21st Century Alchemy: Converting Liquid Metal Deuterium to Plasma

What do stars, lightning and northern lights have in common? All these “objects” are beautiful in their own way, sometimes the observer evokes existential thoughts and romantic feelings. However, from the point of view of physics, they have a common feature - plasma. This ionized gas, considered the fourth aggregate state of matter (in addition to solid, liquid and gaseous), is very common in the vastness of the Universe and is massively produced by people. Today we will consider a study in which scientists were able to convert liquid metal deuterium into plasma. What exactly was required for this and what are the results of this “alchemical” experiment? We will look for answers in the report of the research group. Go.

Background

First of all, it’s worth briefly reminding yourself that there is plasma and deuterium.

Plasma is an ionized gas that is not a substance in a gaseous state. Such a physical pun. The main elements of plasma are free electrons and ions. These guys are very mobile, from which the plasma conducts electric current perfectly.

This state was discovered back in 1879 by the English physicist and chemist William Crookes. He believed that the ionized gas contains the same number of ions and electrons, because the total charge of such a substance will be very small. And this is true - the positive and negative particles (in charge) inside the plasma are in complete equilibrium, that is, the charges of the particles cancel each other, as a result of which the charge of the internal plasma field is zero. Such a neutralization of the charges of each other in particles is called quasineutrality.

Plasma, as I said earlier, is the fourth state of matter, although not all scientists agree with this statement. However, it is worth noting that there are a number of differences from the “ordinary” gaseous state, which give the plasma the right to be called a separate, fourth state. Among these differences are: high electrical conductivity, many independent from each other particles (ions, electrons and neutral particles), non-Maxwellian velocity distribution, collective interaction of particles.

Astrophysicists, electronics manufacturers, and even meteorologists are familiar with plasma. Stars, the solar wind, outer space, interstellar nebulae are plasma. Lightning, the northern lights, the ionosphere and the lights of St. Elmo are plasma. The contents of fluorescent neon lamps, plasma rocket engines, monitors and televisions are also a certain type of plasma. In other words, there is not much plasma.

At the moment, there are several methods for laboratory preparation of plasma, including: heating a substance, ionization by radiation (ultraviolet, X-ray, laser, etc.), electric charge, ionization by shock waves, etc.

Most often, it is the thermal method for producing plasma that is mentioned, that is, by heating a certain substance to very high temperatures. During this process, certain changes take place in the atoms of matter - the electrons detach from their orbits, resulting in separately free electrons and separate ions.



Plasma can also be obtained by passing an electric current through a gas - the gas discharge method. In this case, gas ionization occurs, the degree of which can be changed by manipulating the current parameters. However, the resulting plasma, which is actually heated by electric current, can quickly cool down when it comes in contact with uncharged particles of the surrounding gas.


Plasma in the garage (do not repeat this experiment at home, if you do not want an extra visit from doctors and firefighters).

And now a little about deuterium, but not about simple, but about metal.

For starters, what is deuterium? This is heavy hydrogen (D or ² H), that is, a hydrogen isotope that has 1 neutron and 1 proton in the nucleus (called a deuteron).


A video about how heavy water is obtained from ordinary water - deuterium.

For the first time, deuterium was released in 1932 (1931) thanks to the American scientists Harold Urey and Ferdinand Brikvedde, who distilled 5 liters of liquid hydrogen. The result of this procedure was a 1 ml liquid.

But this is ordinary deuterium, in the study we are considering today we are talking about metallic deuterium. This substance was obtained through exposure to high pressure and high temperatures on deuterium.

In 2015, scientists conducted an experiment to "turn" the insulator into a conductor. It was deuterium that was chosen as the subject. Download link for this study report.

And only after a few years, metallic deuterium became the object of a new study in which scientists decided to turn it into plasma.

Research results

During the study, spherical deuterated carbon shells filled with liquid deuterium were used, which were exposed to several laser pulses (100 ps, ​​picoseconds). This procedure made it possible to obtain a spherically converging shock wave in the liquid deuterium itself (ρ ₀ = 0.172 g / cm ³ ). The laser pulse launched a pulse drive, which initially produced a strong (up to ~ 5.5 Mbar), but not a uniform impact, decreasing in pressure and velocity of the impact during propagation.


Image 1: VISAR * (a complex of a speed interferometer for any reflector) and an optical pyrometer *

were used to measure the profiles of impact velocity and self-emission of impulse impacts inside liquid deuterium.

VISAR * is a time-resolution velocity measuring system that uses laser interferometry to measure the surface speed of solids moving at high speed.

Pyrometer * - non-contact temperature measurement device tel.

Figure 1A shows the results of VISAR: the vertical axis is the impact divided by time (horizontal axis). From this observation it follows that the decay rate is quite low compared to the equilibration time.

Optical analysis ( 1C ) was carried out directly above the shock barrier at a depth of 30-40 nm. These figures were not taken from the ceiling - this is deep enough to observe the balanced state of the plasma, and shallow enough to continuously monitor the changing state of the impact during its attenuation ( 1B ).

Scientists also analyzed the absolute reflection coefficient ( R), isolated from the VISAR laser intensity indices reflected from deuterium during impact ( 1E ). Temperature data were obtained by measuring the spectral radiation of the shock barrier ( 1D ).

During the tests, scientists observed shock attenuation from 60 km / h to 35 km / h, which is equivalent to a pressure range of ~ 5.5 ... ~ 0.5 Mbar. Within this range, the density is practically unchanged (ρ = 0.774, TF = 13.8 eV), however, temperature changes from 3 to 11 eV (1 eV = 11,603 K) are observed. Given the optical properties of deuterium, compressed to 0.774 g / cm ³ , that is, reflection indicators, scientists were able to check its electronic properties.

At low pressure, strong coupling and degeneracy are observed in the sample (Г ≫ 1, ϴ ≪ 1). But with increasing temperature, it is these characteristics that change in the first place. Scientists distinguish two states when these parameters change. In the former, at 0.15 <ϴ <0.4 and 2.6 <T <6, a constant optical reflection of about 40% is observed.


Image 2: Ratio of reflection coefficient and adhesion.

This value is described by the minimum of metal conductivity according to the Mott – Ioffe – Regel rule, when the time for electron – ion relaxation depends on the interatomic distance ( a ) and the Fermi velocity (v _F ): τ _min = a / v _F. The Mott – Ioffe – Regel rule predicts that with full ionization, the minimum coefficient of optical reflection should be 0.38 for light emission at 532 nm. Similar theoretical conclusions are perfectly compared with practical experimental results.

The second state occurs when the value of ϴ exceeds 0.4 (T ~ 5 eV). In this case, the reflection increases to ~ 0.7 at T ~ 11 eV (image No. 2 ). At this moment, the cohesive force decreases when the value of Г reaches 1. At a temperature of 5 eV, full deuterium ionization was expected due to the theoretical dependence of the reflection coefficient and scattering time.

The scientists then decided to test the effect of scattering time (τ) on the observed reflectance. For this, the value of τ was determined for the data recorded using the Fresnel formula and the model of free electrons.


Image No. 3

Thanks to the data obtained ( 3B ), scientists have established that, until T / TF ~ 0.4, a Fermi surface will exist in a metallic liquid. But above this temperature indicator, an increase in the estimated relaxation time implies the absence of a limitation in the permissible speed, and to achieve an increase in the reflection coefficient, a longer relaxation time, that is, higher thermal speeds, is necessary. Therefore, taking into account the relaxation time in the studied area, the scientists found that τ ~ T1.55 ± 0.04.

These figures are very close to the classical nondegenerate limit of an ideal plasma (τ ~ T1.5).

Figure 3A shows the results of comparing the experimentally derived value of electrical conductivity with the values ​​predicted by the two transport models in a dense plasma. These models are reduced to two opposite restrictions: degenerate Ziman and non-degenerate Spitzer. However, they do not indicate the exact position of the crossover * .

Crossover * - a change in the critical indices of a thermodynamic system when external parameters change, during which there are no changes in the system symmetry or jumps in thermodynamic parameters.

This crossover plays a significant role in the thermodynamic and electronic properties of dense conductive liquids. Scientists give the following example: mark system μ chemical potential (T) changes from positive to Fermi-Dirac limit to negative Maxwellian plasma and the heat capacity C _υ moves from C _υ α T / T f in the degenerate limit to C _υ ~ 3 R .


Image No. 4

Finally, scientists compare their creation with similar experiments, but not with deuterium, but with diluted liquid ³He (helium-3) or with ultracold alkaline gases. In these systems, a similar crossover of the temperature dependences of the dynamic properties of the atomic fermionic system already refers to quantum statistics (image above). Despite the difference of 8-12 times the temperature and density indicators, the degeneracy rules in Fermi systems remain common for all systems.

The results of a practical experiment are in excellent agreement with the data of calculations using Monte Carlo methods for dense hydrogen plasma. These calculations showed a significant rearrangement / exchange of electrons in the plasma at T <0.4 TF for different densities. Raising the temperature above this indicator significantly reduces the probability of quantum exchange between two or more electrons. Since the permutation / exchange of electrons is necessary for the formation of the Fermi surface, with increasing temperature, the electrons no longer degenerate, and the Fermi sphere collapses.

For a more detailed acquaintance with the details of the study, I strongly recommend that you look into the report of the research group .

Epilogue

Scientists are extremely pleased with their work. Which is not surprising, considering where their work can be very useful. First, the prediction of the degeneracy criteria in compact astrophysical bodies, which will allow us to determine the boundary between the atmosphere and the degenerate core. Secondly, in the objects of thermonuclear fusion, which will accurately determine the desired temperature range in which the nuclear fuel should be during implosion (an explosion directed inward). In addition, scientists believe that their work will help in the study of quantum phenomena in warm dense matter.

The potential is really great, as well as the number of questions that scientists have yet to answer during further studies of both plasma and such an unusual substance - liquid metal deuterium.



Thanks for watching, stay curious and have a great weekend everyone, guys.

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