The upgraded rf system for the AGS and high intensity proton beams Page: 2 of 7
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bars" on the left side, in parallel with those on the right, and
splitting the loading capacitors to 300 pF on each side, the
frequencies of the higher order modes were increased. Then,
the modes were effectively damped by resistors in parallel
with all the "bus bars". The resistors are 40 0, 25 mm by
600 mm low-inductance power resistors.
II. POWER AMPLIFIER
Power Tetrode
The power amplifier uses a Thomson-CSF TH573 300 kW
tetrode in the grounded-cathode configuration. The anode
power supply can deliver an input power of 390 kW. The
tube must provide 60 kW to drive the four-cell cavity to 40
kV, and provide up to 75 kW of beam power at 6 x 10t3 ppp.
There are ten amplifier/cavities in the system. The surplus in-
stalled power provides the opportunity for future intensity up-
grades but also is instrumental for achieving stability under
the heavily beam-loaded conditions. The choice of a tetrode
is attractive because it leads straightforwardly to a stable
amplifier. The high power tetrode has the additional feature
of a relatively low anode resistance which significantly loads
the cavity, thereby reducing its impedance. This property is
an integral part of rf system's design, as it yields a factor of
three or more of broadband impedance reduction. A benefi-
cial aspect of this type of impedance reduction is that it can
be dynamically controlled by changing the grid bias of the
tube. In this way the loading effect can be enhanced at key
points in the acceleration cycle and then reduced, to conserve
power, when not necessary. Typically the amplifier operates
in class AB1, but is put into class A at peak beam loading
times and is biased to cut-off when the rf system is off
(roughly 60% of the repetition period).
Reactive Power
At times the power amplifier must drive a de-tuned load.
Transients occur at injection, transition, and de-bunching (at
the unstable fixed point) before the slow extraction. The
cavity tuning servos require about 3 ms to settle to the new
compensation current. To maintain constant cavity voltage
during the settling time, the power amplifiers must deliver
reactive current. As much as 50 A average reactive current
could be called for at transition. In this case the power
tetrode would reach 180 A peak during the rf cycle.
Coupling to the Cavity
To fully exploit the low plate resistance of the power tube,
it must couple to the gap without impedance transformation.
Further, the line must be short to prevent standing wave
modes at low frequency. Also, wideband rf feedback requires
a minimum of delay in the feedback path. Figure 1 shows the
coupling line between tube and gap, which is 1.5 m long.
The line crosses a gap and the current returns to ground by
encircling the ferrite of both halves of the cavity cell. Thesingle-ended voltage on the anode appears as an equal
magnitude push-pull voltage on the gap, hence the impedance
ratio is 1:1. Referred to a single gap, however, the tube
impedance is effectively quadrupled because the four cells are
in parallel. The line travels inside the beam tube in the
vacuum, eliminating potential high voltage breakdown
problems. The beam is shielded from the line by a grounded
inner sleeve. To install the coupling line, the cavities were
disassembled into individual cells. This provided an opportu-
nity to upgrade all the vacuum seals of the cavities. Fortu-
nately, neither the ceramic insulators at the gaps nor the
ferrite stacks had to be disassembled. The coupling line is
water cooled with approximately 2 gpm of flow.
III. RF FEEDBACK
At injection the cavities are operated at 1.5 kV/gap, which
requires 0.5 A of current from the power amplifier,(L). At
6 x 10" ppp the rf beam current, (IB), is 6.0 A, implying a
beam loading parameter, IB/I, of 12. It has been shown [1,2]
that when the beam loading parameter becomes greater than
2 the beam control loops, tuning, AVC, and phase, are cross-
coupled and become unstable. RF feedback is needed to
reduce the effective beam loading parameter. Feedback
reduces the perturbations of the gap voltage by the value of
the loop gain, and the beam current, seen from the control
loops, is effectively reduced. Loop gains of 17 dB and greater
(depending on the operating point of the tetrode) are used to
reduce the beam loading parameter to less than 1.7.
Other peripheral benefits follow from rf feedback: 1.
cavity control is linearized, the AVC loop does little; 2.
phasing of the cavities is essentially independent of tuning
servo errors; and 3. the modulation response time of the
cavities is reduced by the loop gain. The hardware of the rf
feedback is described in some detail elsewhere.[3,4] Some
key points deserve emphasis. The feedback amplifier is a
closed-loop circuit itself. It must have wide bandwidth to add
minimal phase shift in the cavity loop. For phase margins less
than 60* the closed loop response will exhibit gain peaking
which, increases the cavity impedance at adjacent revolution
harmonics. Unstable coupled-bunch modes could be driven by
the peaking. Closed loop operation of the feedback amplifier
adds negligible delay to the cavity loop, and phase margins
greater than 600 are maintained. The key component for
achieving the wide bandwidth is the attenuator that senses the
grid voltage. The load resistance on the grid is set to 200 0.
This is a compromise between a low value to keep the Q of
the tuned circuit low and a high value to limit the required
current from the amplifier. Attempts to transform a standard
50 0 load to 200 0 resulted in spurious resonances that de-
stabilized the loop. A better solution was to obtain a special
(Altronics Research Inc.), water-cooled, 200 0 10:1 attenua-
tor which gave a clean spectrum to very high frequencies.
Finally, the control grid of the tetrode constitutes a significant
capacitive load (-1nF) which must be driven to 300 Volts.
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Brennan, J. M. The upgraded rf system for the AGS and high intensity proton beams, article, May 1, 1995; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc697914/m1/2/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.