Conference Chair Welcome Letter The 4th High Energy Density Science (HEDS'15) Ryosuke Kodama Chair, HEDS 2015 Organizing Committee, Director Professor, the Photon Pioneers Center at Osaka University High energy density states created by high power lasers include a variety of states of matter such as warm dense matter (WDM), high pressured plasmas, radiative plasmas, fusion burning plasmas, relativistic plasmas and electro-positron plasmas. These extreme states of matter enable us a lot of applications such as particle acceleration, laboratory astrophysics, material science, nuclear science including medical applications and laser fusion, which are “High Energy Density Science (HEDS). These applications are well in progress with technologies of high power lasers including XFEL and related plasma devices. The applications of these extreme states are expanding as HEDS. The HEDS in the OPIC focuses on two main fields in the high energy density science. One is a field related to high-pressure physics, diagnostics and related fields such as geophysics and laser processing. The other is laser acceleration, plasma photonics and their applications such as high field physics and x-ray sources with a laser-accelerated electron beam. The HEDS15 will focus on the plasma photonics and laser plasma acceleration such as following topical fields; 1) Plasma photonics: Ultra-strong fields, Wake filed plasma, THz radiation, X-rays, Vacuum polarization; 2) Quantum beam generation: Electron, positron, ion, meson and other exotic particle beams, Laser-based X-ray Free Electron Lasers (XFEL); 3) Imaging Technologies: Ultrafast imaging with novel particle and radiation sources. Progresses of high power laser and plasma photonic device technologies make us close to real laser plasma accelerator. Now we have a well-stable electron beam with a pointing stability through a laser-wake field controlled by plasma photonic devices, i.e. plasma collimator and plasma fiber. These technologies could realize a compact GeV accelerator and a laboratory size XFEL in the next decade. Discussion on the feasibility of the compact XFEL with the laser plasma accelerator will be one of the most important and attractive topics of the conference.