WDM (Warm Dense Matter) collaboration

The WDM-collaboration has proposed a set of recommended experiments at  FAIR combining intense heavy ion beams with the kilojoule PHELIX laser facility.

Warm Dense Matter (WDM) refers to high-density finite-temperature regime where free and bound electrons become strongly correlated: the system exhibits long- and short-range order. This intriguing regime takes two broadly distinct manifestations: it occurs when condensed matter reaches temperatures near and above the Fermi temperature and it occurs as plasmas are cooled and/or the density is increased. For example, the plasma can no longer be considered a thermal bath and the behavior of its particles is no longer well described by characteristics of isolated ions. This regime is of importance and an extremely challenging area for scientific inquiry.

The relevance of WDM research arises from a wide occurrence of this state of the matter: it is a subject of investigation in planetary science, cold star physics and all plasma-production devices where the plasma generation starts from cold dense matter (e.g., laser solid matter interaction, heavy ion beam driven plasmas, capillaries, exploding wires, pinch plasmas).

The current knowledge of the plasma behavior is particularly limited in those regions where standard perturbative theoretical approaches are applicable. However, when these methods fail new techniques are needed and this translates directly into the realm of novel experimental science. Indeed, the theoretical difficulties arise from, for example, the importance of density effects (e.g., pressure ionization), as the electrons and ions begin to substantially influence each other. That is, the internal structure of ions becomes a function of the plasma density and temperature while the usually "free" plasma particle exhibit ordering. Experimental problems follow as one must isolate this inherently transient state. This, in brief, is why so little is known about WDM.

In previous laser experiments WDM states have been the subject of investigation but the result have been difficult to obtain for numerous reasons: in-homogeneities, fast time scales (fs) and small spatial dimensions. On the other hand, WDM produced by intense heavy ion beams will have several advantages: large (mm) sample size, long lifetimes (ns), and homogeneity. Typical WDM-temperatures are in the eV range, with temperatures up to the 10 eV range envisaged in future experiments at FAIR for solid density targets. Due to the relatively low temperatures X-ray emission from these dense warm samples is not a reasonable diagnostic. However, the use of the PHELIXkilojoule laser facility that is collocated with the intense heavy ion beams will provide an independent x-ray scattering diagnostic (see Figure 1) yielding on the temperature, density, charge state and ion-ion correlation effects in the WDM. Moreover, inner-shell ionization driven by fast ions might induce Kα/Kβ x-ray emission that will have diagnostic potential.

A particular experimental arrangement for the Warm Dense Matter research is based on a dynamic confinement scheme shown in Figure 2 for solid hydrogen. In the traditional static confinement approach the achievement of homogeneity depends on the fact that the massive outer shell (e.g., Pb) will not move over the observation time due to its inertia. A serious disadvantage, however, is that photo-absorption in the outer confining shell is so large that neither X-ray scattered photons (being produced with the PHELIX kJ-pulse) nor inner-shell transitions can pass the dense shell. Simulations for the dynamicconfinement scheme have shown, that a thin plastic tamper allows almost static parameter regimes, see Figure 3, while permitting x-rays (e.g., Kα radiation of mid-Z laser targets at 4 - 10 keV) to pass through. At an ion beam intensity of 8·1010 uranium ions a temperature exceeding 0.5 eV at solid density is expected for the described hydrogen target.

The world-wide unique combination of intense heavy ion beams with the kilojoule class PHELIX laser-driven x-ray scattering diagnostic will, therefore, allow scientists to perform benchmark experiments in the WDM and dense strongly coupled plasma regime (DSCP) and greatly advance the understanding in this field of research.

The Warm Dense Matter project, denoted the WDM project, proposed in 2004 has successfully passed the scientific evaluation by an international expert committee. In 2005 WDM has then been selected to be one of the recommended experiments in FAIR. In 2006 the WDM-technical design report has been approved.

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Figure 1: Scheme of the implementation of the X-ray scattering diagnostic of heavy ion beam generated Warm Dense Matter targets

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Figure 2: Dynamic confinement scheme accessible by x-ray scattering diagnostic

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Figure 3: Evolution of the mean hydrogen density during the ion beam heating

(c) 2017 FAIR
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