NEXT LINEAR COLLIDER DESIGN STATUS*
N. Phinney, SLAC, Stanford, CA 94025, USA
for the NLC Design Group
Abstract
The layout of the Next Linear Collider (NLC) has beenrevised to provide greater physics capabilities and to
make the design more cost effective. The basic rf unit forthe main linacs uses a solid-state modulator to drive eight
70 MW klystrons with a 3 µs pulse length. The rf power isdistributed with a multi-mode Delay Line Distribution
System (DLDS) to 8 girders of accelerating structures.
This configuration improves the AC to rf efficiency while
reducing the number of modulators and klystrons requiredby a factor of two. Using a new compact design for the
final focus, the NLC now has two different Interaction
Regions (IR), one for high energy (250 GeV-1 TeV cms)
and one for low energy (90-500 GeV cms). The highenergy IR has minimum bending to support an eventual
upgrade to 3-5 TeV cms. The low energy IR is optimized
for precision studies at the Z-pole, W-pair, Top andpossibly Higgs threshold. The design allows for beam tobe shared between the two IRs and the possibility of 180
hz operation is under study. The present design provides
flexibility for a variety of upgrade and staging scenarios
from initial operation at low energy to a future extension
to multi-TeV.

1 INTRODUCTION
The Next Linear Collider (NLC) [1] is a future electronpositron collider that is based on copper accelerator
structures powered with 11.4 GHz X-band rf. It is
designed to begin operation with a center of mass energyof 500 GeV or less, depending on the physics interest, andto be adiabatically upgraded to 1 TeV cms. A schematicof the NLC, which is not to scale, is shown in Fig 1. The
collider consists of electron and positron sources, two Xband main linacs, and a beam delivery system to focus thebeams to the desired small spot sizes. There are two
independent interaction regions (IRs), a high energy IR
eventually upgradeable to multi-TeV and a low energy IR
optimized for precision measurements at 90-500 GeV.
The facility is roughly 30 km in length and has two 13 kmlong X-band linacs, long enough to accelerate the beams
to 500 GeV for collisions at 1 TeV cms. The collider isintended to begin operation at a lower energy, in whichcase the linac tunnels would only be partially filled withaccelerator structures and rf power sources. In addition,
there are bypass lines to deliver low-energy beam to either
IR as desired.
The NLC proposal was started by SLAC and laterjoined by LBNL, LLNL, and FNAL. Work at Fermilab_______________*Work supported by the U.S. Dept. of Energy under contract DE-AC0376SF00515.
†
Email: nan@slac.stanford.edu

Figure 1: Schematic Layout of the NLC.
will focus on the main linac beam line while the efforts at
LBNL and LLNL center on the damping ring complex,
the modulator systems and the gamma-gamma interaction
region. In addition, there has been a close collaboration
with KEK for several years concentrated on X-band rf
development. The JLC X-band linear collider [2] and theNLC have developed a set of common parameterswith very similar rf systems. A status report on the
progress of this collaboration was published in 2000 [3].
Over the last two years, the NLC layout has beenrevised to provide greater physics capabilities and to
make the design more cost effective [4]. The two NLC
IRs are asymmetric, with the high energy IR directly inline with the linacs. The total bending angle is minimal tosupport an eventual upgrade to 3-5 TeV cms. The low
energy IR is optimized for precision studies at the Z-pole,
W-pair, Top or Higgs threshold. This design provides
flexibility for a variety of upgrade and staging scenarios
from initial operation at low energy to a future extension
to multi-TeV. The NLC design luminosity has beenrevised upward to reflect the true design parameters andtake full advantage of recent R&D results. At the sametime, the original cost estimate has been reduced by about
30% through the use of new technologies (permanent
magnets, solid-state modulators, consumable collimators),
more compact optics (bunch compressors, Final Focus,
collimation) and new layouts to share housings.

2 PARAMETERS
The NLC was initially designed to provide a luminosity
in excess of 10 34/cm2/s at 1 TeV cms. To ensure thisluminosity, the design included a large operating spacewith numerous margins and overheads. For example, theinjector system was specified to produce 50% morecharge than required, and the beam emittance dilution
budgets were in excess of 300%. These were based onconservative component tolerance specifications without
including the impact of the emittance tuning techniques
pioneered at the Stanford Linear Collider (SLC) [5],which successfully reduced the emittance dilutions by anorder-of-magnitude. All systems were designed to meet
the tightest tolerances for the full range of parameters.
The design luminosity when all subsystems perform as
specified is roughly a factor of four higher than the initialgoal, similar to the TESLA luminosity estimates which
are based on similar assumptions.
A set of design parameters for the NLC at different
energies are listed in Tables 1 and 2. Table 1 lists theluminosity and IP parameters for 500 GeV and 1 TeV cms
operation while Table 2 gives the luminosity at lower
collision energies.
CMS Energy (GeV)
Luminosity (10 33)
Repetition Rate (Hz)
Bunch Charge (1010)
Bunches/RF Pulse
Bunch Separation (ns)Eff. Gradient (MV/m)
Injected γεx / γεy (10 -8)
γεx / γεy at IP (10 -8 m-rad)βx / βy at IP (mm)σx / σy at IP (nm)σz at IP (um)Pinch Enhancement
Linac Length (km)

500
1000
20
34
120
120
0.75
0.75
190
190
1.4
1.4
50.2
50.2
300 / 2300 / 2360 / 3.5 360 / 3.5
8 / 0.10 10 / 0.12
245 / 2.7 190 / 2.1
110
110
1.43
1.49
6.3
12.8

Table 1: NLC parameters for 500 GeV and 1 TeV cms
To preserve the small beam emittance during
acceleration, the X-band structures must be designed tominimize wakefields, and both the structures and thefocusing quadrupoles must be aligned to very tighttolerances. The wakefield properties of prototype
structures have been measured precisely in the ASSET
test facility and agree well with the calculations.
Structures have been fabricated which meet tolerances far
tighter than those required for NLC. The requiredalignment accuracy has also been demonstrated in
ASSET. Beam-based alignment techniques developed for
the SLC and FFTB have achieved close to the necessary
accuracy, and extensive simulations indicate that thesetechniques are capable of preserving the emittancethrough a 10 km linac with diagnostics and correction
hardware which need to be only a factor of 3 to 5 better
than that used at the FFTB. The FFTB also demonstrated
the validity of the final focus optics and achieved ademagnification of the beam size greater than required for
NLC. All of these results have led to improvements in thedesign and increased confidence in its capabilities.
CMS Energy (GeV)
33

Luminosity (10 )Repetition Rate (Hz)
Bunch Charge (1010)
σx / σy at IP (nm)Beamstrahlung δ B (%)
Photons per e+/e –: nγPolarization loss (%)

92

250

350

3.5
9.4
13.2
120
120
120
0.75
0.75
0.75
630 / 6.2 380 / 3.8 320 / 3.2
0.18
1.1
2
0.49
0.79
0.92
0.08
0.21
0.34

Table 2: NLC parameters for low energy operation
The new NLC layout with two asymmetric IRs lends
itself to consideration of two further options to expand thephysics flexibility of the collider. The baseline design has
always been to deliver luminosity to one IR at a time, but
it may be desireable to have both detectors taking data
simultaneously, particularly if the injectors and low
energy portion of the main linacs could operate at 180 hz
with lower energy beam to one IR. Simultaneous
operation requires pulsed extraction from the linac, andfor different center of mass energies, separate collimation
systems. 180 hz operation requires new damping rings toaccommodate the shorter damping time, possibly two
smaller 90 hz rings. There are also issues with cooling the
X-band klystrons at high rate. Neither option is mature
enough to be part of the NLC baseline design but both arebeing studied as possible future enhancements.

3 OPTICS AND LAYOUT
Recently, a number of changes have also been made to
the optical design to reduce the collider cost and/or
improve the collider performance. The beam delivery
system (BDS) has new designs for the beam collimation,
final focus, and interaction region layout. The new BDS ismore robust and is half the length of the previous design.
The injector systems have also been modified to reduce
cost by minimizing tunnel length.

3.1 Beam Collimation System
The beam collimation system has two purposes: it must
collimate the beam tails to prevent backgrounds at the IP,
including those generated by the collimation system itself,
and it must protect the downstream components against
errant beams. In earlier designs, the beam collimation
section was designed to survive any mis-steered or off-

energy incoming beam. This is a difficult constraint
because the beam density is normally so high that thebeam will damage any material intercepted. The resulting
collimation design for 500 GeV beams was 2.5 km longwith optical tolerances too tight for operational comfort.
In a pulsed linac, the beam energy can change frompulse-to-pulse, but large trajectory changes which are notdue to energy errors are much less frequent. The newcollimation system takes advantage of this fact and isdesigned to passively survive any off-energy beam but toallow on-energy beams with large betatron errors todamage the collimators. The betatron collimators will be`consumable' and can be rotated to a new position after
being damaged [6]. Based on SLC experience, thefrequency of errant betatron errors should be much lessthan 1000 times per year. With this revised design
specification, the collimation system is half as long withmuch looser tolerances and a larger bandwidth [7].

3.2 Final Focus
Conventional final focus designs (FF) for linear
colliders have been based on the lattice of the Final Focus
Test Beam (FFTB) at SLAC which was constructed fromseparate modules for the chromatic correction and madefull use of symmetry. Although this makes the design of
the FF simpler, it has the disadvantage of making the FF
quite long, 1.8 km for 750 GeV beams.
A new design has been adopted where the chromatic
correction of the strong final magnets is performed locally[8]. The new, more compact design has many fewer
elements and better performance. In particular, the new
FF has a larger energy bandpass with comparablealignment tolerances and a more linear transport thatshould make it less sensitive to beam tails. Because of thebetter performance, it is possible to increase L*, the freespace from the final magnet to the IP, from 2 m to 4.3 m.
This will simplify the design of the interaction region andthe interface with the high energy physics detector.
Finally, the length of the FF scales much more weakly
with beam energy in this new design. The new FF canfocus 2.5 TeV beams in only 700 m length, while anequivalent conventional design would have to be 3 km
long. This change makes it much more reasonable toconsider a multi-TeV collider using an advanced highgradient rf system such as the CLIC design [9]. To
capitalize on the multi-TeV potential of the new design, it
was also necessary to eliminate other bending between the
linac and the high energy IP. In the new asymmetriclayout, the linacs are no longer collinear but are oriented
with a shallow 20 mrad angle between them to produce
the desired crossing angle at the high energy IR without
additional bending. The beams to the second IR are bent
by about 25 mrad. This IR has a larger 30 mrad crossing
angle for compatibility with a possible γγ option.

3.3 Interaction Region Layout
The beamline for the high energy IR is 2.5 km from theend of the linac. This distance includes a long 1.4 km

collimation region, the 800-m final focus and an
additional 300 m 'stretch' to accommodate the beamlines
for the low-energy IR. The low-energy IR beamline splits
off at the end of the collimation and includes the 25 mrad
bend and a shorter 500 m final focus. Both beamlines
share the same collimation system but, as a future
upgrade, parallel collimator beamlines could be installedin the same tunnel. In the present layout, the two IRs are
separated by about 20 m transversely and 440 mlongitudinally to provide vibration isolation and shielding
so either IR hall may be accessed while the other is inoperation.

3.4 Injector Systems
There are two separate injector complexes to produce
the electron and positron bunch trains. These have
evolved in response to R&D results and cost optimization.
In the present layout, the electron booster and prelinac arehoused in the same tunnel to minimize infrastructure
costs. The positron drive linac, booster and prelinacs also
share a common tunnel and support buildings. The energyof the second bunch compressor has been lowered to 8
GeV to reduce the overall length and cost [10]. Recent
data from the failure of the SLC positron source indicatesthat multiple positron targets are required to keep theenergy deposited in each target below the threshold formaterial damage. In the present NLC design, the drive
beam electrons are split by an rf separator and directed
onto 3 of 4 multiplexed targets and positron capture
sections. The bunches are then recombined into thedesired 190-bunch train format.
The concept of a central injector complex wasinvestigated for possible cost savings, and many
configurations with and without shared components wereconsidered. Any centralized injector requires long, lowemittance transport lines to bring the beams to the end of
the main linacs and extra tunnels to connect into the linac
housing and into the second bunch-compressor 180°
turnaround. These additions more than offset any
potential savings. The most cost effective location for the
injectors is near the low-energy ends of the linacs as in theoriginal ZDR design. A central injector design is being
developed for the Fermilab deep tunnel site because it has
the advantage of being located entirely on land alreadyowned by the laboratory.

4 X-BAND RF SYSTEM
The rf system for the NLC design operates at afrequency of 11.424 GHz to support the higher
acceleration gradients needed for TeV-scale colliders. The
rf system consists of four main components: themodulators which convert the line ac into pulsed dc power
for the klystrons, the klystrons which convert the dc
pulses into rf pulses at 11 GHz, the pulse compression
system which temporally compresses the rf pulse while
increasing the peak power, and the accelerator structures
which transfer the rf power to the particle beam.

Currently, the NLC rf system is in its third design
iteration. The evolution of the rf system has been driven
by costing models that have been developed for thecollider and by the results from the ongoing R&D
programs. The first iteration of the rf system was based onconventional thyratron switched modulators, 50 MW
Periodic Permanent Magnet (PPM) focused klystrons, theSLED-II pulse compression system and a DampedDetuned (DDS) accelerator structure. This technology isused in the NLC Test Accelerator (NLCTA), which began
operation in 1997. It would be adequate to build a 500
GeV machine today but it is not optimized for a final
energy of 1 TeV or higher.
The present NLC rf design is based on solid-state
modulators, 75 MW PPM X-band klystrons, the Rounded
DDS (RDDS) accelerator structure which has 12% higher
shunt impedance and the Delay Line Distribution System(DLDS) pulse compression scheme which hassignificantly higher efficiency than the SLED-II system.
The rf pulse length has been increased to 3 µs instead of
1.5 µs, which reduces the required number of klystrons
and modulators by a factor of two. The latest
configuration uses an enhancement of the DLDS scheme
where the rf power is propagated in two orthogonal modes
to reduce the amount of waveguide required. In thiscurrent design, the rf system for each 250 GeV linac
consists of 117 modules each of which contains amodulator, eight 75 MW X-band klystrons, an rf pulse
compression unit, and 24 accelerator structures. The
present cost estimate for the rf system has decreased byroughly 50% compared to an implementation with
NLCTA technology.

4.1 Solid State Modulator
Recent improvements in high power Isolated Gate
Bipolar Transistor (IGBT) switches have made it possible
to develop a solid state modulator design. The NLC
modulator uses a stack of 80 induction cores, each wi th
two IGBT switches and a 3-turn transformer to generateover 2 kA at 500 kV. Each modulator will drive 8
klystrons with an estimated cost that is roughly half thatof the conventional modulator and with an overall
efficiency greater than 75%. Another advantage is that thereliability of the system can potentially be much higher.
Failure of a single IGBT should be benign since the core
saturates and becomes nearly transparent to the pulse, andadditional IGBT units are included to offset such a loss.
As a first step to a full-scale solid-state modulator, astack of ten cells was built and used to power a SLAC
5045 S-band klystron. Since then a full-scale prototype
induction modulator has been built by a collaboration of
SLAC, Livermore and Bechtel, Nevada. This unit is under
test using four 5045 S-band klystrons operating as loads.
It will be operated up to the rating of the loads, which is420 kV at 1,800 A peak, 3.2 µs and 120 Hz. It will thenbe upgraded and moved to the NLCTA to power 75 MW
PPM klystrons as they become available.

4.2 75 MW PPM X-band Klystrons
Conventional klystrons use a large solenoid magnet tofocus the beam between the gun and the collector.
Unfortunately, the magnet requires 20 kW of power
which is comparable to the average rf output power,
effectively decreasing the klystron efficiency. To avoid
this a new generation of klystrons using periodic
permanent magnet (PPM) focusing have been developed.
In these PPM klystrons, the focusing is generated withrings of permanent magnet material which are interleavedwith iron pole pieces to generate a periodic axial field.
In 1996, SLAC built a 50 MW PPM klystron which
produced 2 µs long 50 MW pulses with a 55% efficiency.
Next, a 75 MW PPM tube was built and was able toproduce over 75 MW with a pulse length of 2.8 µs and an
efficiency of roughly 55%, consistent with simulations. At
this output level, the pulse length was limited by themodulator output and the repetition rate was limited to 10
Hz due to inadequate cooling of the klystron. A second 75
MW PPM klystron designed to operate at the full 3 µs
pulse length and 120 Hz repetition rate has been built andwill be tested in 2002. This klystron incorporates Design
For Manufacture improvements to make it more suitable
for mass production.

4.3 Delay Line Distribution System
The klystrons most efficiently generate a lower power
and longer pulse than that needed in the structures toaccelerate the beam. To optimize the system, the rf pulse
must be compressed temporally before being sent to theaccelerator structures. The SLED-II system, in operation
at the NLCTA, compresses the klystron pulse by a factorof 6 but the efficiency is only about 70% so the peak
power is only increased by a factor of 4.
To improve on this efficiency, the DLDS system was
proposed at KEK [11]. In this system, the power fromeight klystrons is summed and divided into equal timeintervals. It is then distributed up-beam to eight sets ofaccelerator structures that are spaced appropriately so thatthe beam-to-rf arrival time is the same in each case. The
power is directed to each different group of structures byvarying the relative rf phases of the eight klystrons. The
intrinsic efficiency of this system is 100% although wall
losses and fabrication errors would reduce that to 85-90%.
To reduce the length of waveguide required, a multimode version of this system has been developed in which
the power is distributed through a single circular
waveguide, but in two or more different modes. In thecurrent configuration, each waveguide transports twomodes, reducing the length of waveguide by roughly afactor of two. Future studies will investigate both thepossibility of transporting four modes in one waveguideand the utility of active rf switching techniques whichmight allow all the power to be transported in a single
waveguide. Finally, to test the components at full design
power, the NLCTA has been upgraded to produce 240 ns
long pulses of 800 MW and testing will begin in 2002.

4.4 Accelerator Structures
The accelerator structures for NLC have been studied
for many years, much of this in collaboration with KEK.
A good summary of the structure development history isgiven in Ref. [12]. There are three requirements on the
structure design: first it must transfer the rf energy to thebeam efficiently, second, it must be optimized to reduce
the short-range wakefields which depend on the average
iris radius, and third, the long-range transverse wakefieldmust be suppressed to prevent multibunch beam breakup.
To optimize the rf efficiency, the structure cells arerounded, reducing the linac length by roughly 6% whencompared to a simple disk-loaded waveguide like that in
the SLAC linac. To reduce the short-range wakefields, theaverage iris radius was chosen to be a/ λ ~ 0.18, leading toa relatively large group velocity ranging from 12% in thefront of the structure to 2% at the exit. The long-range
transverse wakefield is suppressed through a combinationof detuning the dipole modes and weak damping. The
damping is achieved through the addition of four singlemoded waveguides (manifolds) that run parallel to thestructure and couple to the cells through slots. The signals
from this manifold can also be used to determine thebeam position with respect to the accelerator structure to
micron-level accuracy.
Four 1.8 m long damped detuned accelerator structures(DDS) have been built with the most recent structureusing rounded cells. Measurements of the rf properties ofthe structures have confirmed: (1) the cell fabrication
techniques which can achieve sub-MHz accuracy, (2) the
wakefield models and wakefield suppression techniques,(3) the rf BPMs which are necessary to align the
structures to the beam and prevent emittance dilution, and(4) the rf design codes which have sub-MHz accuracy.

4.5 High Power Damage
Unfortunately, a major problem in the structure design
was encountered in 1999 once sufficient rf power wasavailable to test the full 1.8 m structures at their design
gradient. The NLC design calls for a gradient of 70 MV/m
to attain a center-of-mass energy of 1 TeV with areasonable length linac. This is well below what has beenachieved in the past with numerous short X-bandstructures which reached gradients of over 100 MV/m. In
long structures, damage was observed at a gradient of 4050 MV/m after 500 hours of operation. These structures
have two primary differences compared to the earlier
higher gradient tests, the structure length and the groupvelocity of the rf power in the structure. A possible model
suggests the damage is correlated with group velocity.
To study the gradient limitation, SLAC and KEK began
an intensive program of R&D on structures with different
group velocities and lengths. One of the 1.8 -m structures
was cut in two and the last 1/4 of the structure, where themaximum group velocity is 5%, was tested. This shortstructure reached a gradient of 70 MV/m 10 times fasterthan the full length structures and, after about 1000 hoursof operation, no evidence of rf damage was observed.

Other structures with different lengths and initial groupvelocities of 3% and 5% have been tested withencouraging results. In addition, different cleaning
techniques during fabrication, different processing
methods, and different structure and coupler designs arebeing investigated. On the assumption that the correlationof gradient with group ve locity is correct, design isproceeding on a replacement structure for the NLC which
could be constructed quickly if testing confirms the initial
results. The replacement structure would be shorter, either
60 or 90 cm long, and have a lower group velocity using aphase advance of 150 ° per cell instead of the standard
120 ° to keep the average iris radius large.

5 CONCLUSIONSIn the last few years, the NLC collaboration has focusedon new technology developments and design changes toreduce the facility cost and enhance its flexibility. The
luminosity has increased to reflect the full design
capabilities. While there has been excellent progress ondeveloping prototype rf components, a high gradient
limitation in the accelerator structure design has been
discovered. An intensive R&D program is underway toinvestigate the problem and find a structure design tomeet the NLC goal of 70 MV/m. Extensive changes to thebaseline rf system and to the beam line optics have
reduced the collider footprint while maintaining theenergy reach of the facility. A significant advantage of the
new collider layout is that it is potentially upgradeable toa multi-TeV collider once an appropriate rf system has
been developed.

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Development, KEK 2000-7, SLAC-R-559 (2000).
[4] The NLC Collaboration, N. Phinney, ed., 2001 Reporton the Next Linear Collider, SLAC-R-571 (2001).
[5] N. Phinney, SLC Final Performance and Lessons,
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[6] J. Frisch, E. Doyle, K. Skarpas, Advanced Collimator
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[7] P. Tenenbaum, et al., Studies of Beam Optics andScattering in the Next Linear Collider Post-Linac
Collimation System, LINAC2000, Monterey, CA (2000).
[8] P. Raimondi, A. Seryi, A Novel Final Focus Design for
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[9] J.P. Delahaye, CLIC, a Two-Beam e +e- Linear Collider in
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[10] P. Emma, Cost and Performance Optimization of the
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[11] S. Tantawi, New Development in RF Pulse
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[12] J. Wang, et al., Accelerator Structure R&D for Linear
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