e Future of High Energy Physics

Executive Summary Working Group Environmental Control (T6)
07/18/2001 5:00 pm

Working Group Conveners

Wilhelm Bialowons (DESY), Chris Laughton (Fermilab), Andrei Seryi
(SLAC)

Organizing Committee
Contacts

Gerry Dugan (Cornell), Norbert Holtkamp (ORNL)

Charge

For the next generation of large accelerators, the civil engineering
of accelerator tunnels and associated underground enclosures will be
a major component of the technical challenge of building such
machines. Because of the large scale involved, the engineering will
be required to be as cost-effective as possible, and issues such as
ground motion and artificial sources of vibration in the environment
will need to be carefully considered. Installation and alignment of
the machine components will be tasks of unprecedented scope, and will
require unprecedented precision. Examine in detail the most important
and most difficult aspects of these challenges, both from the point
of view of performance and cost-effectiveness. In particular, identify
what the site requirements are for the different machines under discussion
(NLC, TESLA, VLHC, Muon source), and describe how tunneling methods
are affected by them. Identify, for the different types of accelerators,
the different length scales that are involved in defining the alignment
tolerances, and what are the tolerances over that length scale. Specify
the R&D efforts needed to define the scope of the most critical
challenges, and prioritize the efforts, in terms of the potential to
provide maximal performance and/or cost-effectiveness. Establish a technology-limited
time line, and the resource requirements, for the most important of
these efforts.

Speakers

Fred Asiri, SLAC;  Ralph Assmann, CERN;  Wilhelm Bialowons,
DESY;   

Reinhard Brinkmann, DESY;  Phil Burrows, Oxford;  John Cogan,
SLAC;  Clay Corvin, SLAC;  Bill Foster, FNAL; Joe Frisch,
SLAC;  Peter Garbincius, FNAL;  Lindemar Hänisch, DESY; 
Linda Hendrickson, SLAC;  Vic Kuchler, FNAL;  Joe Lach, FNAL;
Chris Laughton, FNAL; Catherine LeCocq, SLAC; Tom Mattison, UBC; Rainer
Pitthan, SLAC;  

Johannes Prenting, DESY; Armin Reichold, Oxford; Michael Schmitz, DESY;  

Andrei Seryi, SLAC; Nick Simos, BNL; Steve Smith, SLAC; Peter Tenenbaum,
SLAC

Tunneling
experts (attended the workshop during July 9-10)

Robert Bauer                                     
Illinois State Geological Survey

Philip Frame                                                   
Consultant geophysicist

Donald Hilton                                          
Donald Hilton & Associates

Dennis Lachel                                         
LACHEL & Associates, Inc.

Dave Neil                                                          
NSA Engineering, Inc.

Lars Babendererde                              
Babendererde Ingenieure GmbH

Toby Wightman       American Underground
Construction Association

 

     Scope.
For the next generation of large accelerators, the civil engineering
of accelerator tunnels and associated underground buildings will be
a major component of the technical challenge of constructing such machines.
Between a sixth and a half of the total costs for these machines must
be used for the civil engineering. Because of the large physical scales
of these machines the engineering will be required to be as cost-effective
as possible, and because the considered beam sizes are of nanometer
scale, issues such as structural and thermal stability, ground motion
and artificial sources of vibration in the environment will need to
be carefully studied. The working group concentrated on tunneling, ground
motion, stability, alignment and environmental issues.

      Ground motion. Known information on ground
motion (spectral, correlation) suggests that the considered machines
(NLC, TESLA, VLHC, Muon source) are feasible. Particular concerns for
each of the machine are summarized below.

     In the VLHC the main effect of ground motion
is emittance growth; for the high energy stage, the rms uncorrelated
motion of 0.3nm above ~250Hz would result in doubling the emittance
in ~2.5 hours. This is still a modest growth rate in comparison with
the one for TMCI and resistive wall instabilities that would need to
be cured by feedbacks. The natural ground motion in deep tunnels is
much smaller than 0.3nm above ~250Hz, the concern for VLHC is not the
natural ground motion, but vibrations that may be created by equipment
installed in the tunnels, the enhancement of vibrations by girders and
internal mechanics of cryostats. These issues need to be addressed in
design and further engineering tests. 

     In linear colliders the primary concern is
beam offset at the IP induced by ground motion. In the TESLA and NLC
designs, the tolerance for uncorrelated motion of quadrupoles is about
10nm, though the relevant frequency range roughly defined as f>F<sub>rep/20
is different (f>0.2Hz for TESLA and f>6Hz for NLC). For the NLC
case, even in modestly quiet sites, the motion is below these tolerances.
For TESLA, due to low repetition rate of collisions, the motion, even
in quiet sites, may reach the tolerance limit. However, due to large
separation between bunches, a correction within a bunch train is possible
for TESLA.  An issue of concern for NLC, and to a lesser extent
for TESLA, is cultural noise that may greatly increase vibration in
the tunnel. In an urban area, a deep tunnel solution appears to be the
best alternative. Local geologic factors (soil and rock stiffness, structure
and water table) will strongly influence the in-tunnel vibration characteristics.
Site-specific models of vibration propagation need to be studied in
more detail. In terms of slow ground motion (minutes to months), the
impact on NLC performance is more serious than on TESLA due to higher
RF-frequency. Nevertheless, measured amplitudes are tolerable for NLC
with a shallow site in glacial till being the most critical case. Studies
are planned that would clarify this conclusion.

      Site criteria and technical requirements.
High Energy Physics frontier accelerators are large and complex. Ideally,
they should be constructed close to an existing laboratory site. The
environmental impact of the project is minimized for a tunnel solution
rather than a cut and cover that would involve greater surface disruption.
In many respects, the tunnel design requirements for the beamline housings
are not unlike the requirements for underground rail or metro tunnels.
However, some key requirements, related to stability and watertightness,
are more stringent than those normally associated with underground design.
Meeting such criteria could be difficult to achieve in some ground units
and may require design and construction mitigation measures that are
not currently accounted for within in the framework of the pre-project
plans. Better knowledge of key design parameters of certain ground units
is necessary in order to be able to evaluate, with some confidence,
the types of design mitigation measures that will be needed to meet
stability and watertightness requirements.

     Subsurface ground conditions. None of the
projects have performed site investigations of the subsurface conditions
(borings, seismic work or laboratory testing) along a specific tunnel
alignment. At present, TESLA is the only project that has selected a
tunnel alignment. Site-specific investigation of this alignment is scheduled
to start soon. Confidence in ground conditions along the TESLA tunnel
route is already fairly high given the relatively large amount of existing
geologic, geotechnical and construction reference data available in
the Hamburg area. Based on this data, site conditions along the alignment
are projected to be similar to those encountered during the construction
of HERA. There is only a limited amount of geological, geotechnical
and construction data available to describe some of the ground units
in which the proposed NLC and VLHC tunnels will be sited. For these
ground units there is a need for additional geotechnical data to be
gathered before realistic plans and costs for excavation and tunnel
construction can be developed with confidence. Geotechnical data and
design studies are needed in the following key areas: For the California
and Illinois Tunnels sited in Expansive Shales: The impact of swelling
pressures and/or displacement on the excavation, arch support and foundations
of beamline housings needs to be studied. For the VLHC tunnels sited
in St. Peter Sandstone: The impact of groundwater, in situ stresses
and presence of abrasive minerals on the excavation and support of beamline