Technical Systems
3.3 MODULATORS .1 Overview
The accelerating gradient for the ILC main linacs is supplied by superconducting 1.3 GHz cavities powered by 560 10 MW RF stations, each with a modulator, klystron and RF dis-tribution system. Another 86 similar stations are used in the e+ and e− Sources and RTML bunch compressors. The damping ring RF power is supplied by 650 MHz superconducting cavities powered by 1.2 MW peak power klystrons. These are fed from a DC supply and do not have pulsed modulators. There are also a few special purpose S-band RF stations for instrumentation and a 3.9 GHz RF station to power the crab cavities near the Interaction Point. This section describes only the 1.3 GHz modulators, Damping Ring HVPS system TABLE 3.3-1
Modulator Specifications & Requirements Assuming Klystron µP=3.38, Effy=65%
Specification Typical Maximum
Charger input voltage kV RMS 7.67 8
Charger average power input kW 147.9 161.7
Charger efficiency 0.93 0.93
Charger DC output voltage = Modulator kVin 10.8 11.3 Charger DC avg output current = Modulator Ain 13.26 13.26
Charger average power output @ 5 Hz kW 137.5 150.3
Modulator efficiency 0.94 0.94
Modulator pulse voltage output = Pulse Transformer kVin 10.16 10.18 Modulator pulse current output = Pulse Transformer Ain 1560 1680 Modulator average power output @ 5 Hz kW 129.3 141.3
Pulse transformer step-up ratio 12 12
Pulse transformer efficiency 0.97 0.97
Pulse transformer voltage out = Klystron kVpk 115.7 120 Pulse transformer current out = Klystron Apk 133.0 140 Pulse transformer average power output @ 5 Hz kW 125.4 137.1 High voltage pulse duration (70% to 70%) ms 1.631 1.7 High voltage rise and fall time (0 to 99%) ms <0.23 0.23
High voltage flat top (99% to 99%) ms 1.565 1.565
Pulse flatness during flat top % < ±0.5 ±0.5
Pulse to pulse voltage fluctuation % < ±0.5 ±0.5
Energy deposit in klystron from gun spark J < 20 20
Pulse repetition rate, Hz 5 5
Klystron filament voltage V 9 11
Klystron filament current A 50 60
DRAFT
and associated components.
3.3.2 Technical Description
The 10 MW L-Band RF power stations for the ILC are installed in the support tunnel, spaced approximately 38 meters apart. The L-Band Modulator baseline design was developed for the TESLA Test Facility at DESY, and has been adopted for the European XFEL. Three FNAL units and 5 commercial units have brought online at DESY starting in 1993. The design has a series on-off solid state switch with partial capacitor discharge. The ILC unit varies from this design in two minor ways: (1) A new solid state redundant switch is employed to form the 1.7 msec output pulse, for better reliability; and (2) the input charger will operate from a voltage of 8 kV instead of 480 V to eliminate the AC input step-up transformer in the current design. The modulator specifications and requirements are summarized in Table 3.3-1.
The block schematic is shown in Figure 3.3-1. Photos of current prototypes are shown in Figure 3.3-2. Operation is straightforward: The charger delivers a DC voltage to the storage capacitors of approximately 11 kV. The modulator main switch is then triggered and held closed for 1.7 msec. Capacitor current flows through the switch to the step-up transformer input. At the same time, an auxiliary droop compensation “bouncer” circuit is fired to maintain the pulse top flat to within ±0.5% during the RF drive period. The slightly above 10 kV drive pulse (to compensate for Bouncer voltage) is delivered to the input of the pulse transformer in order to produce at least 115.7 kV 133.0 A to the klystron for rated 10 MW peak output.
FIGURE 3.3-1. Modulator schematic and L-Band RF station block diagram (1 of 646).
The Damping Rings have 650 MHz CW stations using 1.2 MW peak power klystrons, 20 in total for 2 rings. Power is supplied from a DC supply of 2.0 MW delivering 50-75 kV at 17-10 A DC. The RF envelope is controlled by the low level RF and timing to maintain stability and clearing gaps as needed. The station block diagram is shown in Figure 3.3-3.
DRAFT
FIGURE 3.3-2. (a) Capacitor stack, (b) Dual IGBT switch, (c) Bouncer choke, (d) Pulse transformer.
High Voltage
Power Supply Low Level RF
Control & Timing Driver
CW Klystron
Splitter
Circulator
Splitter
Cavity 1 Cavity 2
Splitter
Cavity 4
Cavity 3 8747A22-2007
FIGURE 3.3-3. Damping Ring 1.2 MW RF station (1 of 20).
3.3.3 Technical Issues
3.3.3.1 L-Band
There are no major technical issues with the L-Band modulator as long as the entire sys-tem has sufficient overhead (redundancy) to compensate for a failed station. To achieve an acceptable availability, the linac energy and beam current parameters must be chosen to provide some RF spare stations. Redundancy of internal components such as IGBT switches and sectioning of chargers for N+1 redundancy2 is also important. Currently this is only partially implemented in the prototypes.
2N+1 design segments a single unit such as a power supply into N parallel or series smaller modules components plus an additional spare so one module can fail without interrupting operation. N+1 design is used in stacked or parallel power supplies, capacitors and IGBT’s. Such designs can achieve much higher overall Availability especially if modules can be exchanged without interrupting operation (Hot Swap capability). This is only possible in lower voltage units.
DRAFT
The present design which develops the drive pulse at low voltage and high current has larger losses than would be experienced with a higher voltage design. This is not a major technical issue, but a cost, size and weight issue. Installation and repair during operations will be more difficult with multi-ton components such as the transformer and main capacitor-switch multi-cabinet assembly.
An alternative modulator design is being investigated to address these issues, including the possibility of significant cost reduction. The design would reduce the overall footprint and eliminate the step-up transformer and other oil-filled components.
3.3.3.2 Damping Rings 650 MHz
The Damping Ring stations are modeled after similar stations in operation in Italy, Japan and the US. The power supply systems are very well understood. The only change desired would be to make them N+1 redundant internally for higher reliability. This will be investigated and will not have a large cost impact.
3.3.4 Cost Estimation
The L-Band modulator cost model was derived directly from the latest FNAL design, extrap-olated as needed to fit the ILC specifications. A traditional bottom-up estimate was made and learning curves applied to first-unit costs for an estimated manufacturing cost. Both single and dual source factory models were examined, as well as sensitivity to learning curve assumptions. These costs were also compared with industrial estimates from both Europe and Japan. In general, the US estimated cost lies between the two offshore commercial es-timates. Conservative learning curve exponents (“alphas”3) were used for both parts and labor. Profit and factory support costs were than applied, as well as the staging costs of preparing the units for installation and final system checkout. These costs were compiled in M&S and FTE’s. The factory models were documented in detail for each Area subsystem and given to the responsible managers for the Area rollups.
The modulator and charger costs were based on recent fabrication of units at SLAC in partnership with LLNL. All parts were recently purchased or fabricated at outside shops, and small additional extrapolations were made for the total quantities.
The cost of the HV power supply for the Damping Ring CW tubes was estimated based on recently built PEPII stations at SLAC, and separate estimates from Italy and Japan.
All estimates were in reasonable agreement. The CW power rating needed is 25% lower than for PEP but there will be some additional cost for the N+1 implementation. Again a conservative learning curve was applied for 20 units.
3.3.5 Table of Components
Table 3.3-2 shows the modulator component counts in various Areas.
3“Alpha” refers to the exponential decrease of costs with each doubling of manufacturing volume. For details see section 6.1.
DRAFT
TABLE 3.3-2
Modulator distribution by type and area.
Modulator type Total e− e+ e− e+ e− e+ e− e+ Inj Inj RTML RTML Linac Linac DR DR
10 MW–1.3 GHz–5 Hz 646 13 39 17 17 282 278 0 0
1.2 MW–650 MHz–CW 20 0 0 0 0 0 0 10 10