1. 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

2. 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

3. 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

4. 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

5. 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

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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. LOI Schematic
13/10/2017, ISIS
LOI collaboration

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

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

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