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Introduction

The anti-damper is a transverse feedback system which can be used to mimic horizontal instabilities via positive feedback. The analog damper uses a BPM button pickup along with one plate of the horizontal strip-line kicker. The signal from the button is amplified with a high gain, high bandwidth amplifier in the tunnel. Upstairs, the analog transition module (aTM) turns the doublet signal into a pulse via RF envelope detection and a low pass filter. The module also provides 32db of programmable attenuation to change the damper gain. A simple block diagram of the current analog system is shown below.

Experiments

Run 1

First Analysis MathCad - Antidamper System
First Results MathCad - Antidamper & Octupole Results from 3/30/19

Run 2b

Experimental Proposal IOTA Nonlinear Integrable Optics Experiment: Landau Damping (NIOLD)

Two experimental shifts were taken on 3/19/20 and 3/21/20. Below is a table with parameters for each event collected. For each event we capture 8000 turns of data from each bpm. If there was observable growth in the horizontal position, there is a plot of the TBT data showing measured tune, fitted growth rate, and intensity for bpm E2R. The tune is calculated from a 512 turn FFT window stepped through the data. The growth rate is calculated from a simple exponential fit to the TBT envelope. For events from the 2nd shift, the damper pickup sum signal is also included. This is sensitive to intensity, bunch length, and longitudinal position in the bucket. The damper sum signal will typically increase before we get beam loss, suggesting that the bunch length is getting smaller while the transverse emittance is growing. Note, synchrotron oscillations will appear at 2x the synchrotron frequency. One interesting point is that the beam loss almost always occurs after the initial exponential growth has started to damp down. This suggests the transverse beam size continues to grow after the centroid oscillations roll off or damp down. This is also more prominent with the octupoles powered.

The data suggest that the instability threshold goes down after each event where beam loss occurs. One explanation for this would be the landau damping being reduced to the tails being scraped away. As we don't have a good way to quantify this, one can just look at the first loss event in each store. The assumption here is that the particle distribution will be unperturbed in each case. The upper plot below shows all such events for different octupole currents vs. beam current. They appear relative flat with beam current. The lower plot shows the average instability threshold vs. octupole current. This shows a positive correlation with octupole current. There is a factor of 2 improvement between the 0A and 2A octupole settings which is in good agreement with the first results from Run 1.

The plot below shows the inverse fitted Growth Rate vs Damper Gain for all events where there is an observed oscillation in the TBT data whether it resulted in beam loss or not. To provide some uniformity on the beam parameters and remove suspect events, the following cuts were applied:
  • Bunch Length > 17cm
  • Horizontal Beam size < 300um
  • Vertical Beam size < 500um
  • Damper Gain < 0.5

There appears to be a linear relationship between the observed growth rate and the damper gain as expected. The gain threshold for an instability to grow is less for events with 0A in the octupoles. For events with octupoles powered, the threshold is higher for an instability to start to grow but the initial growth rate is still proportional to the gain.

The following plots show the observed behavior for different events within a store for injections of 1.5mA with octupoles at 0A, 2.4mA with octupoles at 0A, and 2.4mA with octupoles at 1.5A. Each plot shows the measured tune, the position envelope from the TBT data along with growth rate fit, and the damper sum signal (sensitive to intensity, bunch length, and bunch arrival).
The first plot shows peculiar behavior. There is an initial instability which grows as expected but then damps down with no beam loss. At the time it begins to damp down, the sum signal begins to oscillate in a way which could only be caused by bunch length oscillations which likely corresponds with transverse size oscillations as well. In each event where there is beam loss, it occurs at a second instability several thousand turns later which do not exhibit clean exponential growth. This injection is also notable for its initial beam parameters having short bunch length and larger transverse sizes.
The second plot is as expected. There is exponential growth until beam loss occurs. The loss also occurs at much larger position oscillation amplitudes suggesting the transverse emittance had not blow up as much. In each event where there is beam loss, there are oscillations on the sum signal after the loss. It is not possible to say for sure, but these are likely also bunch length oscillations.
With the octupoles on, the behavior changes. There is an initial instability with a growth rate proportional to the gain as shown in the plot above. This then levels off and then damps down if there is no beam loss. There is little tune shift in events with no beam loss. For events were loss occurs, the tune starts to shift after the initial instability levels off and another instability starts. In each event, beam loss starts as the tune hits ~0.28 and the stabilizes at tune ~0.275. Of the 4 events with beam loss, the two with less loss (9% & 14%) do not show oscillations on the sum signal after the loss. The two with larger loss (20% & 22%) do show oscillations on the sum signal.
dev_0A_inj1.5mA.png dev_0A_inj2.4mA.png dev_1_5A_inj2.4mA.png
1.5mA Injection 2.4mA Injection 2.4mA Injection

All the data was collected through ACNET using matlab. The data are stored as *.mat files with each file representing data events taken during one injected beam store in IOTA. The data are available as zip archive with all files for each octupole current setting. Expand the "files..." to see a list of files in the archive.

Archive run2b_octu_0_0A_data.zip

Archive run2b_octu_0_6A_data.zip

Archive run2b_octu_1_1A_data.zip

Archive run2b_octu_1_5A_data.zip

Archive run2b_octu_1_8A_data.zip

Archive run2b_octu_2_0A_data.zip

Archive run2b_octu_-1_1A_data.zip

Archive run2b_octu_-1_5A_data.zip

Papers and presentations

FAST/IOTA Collaboration Meeting talk IOTA Nonlinear Integrable Optics Experiment: Landau Damping 6/10/19