Alternate Current Option:

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Nov 15, 2013 (3 years and 10 months ago)

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Alternate Current Option:


The beam power for the initial installation has been cut in half from that of the
RDR. This allows reduction of both the size of the damping rings and the rf power. The
straight forward approach, aimed at minimizing peak rf

power, was to eliminate every
other bunch, thus halving the bunch train current. This has the effect, for the same
acceleration gradient, of doubling the cavity fill time, thereby increasing the required rf
pulse width by 38% from 1.56 ms to 2.16 ms. This

pushes the specification for the
klystrons and modulators beyond what’s been developed. The longer fill time also
increases the cryogenic dynamic heatload.

An alternative way to achieve the same reduction in average current or number of
bunches per pulse
is to change both the bunch train current and the bunch train length. If
the former is reduced to 69% of the RDR value and the latter to 72.5%, the required rf
pulse duration remains unchanged. The increase in fill time is balanced by the shortened
beam pu
lse. In this scenario, the number of bunches would still be halved, with their
spacing increased by a factor of 1.45, rather than doubled.

More rf sources are required for this option, but still significantly fewer than for the
baseline current and without

extended pulses (which capacity would be unusable in the
event of an upgrade). Furthermore, the cryogenic dynamic heatload can be actually
reduced by a few percent.



option A

option B

upgrade

# of rf units (cavities) per KCS

32 (832)

32 (832)

32 (832)

# of bunches per beam pulse

1,335

1,335

2,670

bunch train current (mA)

4.5

6.21

9

bunch train duration (



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Negative Gradient Cavity Operation


With the undulator at the 150 GeV point in the electron main linac, operation below
300 GeV cms energy requires deceleration of the beam. Specifically, to pick up where
LEP left off, at 20
9 GeV cms, the electron beam must be decelerated by 45.5 GeV
(from 150

GeV to 104.5 GeV) over the section of the linac designed to add 100 GeV (to
250

GeV). This calls for powering the cavities out of phase at
-
45.5% of the design
gradient, or
-
14.33 MV/m
(average).

Reducing the Q
L
’s, the fill time and the rf power by a factor of |V|/V
0

will produce
the desired gradient at beam arrival. The rf pulse can then be terminated, and the
retarding gradient would be maintained constant as the beam loading buildup c
ancels
the decay of the preloaded rf. During the beam, the power extracted from the beam,
equal to

the preceding input rf, leaves the cavities to be absorbed in the loads.

The nominal fill time is 55% of the rf pulse (for either low power bunch spacing
op
tion), which fraction of the pulse energy is nominally deposited in the waveguide
load during cavity fill and discharge. Thus, for operation in a decelerating mode, the
input rf energy is 0.55

(|V|/V
0
)
2

and the rf heat load 0.55

(|V|/V
0
)
2

+ 0.45

(|V|/V
0
)
t
imes the nominal pulse energy. This is less than the nominal heat load up to |V|/V
0

=
0.671 and only 31.9% at the maximum value of 0.455 envisioned above.

It would be simpler to achieve low energy by operating at full reverse acceleration
for the appropria
te fraction of the linacs, leaving the power, timing and coupling
unchanged. However, unless the repetition rate were dropped, this would increase the rf
heat load by 82%, in the local waveguide loads and 29% in the couplers, requiring
greater cooling capa
city to be installed in the tunnel.



Zero Gradient Cavity Operation


To run beam through all or part of the linac with zero gradient without detuning the
cavities, or even changing the Q
L
’s, the rf should be set to arrive simultaneously with
the beam at a

power of ¼ the nominal value. This pulse will effectively be fully
reflected from the cavity into the waveguide load, as the normal decay of the drive
reflection is balanced by the rising beamloading emitted field. Because the cavity is thus
prevented fro
m filling with rf, there is no post
-
beam discharge. The waveguide loads
would receive ¼

0.45 = 0.1125 of the full gradient rf drive energy (where 0.45 is the
fraction of the nominal pulse width which covers the beam), as opposed to 55% of it
(fill time por
tion) in accelerating mode.