This work developed a small-scale processing apparatus for ultra-high temperature and ultra-high-pressure cavitation (UTPC) incorporating a small diameter (0.1 mm) water jet nozzle. This instrumentation comprised a swirl flow nozzle (SFN) installed on the water jet nozzle so as to obtain UTPC from a multifunction cavitation (MFC) setup. Multi-bubble sonoluminescence (MBSL) assessments using two types of photon counting heads were employed to assess UTPC, MFC, ultrasonic cavitation (UC), water jet cavitation (WJC) and SFN-WJC. The SL intensity was found to increase in the order of SFN-WJC, WJC, UC, MFC to UTPC. Because UTPC produced the most intense emissions, this process evidently attained the highest processing temperature. Assuming a UC bubble temperature of 4000 K, the temperatures associated with UTPC, MFC and WJC were determined to be 5400–5900, 5300 and 3200–3300 K, respectively. The energy density of a single bubble during UTPC was
calculated using the Rayleigh–Plesset and Planck equations for an initial bubble radius of 100 μm together with photon measurements from many bubbles and employing Planck's law. The highest SL intensity of UPTC is thought to exist due to the high energy density of UTPC. This research demonstrates that it is possible to increase the energy density of cavitation bubbles within a small reaction area.