Answers to Reviewer #1 (Reviewer’s text in bold)

Although the paper describes certainly an interesting idea from the basic physics point of view, selling this as a potential primary energy source is too early and at this stage scientifically not justified.

We cannot find in the paper a statement that our idea is a “potential primary energy source … at this stage.” On the contrary, the Conclusions affirm that “The program is only an intermediate stage” and this is our general approach, also considering the limited budget available for this project. In fact, the title “Progress of the Plasma Centerpost” explicitly limits results to centerpost while works are still in progress.

The paper introduces the relevance of plasma studies independently of fusion energy and cites other plasma applications we are investigating as plasma torches, material treatments (including limiters and divertors), astrophysical phenomena,space propulsion. In fact, “The success of PROTO-SPHERA could provide relevant contributions in many plasma applications,” but they are “Potential Developments” (title of Section 2).

The sentence “Nuclear fusion could ensure a large-scale, safe, environmentally-friendly and virtually inexhaustible source of energy,” clearly refers to nuclear fusion in general. Even though the practical feasibility of fusion energy can be questionable, this is accepted by most of researchers.

It is true that it is written “[Compact] devices may develop an alternative and faster path to fusion energy,” but it is a hypothesis (“may”) reported from the cited references. Maybe “faster” (namely before 2050 of DEMO roadmap) is too optimistic, but based on several scientific experiments (MAST, NSTX and so on). Anyway, these experiments could support and accelerate the ITER/DEMO path.

Finally, if the impression of a ready primary energy source was induced by the journal title (“Energies”), please consider that the paper was submitted to this journal as invited for the “Special Issue for Selected Papers from 15 IEEE International Conference on Environment and Electrical Engineering (EEEIC 2015).” On the other hand, if this journal considered only ready primary energy source,nuclear fusion should be ignored (whereas there are special issues about it).

Starting with the introduction, the authors refer to a number of fusion experiments and concepts, not discriminating between those which have a sound scientific basis and those which lack any scientific confirmation of their claims.

This is actually an introduction of the general scenario for readers not expert in fusion in a not fusion-oriented journal. The scope is to emphasize the renewed interest in nuclear fusion, even though some approaches are less rigorous than others.

In general, the reader’s attention is drawn to this. For example: “unfortunately they are not yet economically viable;” the word “betting” means no scientific evidence; “outside the scientific community;” “the lists consider only the most rigorous approaches;” “prediction is not based on scientific considerations.”

In order to be more explicit on the last statement, we add your sentence “discriminating between those which have a sound scientific basis and those which lack any scientific confirmation of their claims”.

An example is the reference to the Lockheed Martin website (reference 18) which is used twice. In an unprecedented advertisement campaign, Lockheed Martin claimed to have found a new compact plasma confinement concept which could be developed to a fusion power system within a few years. Looking at the Lockheed Martin proposal it becomes clear that they simply combine old confinement concepts (magnetic cusp and magnetic mirror) which have been given up already decades ago because of their significant drawbacks. In addition, Lockheed Martin has not addressed even the most basic questions associated with deuterium-tritium fusion such as the shielding of the coils from high energy neutrons.I am afraid that the paper tries to suggest in a similar way to have found a new confinement concept (“… may be a milestone of such a path.”) without even having proven the most basic requirements of high temperature plasma confinement.

We are aware about Lockheed Martin. We tried to contact them in several ways to share our knowledges and experiences, without answers.

The paper does not suggest that Lockheed Martin’s approach works (in producing energy). The paper only reports that Lockheed Martin’s approach exists. It is a confirmation of renewed interested in fusion.

As explained before, the path is that of compact tokamaks. Lockheed Martin’s proposal is not a milestone of such path (certainly not at this stage and without sharing information). Its partial similarity to PROTO-SPHERA is a fact that could be interesting in a (extensive) paper about PROTO-SPHERA.

However, since this reference could be misunderstood, the entire sentence was removed from the Introduction. This was moved in Section 2 writing explicitly that it is just a curiosity.

Another example is the reference to cold fusion (reference 19). The corresponding IEEE news (not a scientific publication) reports about renewed interest in cold fusion. The only catch is that this was 2004, more than 10 years ago, and since then no break-through was reported. This kind of referencing is not what I expect from a sound scientific paper.

Again: this is a “bet” and only a confirmationof renewed interest.

We recognize that in this case the citation is old and is not possible to provide scientific references (even though there is an increasing interest by investors also in cold fusion). Therefore, this reference was removed from the paper.

In the second chapter the authors continue in this direction by suggesting that larger tokamaks weren’t built because of the problems with disruptions and instead mainly spherical tokamaks were taken into operation. While it is indeed correct that disruptions are an issue when going to large size tokamaks such as ITER or a demonstration power plant, the presented statement is not correct. Since 1994 several (superconducting) tokamaks started operation, EAST in China and KSTAR in Korea, and the superconducting tokamak JT60-SA which is the size of JET is under construction in Japan will start operation in 2019.

You confirm the information that only alternative configurations were built ex-novo in Europe and USA. The words “in Europe and USA” makes evident that they were built elsewhere. In fact, JT60-SA is cited several times in the paper, including the information that it will be completed in 2019 (also with a fundamental contribution by ENEA).

The reason is not well defined. Probably it is mainly economic, but “maybe also for this reason.”

The elaboration of the dependence of the fusion power amplification (Q) on the aspect ratio is confusing and partly trivial. The real interesting question, how Q depends on the aspect ratio is not really discussed.

You are right: this part was weak. In fact, this is a really interesting question (and “an unresolved problem”) but it is beyond the scope of the paper. The formula (5) was deleted. Some references dealing with this question were kept, together with the information that it is an unresolved problem.

Another point of criticism is that the central question concerning the stability of the proposed configuration is not properly addressed. The authors refer to the achieved beta values of spherical tokamaks. However, those devices have a much higher externally applied toroidal magnetic field, operating at a much higher safety factor. Therefore, the stability limit of a spherical tokamak is not necessarily a good reference for the proto-sphera device. Later in the chapter references to the ideal stability of the proto-sphera configuration are found, but they are up to now purely theoretical considerations where the paper lacks a clear explanation of the assumptions made. Typically, resistive instabilities produce limits which lie below the ones of the ideal instabilities. And plasma resistivity must play a role for many reasons, one being that the formation of the configuration by magnetic reconnection is impossible without finite resistivity. Besides, so far no experimental evidence exists demonstrating the stability properties. The experiments presented in this paper are far from even producing the anticipated configuration (as explained in figure 1).

This discussion is out of the scope of the paper but many references covering these concerns are provided in the bibliography [].

The papers published about the ideal MHD stability of PROTO-SPHERA were meant only to assess the toroidal plasma current that can be produced in the experiment with 60 kA of centerpost current, in order to design the compression poloidal field coils and the power supplies. The resistive MHD stability and the confinement remain unassessed and their experimental verification is one of the main goals of the experiment. However, the creation of the spherical form around the plasma centerpost was experimentally obtained in 1993 on the TS-3 experiment at Tokyo University (this information and a proper reference have been added in the paper).

Furthermore, the inductive flux that will be obtained from the PROTO-SPHERA compression coils at the time of the spherical torus formation will guarantee 120 kA of toroidal current inside the spherical torus. The ? increase to 240 kA really depends on the efficiency of helicity injection at formation time. This is an unknown before the experiment, but it has been estimated [NF 2006] from the data of the helicity sustainment of HIT-II and from the computer simulation by Ricardo Farengo [?].

The TS-3 experiment sustained the spherical torus plasma for about 100 Alfven times but could not asses its resilience up to a resistive timescale: this is the second main goal of PROTO-SPHERA. The experiment was designed trying to obtain a spherical tokamak safety factor profile (not a spheromak one) but what will actually happen depends on the efficiency of the helicity injection from the centerpost to the spherical torus: if it is low, the spherical torus plasma will decay; if it is too high, the resistive MHD limit could be ?.

The way the references to “natural phenomena” are made (figures 2-4) is utterly unscientific. Again, one gets the impression the authors want to replace a proper scientific discussion by oversimplified analogies. While all three phenomena presented are certainly scientifically very interesting, showing them in the context of the proto-shera device only because they share a similar topology, but without a further explanation, does not mean anything.

The analogy of what is attempted in a laboratory with natural phenomena (in this case configuration of the Crab Nebula) has always been a very precious guide for scientific and technical developments. The authors believe that many of the present problems of magnetically controlled nuclear fusion are related to the fact that the attempted configurations do not have a correspondence in nature. The establishment of the spherical torus and its beta values are the aim of the experiment: if these were known facts, the proponents of PROTO-SPHERA would simply not have been interested in building the machine.

Finally, the technical part of the paper, describing the experimental setup and the results achieved, reports about the progress made with establishing a plasma centre post. On the way to a spherical torus plasma this is certainly an important step. However, so far evidence that this is actually achievable, not to mention the question which beta-values can be reached, is lacking.

This scope of the paper is well identified by the title.The results presented in this paper deal only with the centerpost plasma as the poloidal field compression coils are not yet inserted in the present phase of the experiment. In fact, the experiment was scheduled to have two phases: the second one will start only when all the goals of the centerpost plasma (8.5 kA for 1 s) will be fully obtained.

Answers to Reviewer #2 (Reviewer’s text in bold)

This is a nice overview of the status of the PROTO-SPHERA Spherical Tokamak. The plasma configuration is attractive with no magnetic coil linking plasma. I am glad to see the machine is working as designed, however the manuscript is relatively sparse in the experimental data. For the manuscript improvement, I would like to see some latest experimental data of this new experimental facility. For example,

  1. Are there any discharge traces of the plasma such as the plasma current, and other parameters?

Yes. The new Figure 18 showing the current and voltage traces was inserted in the paper.

  1. Are there any plasma diagnostics measuring plasma density and temperature, for example?

Not yet. A specific system was prepared to be inserted in the next campaign.

  1. Are there any magnetics? It would be helpful for example to see the magnetic fields produced by the plasma current.

There are 72 magnetic probes. ? only the plasma current going through the PF2 coils, measured by Rogowski coil, are displayed. Si possono vedere i magnetic field?

  1. Any comparison of experimental data with modeling projection would be also quite helpful.

You are right.

The shape of the central column obtained in the experiment is exactly the same as it was calculated in the design ? of the machine.Even the triple X-points that are present on the top and bottom of the machine have been observed.

Also this observation was inserted in the “Summary of Main Experimental Results” and the new Figure ? was inserted to show it.

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