Y. Irie, KEK for the LOI collaboration LOI Experiment Proposal: New cavity tuning scheme to increase the RF voltage in the later acceleration cycle Y. Irie, KEK for the LOI collaboration 13/10/2017, ISIS LOI collaboration

Present RF voltage pattern: Cannot produce higher voltage at later cycle due to Buck Regulator overcurrent. Aim higher RF voltage by decreasing the BR current with an optimized cavity tuning. Ch 1_Gap volt envelope (1V = 1.2kV) Ch 2_LOI cavtune PD output (1V = 10deg.) Ch 3_Buck regulator output (30Amp/V) 13/10/2017, ISIS LOI collaboration

Motivation to aim higher RF voltage LOI was proven to give very low output impedance (<35W) at the experiment in 2011. The LOI scheme is, however, categorized as the high power solution to the beam loading problem when compared to the beam feedforward scheme (BFF) as the low power solution. The LOI has an advantage over the BFF that the influence of valve parameter change, such as amplification factor, is small, although it is huge to the BFF. Then, saving energy and efficient production of higher RF voltage are interesting theme of the LOI for the credit. Cathode follower is also classified into the high power solution. Amplification factor changes with the long-term drift or change of operating point which depends upon the strength of beam loading. 13/10/2017, ISIS LOI collaboration

Current flow through the final triode Triode tube absorbs the current fed by the driver amplifier as well as the cavity load current, and emits the current into the grid as well as the cavity load: the load of the triode is not only the cavity, but also the feedback impedance: Oh, my poor boy, Triode! 13/10/2017, ISIS LOI collaboration

Equivalent circuit of the final stage Two resonant states: cavity resonance and resonance as a whole. Because the triode current monitor is not available at present, viability of grid volt and grid I/P current monitors is also considered in the following. 13/10/2017, ISIS LOI collaboration

Abbreviations cavity resonance  minimum cavity I/P current : MCC. resonance as a whole  minimum triode current : MTC. 13/10/2017, ISIS LOI collaboration

Cavity inductance difference between two resonant states Solid line: analytical calculations Marker: TopSpice simulation For the triode minimum current operation, cavity inductance should resonate with all the capacitors more than the gap capacitance. The cavity inductance is then lowered for the MTC operation. The cavity inductance is controlled by the ferrite bias supply. How much bias current more required for the reduced cavity inductance: 0.377uH to 0.335uH? 13/10/2017, ISIS LOI collaboration

Current minimum operations at resonance Ferrite bias current is tuned to have triode current minimum at the 2RF frequency for a given time. 13/10/2017, ISIS LOI collaboration

Currents/1kV gap voltage at MTC and MCC Minimum triode current (MTC) Minimum cavity I/P current (MCC) Cavity I/P Current Triode Current X 1/3.4 Driver Current Driver Current Triode Current Cavity I/P Current Driver current is similar for both cases since it behaves like a constant current source: driver stage impedance is much higher than the input impedance seen looking into the triode grid. Triode current can be reduced by 1/3.4 for the MTC tuning than MCC. Driver current is not affected by the tuning because of its constant current operation. 13/10/2017, ISIS LOI collaboration

Tuning sensitivity at resonance Phase variation to bias current error, ie 𝝏(𝜟𝝋) 𝝏 𝒇 𝒓𝒆𝒔 [ Degree/MHz ] Gridvolt phase (Df) to give minimum triode current 13/10/2017, ISIS LOI collaboration

Phase difference and tuning sensitivity @ MTC Phase difference (Df ) from gap voltage Tuning sensitivity almost 180 degs. Triode Current vs. Gap Voltage is the best choice for MTC tuning. For the second best, Grid Voltage vs. Gap Voltage should be used. 13/10/2017, ISIS LOI collaboration

Phase difference and tuning sensitivity @ MCC Phase difference (Df ) from gap voltage Tuning sensitivity Again, phase difference and its sensitivity of the grid current are very similar between MTC and MCC cases. Cavity I/P Current vs. Gap Voltage is the best choice for MCC tuning. 13/10/2017, ISIS LOI collaboration

Signal processing before CAVTUNE input @ MTC Check the CAVTUNE output polarity for bias control. 2RF component by the beam loading distorts the grid voltage (and, possibly, triode current although not confirmed yet). FPGA（field-programmable gate array） Swept delay-time function in the delay equalizer is necessary for precise tuning. 13/10/2017, ISIS LOI collaboration

Probing the triode current Anode option: resistor at high potential Cathode option: resistor, Rogowski coil. contaminated with grid-cathode current. 13/10/2017, ISIS LOI collaboration

Conclusions We propose the MTC tuning to increase the RF voltage in the later acceleration cycle. It is interesting to compare MTC and MCC methods from view point of system stability, including the case with beam loading. Preparations to carry out the experiment: 1. Install the triode current monitor. 2. Implement swept delay-time function in the delay equalizer for Df detector for precise tuning. 3. Safety check of the whole LOI system after a long break. 4. anything else? 13/10/2017, ISIS LOI collaboration

LOI Schematic 13/10/2017, ISIS LOI collaboration

TopSpice Schematic of LOI 13/10/2017, ISIS LOI collaboration

Difference of ideal and actual triode current monitor 13/10/2017, ISIS LOI collaboration

Input impedance (Zin) seen looking into triode grid MCC MCC MTC MTC 13/10/2017, ISIS LOI collaboration