RF controls based on carrier suppression detection with attosecond resolution

This paper presents a radio frequency (rf) control system with attosecond resolution based on a carrier suppression interferometer operating a superconducting cavity at the Cryo Module Test Bench (CMTB). This novel application of the carrier suppression detector extends conventional heterodyne methods and improves the residual jitter of the regulated rf field in the cavity by more than one order of magnitude. The cavity operated at 1.3 GHz with a gradient of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mn>8</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi>MV</a:mi><a:mo>/</a:mo><a:mi mathvariant="normal">m</a:mi></a:mrow></a:math> and a loaded quality factor of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:msup><d:mn>10</d:mn><d:mn>7</d:mn></d:msup></d:math>. The setup achieved out-of-loop phase noise detection values of <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mi mathvariant="script">L</f:mi><f:mo>=</f:mo><f:mo>−</f:mo><f:mn>180</f:mn><f:mtext> </f:mtext><f:mrow><f:mi>dBc</f:mi><f:mo>/</f:mo><f:mi>Hz</f:mi></f:mrow></f:math> at 10 kHz and <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mi mathvariant="script">L</i:mi><i:mo>=</i:mo><i:mo>−</i:mo><i:mn>165</i:mn><i:mtext> </i:mtext><i:mrow><i:mi>dBc</i:mi><i:mo>/</i:mo><i:mi>Hz</i:mi></i:mrow></i:math> at 100 Hz with a time resolution of 189 as within an offset frequency range from 10 Hz to 1 MHz. The phase noise budget of subcomponents such as in-loop and out-of-loop detectors, high-power drive, microphonics, and the reference source is reported. The facility rf reference phase noise in the offset frequency range from 1 to 100 kHz is identified as the key noise contributor. Furthermore, the narrow-band cavity reduces the phase jitter experienced by the beam to just 116 as. The presented research combining conventional receivers with carrier suppression detectors in continuous wave operation is a key milestone toward attosecond resolution, in particular relevant for pump-probe experiments in free-electron laser facilities.