and use the first sweep as a reference. If the plasma environment can be assumed to be stable for a few tens of seconds, then the effects of contamination on the I-V curves can be accounted for in the calibration. Measuring with multiple probes would allow acquiring continuous measurements data even if sweeps are stopped and restarted regularly (if this is done alternatively for both - or more – probes).
  
  Onother possibility is to perform increasing and decreasing sweeps with different duration as in (Lebreton, et al., 2006) and to model the contamination layer as an R-C parallel circuit as in (Fang et al., 2018 and Piel et al., 2001). If properly modeled and combined with a database of PIC simulations of contaminated probe surfaces for different R, C and plasma parameters values, this could be used to retrieve the uncontaminated I-V curve. The study of the effect of the contamination, on a single sweep and on consecutive sweeps, is an on-going work that will be subject of a follow-up publication. Hence in this article, we acknowledge that there are uncertainties in the determination of the plasma parameters, but their quantification is out of scope of this paper.
  
  Comment 7:
  
  We propose to add the following text:
  
  Similar effects (i.e. reduced current at a given probe potential, the derived plasma potential becomes more positive, the derived electron density decreases and the derived electron temperature becomes hotter) have been reported in (Samaniego, J. I. et al., 2018) for several probe coatings (including Gold and DAG) both when the probes were exposed to oxygen-rich environments and without exposition to oxygen-rich environment. In this latter case, the sources of contamination were assumed to be the deposition of vacuum pump oil and moisture from the air, similarly to the contamination of the SLP probes reported in this Section. Then, although Gold should be insensitive to atomic oxygen, it was found that the O+ and O2+ bombardment induced a moderate but noticeable oxidation effect on the probe measurements, in addition to the effects of the contamination alone.
  
  As suggested, the following reference will be added:
  
  Samaniego, J. I., Wang, X., Andersson, L., Malaspina, D., Ergun, R. E., & Horányi, M. (2018). Investigation of coatings for Langmuir probes in an oxygen-rich space environment. Journal of Geophysical Research: Space Physics, 123, 6054– 6064. https://doi.org/10.1029/2018JA025563
  
  Citation: https://doi.org/10.5194/gi-2022-7-AC1
RC2:
'Comment on gi-2022-7', Anonymous Referee #2, 19 Jul 2022

Review Laboratory Measurements of the Performances of the Sweeping

Langmuir Probe Instrument Aboard the PICASSO CubeSat

This article reports on laboratory measurements of a LP sensor aboard a cubsat.

Such measurements are important and necessary to fully exploit the data analysis of this

instrument.

The article is worth publishing, after minor comments to be addressed by the authors:

(1) it would be useful to mention the status of the PICASSO satellite to put the article into context.

The article only mentions that this is an in-orbit demonstrator launched in September 2020.

(2) Is it possible to briefly explain what

"the traditional Orbital-Motion-Limited (OML) collection theory" is for? (page 2)

(3) Figure 1 caption: clarify in which V range exactly the current is multiplied by 3.

(4) In the text sometimes it's PI-CASSO or PICASSO. Update for coherence.

(5) Page 7: SPIS (Spacecraft Plasma Interaction System) simulations were performed. it would be useful to mention here

all assumptions for these simulations (surface properties etc..).

Citation: https://doi.org/10.5194/gi-2022-7-RC2
- AC2: 'Reply on RC2', Sylvain Ranvier, 07 Oct 2022
  
  Dear Referee,
  Thank you for your comments that will help making the paper clearer.
  
  Comment 1:
  The PICASSO mission ended in June 2022 because of issues with the platform. Although a limited amount of time has been allocated to the payload commissioning, it has been possible to validate all the measurement and diagnostic modes of SLP.
  Comment 2:
  
  The Orbital-Motion-Limited (OML) collection theory can be applied if, inter alia, the cylindrical probe radius is small compared to the Debye length and the length of the probe is much longer than the Debye length (in practice more than 10 times the Debye length). If all assumptions for the OML theory are met, the equations (1) and (2) can be used to determine the electron density and temperature from the I-V characteristics. In principle, the electron retardation region obeys the OML theory, irrespective of the probe geometry, hence it can always be retrieved, if one can properly isolate the electron current contribution to the I-V curve.
  Comment 3:
  
  Figure 1 has been replaced by a figure where the scale is kept constant throughout the bias range (no current multiplication).
  Comment 4:
  
  The text has been updated for coherence, it is PICASSO everywhere now.
  Comment 5:
  
  Surface properties:
  
  Cube: AL2K (Aluminium according to NASCAP-2k). Probe: AL2K (Aluminium according to NASCAP-2k)
  
  No photo emission
  Absolute spacecraft capacitance: fixed at 200 pF
  
  plasma density: 2e10 /m³
  
  electron and ion temperature: 0.05 eV
  
  flowing plasma (V= 7.5 km/s)
  
  Ions treated as PIC
  
  electrons treated as a Maxwell Boltzmann equilibrium distribution
  
  Citation: https://doi.org/10.5194/gi-2022-7-AC2

Sylvain Ranvier and Jean-Pierre Lebreton

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Month	HTML	PDF	XML	Total
May 2022	137	21	9	167
Jun 2022	105	11	3	119
Jul 2022	104	11	2	117
Aug 2022	70	3	0	73
Sep 2022	61	6	0	67
Oct 2022	79	14	4	97
Nov 2022	32	8	1	41
Dec 2022	7	1	0	8

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Short summary

The Sweeping Langmuir Probe on board the PICASSO CubeSat was designed to measure the plasma parameters. Before launch, the instrument has been tested in a plasma chamber. It is shown that the traditional method to interpret the data cannot be applied directly for this type of probes, and an adaptation is proposed. It is reported how, with a reduced number of data points, the plasma parameters can still be retrieved. Finally, the effect of the contamination of the probe surface are investigated.


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