Refining narrow-band noise signals for transversal beam excitation to improve J-PARC MR slow extraction

Schnase, A.; Tamura, Fumihiko   ; Tomizawa, Masahito*; Toyama, Takeshi*; Yoshii, Masahito*

At the 2009 LLRF workshop, we showed procedures to create complex modulation sequences for well-defined narrow-band signals. A DSP controlled the modulation data for a DDS-synthesizer. In 2010 such noise signals fed to the MR-transversal exciter were used to study the duty factor improvement of the extracted proton beam from J-PARC Main-Ring (MR). According to the experiment group higher noise frequencies are preferred, where the DDS is at its limits. A commercial vector signal generator with built-in digital IQ modulation was examined. It stores several million sample long IQ-modulation sequences and can generate higher frequency signals. Signal preparation is similar to the DDS board. We present properties of the narrow-band noise signals generated by the vector generator, and Nov. 2010 extraction beam study results where we used such signals. The duty-factor improved by transverse noise near 20 MHz, but there was pressure rise due to multi-pactor in the horizontal exciter. The multi-pactor problems are solved, giving more freedom for the operating frequency. This helps future beam tests with transversal noise aiming at higher frequency. The IQ-modulation sequence creating process is refined to make sure that the peak power is only 3 dB higher than the average power. A verified signal level prevents power amplifier problems. We describe synthesis details, output signal measurement methodology, and results.