Heavy Ion Beam Probing Diagnostics on the TUMAN-3M Tokamak for Study Plasma Potential and Electric Fields in New Operational Regimes

Round 1

Reviewer 1 Report

The English is poor, spelling errors and grammar errors exist all over the paper. Must be properly corrected.

The paper title is about improving the HIBP for performing plasma potential measurements during Heating Beam co-injection with plasma current. The modifications on the source power supply and on the acceptance angles of the energy analyzers are noted but are not a scientific breakthrough. The paper should be supported with more examples of measurements and more complete plasma data in order to infer on the effects of the H-L transition.

The paper does not add much to the already published work: DOI:10.1063/1.1787954

The author must clearly prove the substance of novelty of the present paper in regard to the published work.

In addition the following comments must be addressed:

(...)reaching values of 500 - 1000V due to the capacitive coupling between the chamber and the toroidal field coils.

Question 1: - Can the author explain what it means by capacitive coupling? 500-1000V is much higher than plasma potential, the measurements shall be totally masked by these floating voltages that may offset the plasma potential in an undetermined way. How does the author account for this?

(...)The change in the beam width with its propagation along the trajectory in a given magnetic field of the tokamak is determined by two factors: focusing with an electrostatic lens in the injector and beam deformation in the inhomogeneous magnetic field.

Question 2 - Equally important is beam scattering due to collisions with plasma ions. Has the author estimate this effect on beam profile when reaching the detector? This may affect the plasma potential measurements accuracy

Author Response

Author Response File: Author Response.docx

Reviewer 2 Report

The article reports on the development of the Heavy Ion Beam Probing (HIBP) diagnostics on the TUMAN-3M tokamak, namely, on changing of the direction of neutral beam injection (NBI) from counter-NBI to co-NBI and the corresponding transformation of the entire diagnostic complex, and shows the first results of measurements. Describing the principles and tools of HIBP can be very helpful to a wide audience in the controlled thermonuclear fusion community. Therefore, I recommend publishing this manuscript after the minor revisions suggested below.

 Main shortcoming

 The shortcoming of the present manuscript, which aims at a detailed description of the diagnostics in order to show the relevance of changing the direction of injection, is common to all works on this topic. It would be nice if the authors of this article contributed to clarifying this issue.

On the one hand, the authors state in lines 167-172 that there is difficulty in interpreting the measurement data: “HIBP is a direct method for the plasma potential measurements, since it does not require any a priori assumptions and models of the interaction between the plasma and the probing beam, except for the fundamental principles - the energy conservation low and the potentiality of the electrostatic field. At the same time, there are currently no experimental methods for determining the position of the potential measurement point.“ The authors describe in detail the work aimed at determining the parameters of the primary diagnostic beam, including the coordinates of the beam entry point. However, the authors do not describe the inverse problem of reconstructing the ion beam trajectory in a plasma with an unknown spatial profile of the magnetic field. Not only the position of the birth of the secondary beam is unknown, but also the spatial profile of the magnetic field, including the poloidal and toroidal magnetic fields, along the entire trajectory of the secondary beam is unknown. These spatial profiles are usually reconstructed by solving the inverse problem for force equilibrium equations, including the Grad-Shafranov equation for radial force equilibrium. However, the accuracy of this reconstruction influences the accuracy of the HIBP measurements, and, strictly speaking, both inverse problems — (i) reconstruction of the spatial profiles of the magnetic field from the experimental data for the plasma equilibrium and (ii) local measurements of the potential from the experimental data of the HIPB — should be performed simultaneously, since the spatial profiles of the plasma rotation velocity, macroscopic electric potential and magnetic field are interdependent. Therefore, the authors are invited to supplement their description of HIBP with a description of these issues and their estimates of the accuracy of their results for the potential, taking into account the accuracy of calculating the beam trajectory in an indirectly measured magnetic field.

 Minor corrections

 Line 34: “On LHD stallarator “ replace with “stellarator”

 Line 38: “has been operating for many years the HIBP”, insert “with” to make “has been operating for many years with the HIBP”

 Line 45: without proper explanation the reference [11] is not applicable here

 Line 58: “a beam of accelerated ions with energy and mass;” Probably the authors assumed ”ions with high energy and large mass”

 Lines 74-76: the second part of the sentence should be corrected as follows: “The next section briefly describes the composition of the HIBP 74 diagnostic complex at the TUMAN-3M tokamak and presents an algorithm for modeling the trajectories of primary and secondary ions”

 Line 80: replace “control system for primary beam angle of entry and position” with “control system for angle of entry and position of primary beam”; similar changes to avoid overly long terms are recommended throughout the manuscript, e.g. “Autonomous isolated thermoionic source filament heater power supply” at line 130

 Line 170: replace “low” with “law” :)

 Line 206: replace “volage” with “voltage”

Author Response

Author Response File: Author Response.docx

Reviewer 3 Report

The issue of diagnostics of  hot dense laboratory plasma (including plasmas in different magnetic confinement fusion devices) in order to study a dynamics of the electric and magnetic fields, plasma density etc (in the author’s formulation) has nowadays attracted considerable attention. The work is devoted to the application of the Heavy Ion Beam Probing (HIBP) method to study the electric field in a hot dense plasma of modern toroidal magnetic confinement devices. As the authors have indicated, on the TUMAN-3M tokamak HIBPs have been used in regimes with improved plasma confinement to elucidate the role of the radial electric field during the transition to good confinement regimes. The authors presented and discussed in detail the results of the first plasma potential measurements in the so-called co-NBI scenario based on the upgraded HIBP diagnostic technology.  The title is quite adequate and appropriate. The introduction is a very useful tour d'horizon to works on diagnostics of hot dense plasma in different magnetic confinement fusion devices using the HIBP method.  The abstract and conclusions contain the essential information of the article; it is written clearly. The parts describing the HIBP diagnostics at the TUMAN-3M tokamak as well as the measurement details are clearly and correctly written.

The paper is very interesting, original and novel and adds significantly to results that are already published. My understanding is that the paper by Leonid Askinazi et al is aimed for the Journal “Atoms” (MDPI), so its style is adequate to the purpose, and the length of a paper is justified by its contents.

The only minor editorial remark is related to the possible expansion of the list of references in order to take the possible questions of readers into account. It would be useful for readers to add a few references to books and reviews that describe the general aspects of the dynamics of the potential of electric and magnetic fields in a hot dense plasma (including the hot plasma of modern toroidal magnetic confinement devices). The possible refs are added (look the enclosed pdf file)   solely at the discretion of the authors. Thanks for minor revision.

Recommendation: The scientific merit of the paper is very high. The paper should be definitely recommended for publication in the Journal “Atoms” (MDPI) provided the authors comply with the single editorial minor point listed above.

Comments for author File: Comments.pdf

Author Response

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Paper has been revised but I have a few questions

1 - The statement: "Examples of theoretical calculation of plasma current profile effect on the toroidal displacement of the secondary beam in the TUMAN-3 may be found in [19], while implementation of these approach to ISTTOK tokamak experiment is given in [10]."

The paper in reference [19] rightly describes the mathematical problem but does not provide a univocal practical solution as the one obtained in ISTTOK. The solution presented in ISTTOK is shown for the first time on a paper in 1994 and improved in 2000. The reference [10] revisits this method (its an overview paper and gives references to the above mentioned papers).  Nowhere in the paper in reference [19] is the description of retrieving of equations as presented in paper reference [10]. We suggest the authors to not induce that ISTTOK applied a solution proposed in the paper [19]. That is not the correct connection. ISTTOK developed for the first time the practical solution for the problem of removing the poloidal magnetic field integral path effects between two consecutive beams collected all along the plasma diameter. This method cannot be applied in the HIBP installations that do not use multiple cell detector due to the impossibility of "freeze" of multiple ionization points during one measurement. It is not possible in good truth to recover the magnetic field profile by scanning the plasma point by point while changes in plasma current and beam energy occur. Even if situation in the plasma was steady it would be practically impossible to regain the magnetic field profile by scanning the primary beam and measuring point by point. The measurement must be performed with a full collection of secondary ions from plasma entry until some point in the plasma, preferably covering the full diameter (or radius at least).

2 - Regarding the statement: "However, for typical plasma parame-ters of the experiments on the TUMAN-3M (Te ~ 500 eV, ne ~ 2 1019 m-3, a = 0.25 m), using tabulated cross-sections for K+ and K2+ ions [22], an estimation for the two exponential factors in (2) gives 0.994 and 0.996, that is close enough to 1."

I am not sure what the authors mean. Firstly, we cannot use directly the cross sections, we need to account for Maxwellian distribution of electrons (as the authors recognize). Secondly, the estimations on primary beam attenuation are clearly overstated to my reading. It means that the primary beam is attenuated only a fraction of 0.006 of its initial current. It would be rather impossible to measure in a practical environment of an HIBP installation the secondary current emerging from a 1 cm sample volume of: (Ip x 1x10^-2 )/ (250) * 0.006 = Ip x 2.4 x 10^-7 A. The given figures need some clarification.

3 - The authors must include in the text the answer they gave regarding the chamber floating voltage. The fact that the HIBP gun is floating regard to the tokamak vessel moves the reference point of plasma potential measurements to the ground of the HIBP power supplies. This affects the absolute value, maybe not the Electric field as it is the gradient of V. This must be made clear. When there is the steady state situation values of the V_vessel should be indicated (is it 10 V or 150 V?).

Author Response

Our response to the reviewer's comment is in the file attached

Author Response File: Author Response.pdf