Plasma phase transition in warm dense hydrogen

Self Cite

Warm dense hydrogen is studied in the region of fluid-fluid phase transition within the framework of the density functional theory. We report a procedure of obtaining metastable states and calculate the equation of state. Metastable states are diagnosed by pair correlation functions and values of conductivity. We obtain a strong overlapping through the density of metastable and equilibrium branches of pressure isotherms. This indicates the plasma nature of the phase transition. KEYWORDS density functional theory, equation of state, metastable states, warm dense hydrogen 1We present a method for the calculation of molecular metastable states of warm dense hydrogen within the framework of the DFT. The metastable states are diagnosed by PCFs and values of conductivity. The existence of metastable states indicates that the phase transition is a first-order phase transition. Note the following results:1 The metastable states are obtained for 700 and 1000 K isotherms. The isotherms have a peculiar sloped form with a strong overlapping of equilibrium and metastable branches and a rather small difference of specific volumes between branches. The overlapping found corresponds to the prediction [17] for the plasma phase transition. 2 The small jump of the specific volume at the phase transition prevents us from resolving metastable states at higher temperatures. 3 The phase coexistence line on the specific volume-pressure plane looks like a long and very narrow tongue. In this case, the accuracy of calculations does not permit us to locate the critical point within the interval of several thousand Kelvin. 4 The transition is overturned by a sequence of van der Waals loops, when an increase in temperature leads to a decrease in pressure, at which point the transition occurs as opposed to the conventional picture. 5 The findings from this study present arguments, in addition to, [5,8,49] in favour of the plasma nature of the fluid-fluid phase transition in warm dense hydrogen.

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Metastable states are diagnosed by pair correlation functions and values of conductivity.…”

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confidence: 60%

Section: First-order Naturementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

“…[3]. [5,8], the structural change of the fluid is supposed to be related to the ionization process with the formation of H + 2 and H + 3 ions. It is also noted in refs.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Metastable molecular fluid hydrogen at high pressures

Norman

Saitov

Sartan

2019

Self Cite

“…[40] Nevertheless, using the PBE functional allows obtaining better agreement with the experimental phase coexisting curve obtained on the DAC experimental data. [55] can be applied to handle the results obtained in refs. The results obtained in this work are qualitative rather than quantitative and concern the mechanism of the phase transition, which is derived from the analysis of the proton-proton PCFs.…”

Section: Calculation Methodsmentioning

confidence: 99%

Plasma phase transition (by the fiftieth anniversary of the prediction)

Norman

Saitov

2019

Self Cite

At first, we present a brief review of the problem. Then, we consider plasma phase transition (PPT) as a mechanism of the first order fluid-fluid phase transition in warm dense hydrogen. The pros and cons are analysed. The properties of warm dense hydrogen are investigated by ab initio methods of molecular dynamics using the density functional theory. Strong ionization during the fluid-fluid phase transition in warm dense hydrogen makes this transition close to the prediction of the PPT. Finally, we present differences in the real phase transition from the prediction 1968-1969. Structures are observed with inter-proton separations that are equal to the distances between protons in the H + 2 and H + 3 ions. The transition is not only ionization, but also structural. An analysis of the phase transition counterpart in solid hydrogen under high pressure allows us to reveal partially the character of the new structure. The ionized phase includes complex cluster ions. Van der Waals loops are of abnormal inverted form. KEYWORDSdense hydrogen, density functional theory, pair correlation function, plasma phase transition INTRODUCTION: THREE PREDICTIONS OF 1968-1970The plasma phase transition (PPT) was predicted by Norman and Starostin [1] in 1968. The strong overlapping in the density of the equilibrium branch of one phase with a metastable branch of another phase was predicted in ref.[2]. The third prediction was a triple point on the melting curve. [3,4] Nature of the transitionThe hypothesis of PPT is advanced in ref.[1] by analogy with the van der Waals equation of state, where a first-order phase transition arises as a result of the competition between the long-range weak attraction and short-range strong repulsion between neutral particles, and temperature. In plasmas, due to the polarization of spatial charge distribution, the average Coulomb interaction energy of particles is negative and, therefore, has a general character of the long-range weak effective Coulomb attraction. This interaction is introduced into the expression for the free energy of plasma in the form of Debye-Hückel correction.where Γ = (4 /3) 1/3 e 2 n e 1/3 /kT is the Coulomb non-ideality parameter, T is the temperature, e is the elementary charge, k is the Boltzmann's constant, n e is the charge number density. In the region Γ > 1, the system becomes thermodynamically unstable. To stabilize the system of charged particles, a short-range effective quantum repulsion between free electrons and ions is taken into account in ref.[1], along with the Coulomb interaction. The localization of free charges at close distances leads to an increase

Divertor design through adjoint approaches and efficient code simulation strategies

Dekeyser

Blommaert

Ghoos

et al. 2018

Divertor power exhaust is one of the main issues to be addressed on the way to reliable fusion power plants. It is generally accepted that at least partially detached operation is essential to mitigate the heat and particle loads to the divertor target plates. These highly dissipative regimes pose serious challenges to the plasma edge codes used for the design of these components, leading to exacerbated runtimes and convergence problems due to the presence of statistical noise in the code system. Moreover, the large number of design variables precludes investigating a wide range of designs and operating points. Building on a framework of code speed‐up based on the minimization of numerical errors in coupled finite‐volume Monte Carlo codes, we develop an efficient discrete adjoint technique for the automated solution of divertor design problems. We are able to compute the sensitivity information with very low variance, despite the statistical noise. By integrating the sensitivity analysis into a one‐shot optimization algorithm, robust convergence of the optimization problem is achieved at a cost independent of the number of design variables. The results are encouraging for the development of adjoint‐based optimization techniques for plasma edge codes such as SOLPS‐ITER.