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An instability can be driven by narrow band impedances or by broad band impedances. In the case of narrow band impedances, only a small number of couple bunch modes will be driven. Narrow band longitudinal impedances are often due to the RF cavities.

The coupled bunch (CB) mode longitudinal damper system in Booster is a narrow band feedback system. Mode lines were observed in Booster using a simple measuring system that included mixing down the beam signal and then displaying the sampled frequency spectrum on a vector analyzer. Narrow band dampers were then built at the mode frequencies that exhibited the largest amplitude. The Booster had several mode lines with undesirable amplitudes, mode 1 (at 52.813 MHz + »620 KHz), mode 48 (at 52.813 MHz + »48*620 KHz), mode 49 (at 52.813 MHz + »49*620 KHz) and mode 50 (at 52.813 MHz + »50*620 KHz).  The 620 KHz is the revolution frequency at flat top.

Mode 1, which is nearest the cavity harmonic frequency, was observed to be the most destructive couple bunch instability (this is based on by beam emittance measurements). The mode 1 damper was found to be important when running Booster current above 3.7E12. The other three dampers were needed (mode 38 and 39 more then mode 40) when running above 4E12 protons.

Below is a block diagram of a Booster Couple Bunch Mode Damper:

The basic operation of the Booster couple bunch mode dampers is similar to those used in the Main Injector and Tevatron. A resistive wall pickup (long 18) signal is fed into a superheterodyne receiver. (The superheterodyne receiver is a common and useful RF processing technique. The front end of the receiver is an analog I/Q demodulator. The inputed beam signal is split and mixed with a local oscillatorís quadrature signals. The 90 degree mixing provides the mechanism to distinguish the I and Q components of the beam signal. After processing, the I/Q signals are mixed back up to RF.) Each damper has a DDS unit whose quadrature outputs are used to mix down the beam signal to base band. Because the I/Q signals at IF (intermediate frequency) will have side bands at the synchrotron frequency, band pass filters can then be used to detect the error signals. The dampers have front panel outputs which include the two (I and Q) low frequency errors. The CB damper IF signals are displayed both on a scope in the low level room as well as through the control system.

Low Level Room CB Damper Layout (only one damper shown):

  The initial tune up of the damper system requires adjusting the delays between the dampers and the low level system as well as balancing the legs of the I/Q demodulator. Once the initial tune up is complete, tuning is then done to minimize the band passed synchrotron frequency error signals. The tuning is simply an adjustment of the phases of the error signals before mixing back up. The phase adjustments are done by tuning a time table in the 465 cards which results in a phase shift (Because the phase shifter card has a fixed voltage/degree relation, a software transform in the 465 card displays the time table in degrees.

Dipole Motion

Parts List

1) Booster NIM double wide Damper Cards (used for mode 48, 49, 50):

The version that is presently in use was assembled 6/94. There are several newer versions, but all work about the same. (Main Ring, Tevatron have used the same layout.) The DDS unit model ADS 431 203 is no longer made, although we have lots of them. The cards cannot be interchanged without changes!

2) RF Amp Card:
This is an off the shelf RF card that is also used by the RF group. It has two separate input/output. We use one for the clk signal (booster RF) and the other is used to amplify the pickup signal for modes 48, 49, and 50.

3) Phase Shifter Cards:
The Æ shifters have many uses and can be found in Main Injector, and Tevatron as well as in Booster. They have about 1.2 MHz of band width. They are slightly noisy 2MV/Ö Hz. These cards can be interchanged.

4) The 465 cards:
These cards are used to control the phase shift of the applied damping error. A phase shift is necessary because the frequency is sweeping from ~50 MHz to 52 MHz (only used after transition).

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