The hollow cathode magnetron (HCM) is a new type of high-density plasma device developed for ionized physical vapor deposition (I-PVD). A novel magnetic geometry provides the confining magnetic field to sustain a magnetron discharge within a cup-shaped hollow cathode and the means of ion extraction from the source to the substrate. The use of a "cusp mirror" to reflect most of the escaping electrons back into the hollow cathode cavity has allowed the HCM to achieve extremely high plasma density (10@super 12@ - 10@super 13@ cm@super -3@). The HCM source is highly scaleable and has been successfully implemented in sources ranging from 19 to over 380 mm in diameter. Although the HCM has proven to be a very successful I-PVD source, there is a lack of understanding about its detail working mechanisms. Recent progress in three dimensional electrical probe measurements together with plasma modeling have revealed a different physics picture from our previous belief. Strong radial electric field on the order of 400 V/m was measured inside the hollow cathode. In conjunction with the confining magnetic field, a large ExB drift current is established because of the magnetron effect. As a result of this current and the incomplete shielding of the cathode voltage, the measured plasma density profile inside the cathode is hollow and funnel-shaped. The density profile becomes Gaussian as the plasma emerges through the magnetic null. Unlike other downstream plasma sources where the plasma density near the source is much higher than that downstream, no significant difference in plasma density is observed for the HCM. With the exception of the plasma edge where the presence of an energetic electron tail was evident, the electron energy distribution (EEDF) was approximately Maxwellian. Despite more than two orders of magnitude variation in plasma density, the electron temperature profile is relative flat throughout the entire plasma.