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Published byLane Thornsbury Modified about 1 year ago

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Measurements of adiabatic dual rf capture in the SIS 18 O. Chorniy

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2 Contents Introduction Dual harmonic rf capture, scheme 1 Dual harmonic rf capture, scheme 2 Single rf capture Summary and Outlook

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3 Introduction In dual harmonic rf bucket (in comparison with single harmonic rf bucket): The flat voltage leads to a longer bunch. RF bucket area is larger then for single harmonic. It reduces maximum density in the bucket center. Bunch in a dual rf bucket is more stable. Nonlinearity created by dual harmonic rf bucket increases Landau damping. Longitudinal density Dual harmonic rf signal

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Dual harmonic rf capture, scheme 1

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5 RF amplitude ramps for different harmonics starts with time delay Dual harmonic rf capture, scheme 1 Features of the measurements: Constant (injection) energy Ion Intensity, part. Pickup RF cavity (h=4) SIS RF cavity (h=8) Fixed rf capture for different intensities

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6 Diagnostics in time and frequency domain Longitudinal beam profile measurements Longitudinal schottky measurements

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7 Longitudinal bunch profiles in a single and dual rf buckets Maxwell-Boltzmann beam profile RF potential in general form Dual harmonics rf Single harmonic rf -represent rms bunch length for short bunches

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8 Longitudinal bunch spectrum in a single and dual rf buckets From bunch spectra in a single rf bucket From bunch spectra in a dual rf bucket Synchrotron frequencies should be equal expected

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9 Longitudinal bunch profiles at different intensities single rf bucketdual rf bucket One of the reasons for the bunch lengthening can be increase of momentum spread at injection. Another reason can be the mismatch of rf frequency.

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10 Momentum spread of the injected coasting beam at different intensities Low intensity momentum spreadHigh intensity momentum spread Thus the emittance (and as a result, bunch length) may grow due to filamentation of the part of momentum spread which is not matched with synchronous energy

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11 Dilution factor Longitudinal phase space before and after rf capture rf bucket Dilution factor dilution factor (experiments) dilution factor (ESME result) Bunch profile (dual rf bucket) Schottky spectra The dilution factor was calculated for lowest intensity

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Dual harmonic rf capture, scheme 2

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Amplitude ramps for different harmonics starts simultaneously Pickup RF cavity (h=4) SIS RF cavity (h=8) Features of the measurements: Constant intensity Capture time Dual harmonic rf capture, scheme 2 Momentum spread: Different rf capture times for fixed intensity

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14 Capture efficiency at different capture times Short capturing time influences noise in the stationary bunch form. Possible explanation: mismatch of rf frequency. Emittance increase by factor of 2 even for “adiabatic” times. Measured Dilution factor= 200ms50ms10ms

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15 Comparison with simulation results Simulations (ESME):Experiment (SIS) vs simulations (ESME) A significant deviation of the simulation results from the measurements was found The difference cannot be explained by the error in the value of the initial voltage Simulation results provide us the value of frequency mismatch of 500 Hz

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16 Dilution factor vs initial conditions Phase space, coasting beam initial rf bucket max 00 / p dp p rms Simulation results for different ions (done by T. Shukla) In process: Analytic description of the curve Dilution factor( ) Initial conditions can be described by:

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17 Comparison between the different dual harmonics rf capture schemes Bunch length 218 deg Bunch waterfall plot in the case of rf capture with delay between harmonics Bunch length 206 deg Bunch waterfall plot in the case of rf capture without delay between harmonics Measurements at high intensity : Long capture time : 100-200 ms

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Single rf capture results

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19 RF capture in single rf bucket 200 ms100 ms 10 ms Almost constant bunch length for all capture times. Capturing time has influence on the presence of persistent longitudinal oscillations (in dual harmonics rf scheme only small noise).

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20 Spectra of the bunch in single rf bucket Do we expect similar strong longitudinal oscillations in dual rf bucket at higher intensities? Will it lead to instabilities? (Future work) Beam spectra Synchrotron frequency Dipole frequency Frequency of persistent oscillations Oscillations cannot be damped by dipole feedback system.

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21 Conclusions To save the machine cycle time both harmonics can start at the same time. Dilution factor can be reduced by rf capture at different rf frequencies. For the intensity of 10^10 ramping time can be down to 20-50 ms (only for dual rf harmonics regime) Dual rf capture experimental studies will be continued for higher intensities

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