Principle of Global Decoupling with Coupling Angle Modulation

The global betatron decoupling on the ramp is an important issue for the operation of the Relativistic Heavy Ion Collider (RHIC). A new scheme, coupling angle modulation, was found. By modulating two orthogonal skew quadrupole families, an extra rotating coupling is introduced into the coupled machine. The skew quadrupole modulation frequency is about 0.2Hz for the RHIC ramp, and 0.5Hz at injection and store. The eigentune changes are tracked with a high resolution phase lock loop (PLL) tune measurement system. The global coupling correction strengths are determined by the modulation skew quadrupole strengths at the minimum eigentune split multiplied by a factor k. k is determined by the uncoupled eigentune split and the maximum and the minimum tune split during the skew quadrupole modulation. This decoupling scheme is fast and robust. It had been verified at the RHIC and has been applied for the RHIC global decoupling on the ramp. In this article, the principle of the coupling angle modulation is presented in detail. Simulation results are also shown.