Scanning of the interaction-point consequences for the RHIC storage RF system Page: 3 of 9
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power there exists a hard limit for the maximum possible phase deviation PHIMAX of the
beam, namely the phase at which the full available voltage of the individual cavities is used
to re-establish quadrature between beam and overall combined voltage, see fig 3.
PHIMAX=ASIN(NCAV.IND/NCAV.COM) = ASIN(2/6)=0.3398
The maximum geometric deviation LMAX follows from
LMAX = PHIMAX * LAMBDA /(2 * PI) = 82.7 mm
The phasor diagram for phase deviations smaller than PHIMAX at a cavity that has
restore (rather then sink) beam energy is shown in fig 4.
It is supposed that the cavity tuner remained in the optimum position for zero beam offset
due to the anticipated rapidity of the scan. The amplifier has to deliver a sizeable additional
rf current which ultimately hits the limits of the power tube and/or the DC power supply.
Numerical values of several key parameters are listed in the annex as a function of beam
offset, for both the accelerating and the decelerating cavities.
The limiting value in the present case is the peak current capability of the amplifier
tube. The amplifier has been tested at CERN for peak cathode currents up to about 60 A;
the unit can no longer be operated CW but has to be pulsed, at 20% maximum duty factor
and pulses of max. 1 sec duration.
That limiting peak cathode current of 60 A corresponds to a beam offset of about
+64 mm. The combined rf voltage URF.STO is reduced to 7.05 MV under these conditions,
the DC supply current in an accelerating amplifier attains 16.8 A, the necessary rf drive
power 5.2 kW.
The decelerating counterpart in the other ring draws a DC current of only 3.83 A for
the same (negative) beam offset. If the scan of the interaction point is rapid and double-
sided, such that the individual cavities of a ring are alternating acceleration and deceleration
in sufficiently fast sequence, then the currents for the two modes can be averaged in a
storage capacitor and the requirements on the power supply remain essentially unchanged.
A maximum beam offset of about +-64 mm can be achieved at 20% duty factor for
pulse length of 1 second. The peak cathode current of the final rf amplifier is the limiting
factor. Amplifier drive chain and DC power supply have to be designed with considerable
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Pirki, W. Scanning of the interaction-point consequences for the RHIC storage RF system, report, April 1, 1993; United States. (digital.library.unt.edu/ark:/67531/metadc870376/m1/3/: accessed June 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.