RESUMO
We theoretically investigate the quantum phase transition in the collective systems of qubits in a high quality cavity, where the cavity field is squeezed via the optical parametric amplification process. We show that the squeezed light induced symmetry breaking can result in quantum phase transition without the ultrastrong coupling requirement. Using the standard mean field theory, we derive the condition of the quantum phase transition. Surprisingly, we show that there exists a tricritical point where the first- and second-order phase transitions meet. With specific atom-cavity coupling strengths, both the first- and second-order phase transition can be controlled by the nonlinear gain coefficient, which is sensitive to the pump field. These features also lead to an optical switching from the normal phase to the superradiant phase by just increasing the pump field intensity. The signature of these phase transitions can be observed by detecting the phase space Wigner function distribution with different profiles controlled by the squeezed light intensity. Such superradiant phase transition can be implemented in various quantum systems, including atoms, quantum dots, and ions in optical cavities as well as the circuit quantum electrodynamics system.
RESUMO
Ion cyclotron resonance heating (ICRH), one of the main auxiliary methods, for high-power and long-pulse plasma heating had been developed in Experimental Advanced Superconducting Tokamak (EAST). An impedance matching system, one important part of ICRH, had been developed for high-power injection and transmitter protection by reducing the reflected power from the antenna. The input impedance in the outlet of the stub tuner can be measured by voltage-current probes installed on the coaxial transmission line between the antenna and triple liquid stub tuners, and the optimum liquid levels in the stub tuners can be calculated based on the input impedance. The calculation and adjustment process of the optimum liquid levels are described comprehensively in this article. Finally, impedance matching had been achieved between two shots during EAST experiments. In the near future, a real-time impedance matching system will be developed to prevent large variations of the ICRH antenna impedance and achieve steady-state and long-pulse operation with the ICRH system.
RESUMO
A passive and noninvasive diagnostic system based on high-frequency B-dot probes (HFBs) has been designed and developed for the measurement and identification of ion cyclotron emission (ICE) in the Experimental Advanced Superconducting Tokamak (EAST). Details of the hardware components of this system including HFBs, direct current blockers, radio frequency splitters, filters, and power detectors as well as data acquisition systems are presented. A spectrum analyzer is used in addition to the ordinary speed acquisition card for data registration and analysis. The reliability of a HFB based diagnostic system has been well validated during the 2018 spring experiments on the EAST. ICE signals corresponding to fundamental cyclotron frequency of hydrogen ions and harmonics of deuterium ions were observed in experiments where deuterium plasmas were heated with deuterium neutral beams. The field dependence of ICE has been verified by recent experiments with three different background magnetic fields. The observed ratio of the ICE frequency is consistent with the ratio of the magnetic field intensity within measurement errors of a few percent.