1. V. Paramonov, on behalf INR team.
Realization of RF input into coupling element of the accelerating cavity.
V. Paramonov, on behalf INR team.
Institute for Nuclear Research, Moscow

2. Motivation
Accelerating structure of the main part of INR proton linac - 27 four-section Disk And Washer (DAW) cavities, f = 991 MHz, was constructed by INR in house in 80-th.
1 – accelerating sections, 2 – bridge coupling cavities, 3 – RF input, 4- focusing elements, 5 – cooling 2

3. The first cavity, b=0. 43, 100 MeV, was the first in the line
The first cavity, b=0.43, 100 MeV, was the first in the line. Last time the cavity is losing reliability of operation and restricts the average beam power of our linac We have to replace it and now develop technical proposal For the single cavity the direct DAW repetition is too expensive We consider another structures and develop proposal with limitation – the minimal changes in another systems.
Such scheme is common for NC hadron linacs. Not usual – bridge coupler from standard rectangular WG – the simplest solution
Solved now
SCS DAW ACS CDS
Ze % % % %-107% kc Ph, kW/m < < Vac. cond HOM night mare R/l ~ ~ ~ ~ RF tuning after brazing ac nn nn a ? Wat./vac yes yes yes no!
f=const
Our choice now is CDS due to smaller transverse dimensions and tolerable another parameters.

4. Cavity scheme
Coupling with a slot
1 – accelerating sections (accelerating cells) – 4 units, – bridge couplers (coupling cells) – 3 units, bellows, 4-tuners, 5- end shorts Total seven cells. In the spectrum there are seven modes and operating one is central Field direction in adjacent sections (accelerating cells) is opposite All symptoms of operating p/2 mode! But for a simple p/2 mode the field in coupling cells is zero and RF input is impossible! But it is realized Moreover, it works! To realize RF input we must have sufficient field strength in this point!

5. Methodical basement 1
Next AC mode
Operating AC
Previous AC mode
Mode p/2 is from single mode periodical chain of cavities We have to see wider – periodicity is more complicated and it reflects directly in cavity spectrum. Each mode in accelerating section provides own band in the cavity.
Analysis of slot coupled cavities in multi mode approximation was developed.
Based on V. Kaljushnij, JTPh, 1978

6. Now cells are sections and bridge couplers, in which we consider several modes. We can estimate coefficients from numerical simulations for separated elements. Slots: provide identifiable shift for each mode; couples all modes inside cell; couples all modes of the cell with all modes in adjacent cells.
But at operating frequency now in the bridge cavity is:
There is a mixture, but not zero required component! The method is powerful and helpful…but difficult for simple physical treatment..

7. L/4 Interaction of modes in the cavity near operating frequency
Now we apply scheme to CDS
Slot & transition
L/4
Structure sections df
Regular waveguide WG
Yellow curves – even (with respect WG middle plane) modes, blue curves – odd modes, df – is the separation in frequency between modes in sections, L – wavelength in the waveguide. We can interesting with two regions of interaction, 1 or 2, depending on required parity (phasing) of sections.

8. Changing the slot dimensions and slot vicinity in the transition, we can manage the region of modes interaction, tune for operating frequency and change modes separation in frequency To select operating frequency we need repeatable specific points at the plot.
L=dw
Two options for selection are possible: a) – symmetrical positions of side modes with respect operating one; b) – point d2f/dl2=0; The last one corresponds to the minimal field in the regular part of th bridge! We need physical ideas for treatment.

9. f e,m=f m,e or f e,e = f m,m Methodical basement 2
Let us define period in the total accelerating cavity,
and ask cavity to be compensated system. At operating frequency should coincide frequencies of two modes with conjugated parity of field distribution.
Section is based on compensated accelerating structures, SCS, DAW, ACS or CDS and already satisfies conditions of compensation.
f e,m=f m,e or f e,e = f m,m
We have to select required parity (for RF input into bridge), and, playing with last end cell of the section, slot, transition and regular part of bridge coupler, tune period at operating frequency and symmetrical position of side modes.
In such treatment the bridge coupler is the part of super-period with accelerating mode and there is no prohibition to the field strength The physical point – magnetic field at the slot sides is continuous It is our technical task to extend field into bridge.

10. Practically for the same Ez strength in the sections,
which means practically the same magnetic field at the slot, playing with slot and transition,
we can control magnetic field value in the bridge coupler more than in order, providing sufficient value for RF matching After that detailed analysis pro and contra – what is the best compromise in the field strength.

11. Summary
We physically think in frames of some ideas and models for our description and understanding of physical processes. The level of problem understanding defines our ability to control it.
For careful, precise and non-contradictive description of the cavity we have to extend the model for presentation of our accelerating cavity: a)- as periodical system (big period – one section + 2 half brides) the cavity operates in the compensated p mode; b) - the bridge is the part of the big period working in accelerating mode and there is no limitation for the field strength in this part; c) –joint of representation as compensated system and multi mode slot coupled cells provides guide line for application of this bridge together with any compensated structure and correct cavity tuning; d) - technologically the bridge device made from standard WG is the simplest solution as compared to analogues e) – technical solution for coupling between section and bridge device allows select ratio of fields in sufficiently wide range providing both possibility of RF input and cavity fine tuning.

12. Thank you for attention!12