PEARL

Pan-Oceanic Environmental and Atmospheric Research Laboratory:
A Consortium Proposal for a National Facility for Marine and Atmospheric Research

INTRODUCTION

The growing population of the earth and the attendant human activities are placing increasing burdens on the earth's atmosphere and coastal regions of the ocean. Knowledge of this anthropogenic loading causing pollution and its capacity for environmental change is critical to global climate studies. In addition, loading of the atmosphere and the upper ocean affects performance of optical systems used for civilian and military operations. Further, recognizing the need to eliminate or minimize the effects of anthropogenic pollutants on the atmosphere and upper layers of oceans, policy makers have often rushed to judgement without adequate factual information regarding the details of sources and sinks and the complex interactions among the anthropogenic and naturally occurring pollutants. Often this shortcoming is a direct result of the lack of availability of appropriate and efficient schemes for obtaining the requisite data. However, recent developments in optical technology present an exceptional opportunity for a substantial improvement in the ability to detect and understand the complex interactions among various compounds in the marine and atmospheric environment. These developments will also permit the study of processes taking place in the high atmosphere and upper ocean that have heretofore been inaccessible to scientific study.

We, a consortium of universities and industries, propose to construct, deploy and manage a unique mobile laboratory facility for the study of the upper ocean and atmosphere. We have named this unique seagoing mobile platform PEARL (Pan-Oceanic Environmental and Atmospheric Research Laboratory) and propose that it be designated a national facility for the study of the ocean and atmosphere by members of the consortium institutions and other interested scientists. We expect that this facility be assembled from components that were largely acquired and/or constructed for other purposes and that are now in storage. In this way we can build the laboratory at a total construction cost that is less than one fourth of the cost that would be necessary were the facility to be constructed from scratch.

The purpose of the PEARL is to increase our understanding of the scattering and attenuation of light in the ocean and atmosphere and to improve our understanding of the dispersal and reaction of anthropogenic and naturally occurring compounds in the earth's atmosphere, to identify the sources and sinks of these materials in both the atmosphere and the upper ocean, and to increase our systematic understanding of the circulation, biology, and mixing within the upper 100 meters of the ocean. We would also expect to provide a real test and evaluation of the integrated optical technologies and identify areas where these technologies need improvement or modification. In addition, we expect to address several key issues in optical oceanography and atmospheric physics that are of keen interest to the U.S. Navy and other branches of the Department of Defense.


Figure 1. Schematics of the PEARL platform indicating some of the research capabilities.

The heart of our proposed effort will be an existing free electron laser light source (FEL) that will be installed and operated from an existing ship (Figure 1). This mobile platform would be able to attack outstanding scientific problems in a number of different environments as diverse as the origin of noctilucent (night luminous) clouds in the polar upper mesosphere (altitude of 80 to 90 km), the concentration of water vapor in the stratosphere, the dispersal of Arctic haze, the development of the Antarctic ozone hole, the sources and dispersion of Los Angeles basin smog, and the effect of volcanic emissions on the albedo and temperature of the earth. Along the track between these remote sites, or as targeted studies in and of themselves, the facility could gather precise information about the bathymetry of coastal regions to a depth of 200 meters, the distribution of phytoplankton and other particulate matter in the upper ocean, the variation of the optical properties of the sea in various environments, the variation of water vapor in the atmosphere as a function of height and lateral position, and perhaps even contribute to the understanding of the recruitment of fish stocks for species of fish currently under substantial fishing pressure.

A FEL aboard a ship promises to be a revolutionary tool for environmental research for it would have the ability to project beams over large expanses of ocean or over basin sized land regions. It would have an immediate application to global change research. The PEARL system's studies of the atmosphere would provide a cost effective source of unambiguous critical data that cannot be obtained by current means. The same unprecedented capabilities could also be applied for other remote sensing applications such as finding manufacturing sites for illicit drugs and weapons of mass destruction, generating nearly real-time maps of the air pollution over large urban areas and establishing the variation in atmospheric moisture content in the vicinity of the land sea interface.

Remote sensing capabilities of the FEL aboard the PEARL are ideally suited for a variety of defense related applications both in peace time as well as in potential conflicts. There is now an increased emphasis on deployment of remote detection and characterization systems for identifying biological and chemical agents of warfare.

The cost of the PEARL will be held down by using an existing ship, FEL hardware contributed by Rockwell International, a surplus aerostat system, etc. The proposed PEARL will have "reach" that will provide many kinds of data that are not accessible with "passive" measurements from satellites and will provide data that unlock the meaning of satellite images.

Considerable effort and expense already have been expended to develop the facilities that are proposed to be incorporated in the PEARL. It is only fitting, therefore, that the agencies that were responsible for the initial development of the individual hardware items should also benefit. Thus we propose that several areas of Navy and DOD relevance also be addressed in the development and deployment of the proposed facility.

RATIONALE

The world population has doubled since 1950. It stands at more than 5.8 billion as of April 1, 1998 and is growing at a rate of 85 million people per year. This growth coupled together with increased per capita consumption of energy and an increasing rate of utilization natural resources has created an exponentially increasing burden of anthropogenically generated byproducts on our environment. These anthropogenic products and a variety of naturally occurring chemical compounds interact in a very complex way in the earth's atmosphere and its oceans. Understanding this complexity demands a detailed and often simultaneous measurements of many key compounds in various stages of their evolution to provide a complete and systematic understanding of the impact of various human activities. These impacts lead to both local and global climate changes. In-situ measurement techniques are reasonably well developed but they suffer from a cost and time disadvantage when we begin to focus on large land and sea areas and volumes.

For these studies, optical techniques provide mechanisms by which remote sensing over large land and sea masses can be carried out effectively. Recent developments in optical technology present an exceptional opportunity for a substantial improvement in the ability to detect and understand the complex interactions between anthropogenic compounds in the marine and atmospheric environment. It is the goal of this proposal to better understand the attenuation, scattering and fluorescence of light over a wide spectral range (300 nm - 20 Ám) in the upper ocean, marine boundary layer, and the atmosphere. Additional goals are to use the light for probing the vertical structure and spatial and temporal variability of biogenic process in the upper ocean, to better define the anthropogenic loading in the atmosphere and coastal environments, and to provide insights into the reactions that remove these materials from the atmosphere and upper ocean. Concurrent observations of the upper 200 to 500 meters of the ocean will enable studies of bathymetry and natural circulation in coastal regions and in general contribute to the understanding of the distribution of marine life and particulate material in the upper ocean.

Scattering and attenuation of natural and artificial light in the marine boundary layer (MBL) are of concern for both civilian and military operations. Atmospheric attenuation adversely affects laser ranging, free space optical data communications, remote sensing and visibility. The main factors in the atmospheric attenuation are aerosol composition and abundance, Rayleigh scattering, Mie scattering, water vapor content, and temperature inhomogeneity. Often aerosols are the most uncertain factor in modeling optical attenuation in the visible and near infra-red (IR) region. In the upper ocean and coastal areas, the light scattering and attenuation affect underwater visibility, bathymetry, underwater laser communication, mine hunting as well as photosynthesis by phytoplankton - the primary carbon fixing process in the ocean. The variability in the optical properties of the upper ocean are regulated by the amount and quality of particulate and dissolved components resulting from phytoplankton production in the ecosystem. There is a tight coupling between the optical and ecosystem properties of the oceans.

The detailed and simultaneous observation of the vertical structure of the ocean and the atmosphere in real time are not possible with existing instruments, including light detection and ranging (LIDAR) systems because of a combination of constraints imposed by low power and limited wavelength tunability. The proposed PEARL with a powerful tunable laser will enhance our capabilities in marine and atmospheric research.

PROPOSED PROGRAM

The proposed technical and scientific program is too large and complex for any single institution to undertake, but through a consortium incorporating the talents several institutions, the diverse topics mentioned above can be addressed through the new national resource. Scientific and engineering expertise in four institutions have been brought together to exploit this new and unique capability. We believe that solutions to many diverse topics from the set described below can come from the observations that this new facility will make possible and that many other uses will develop as scientists from other institutions become more familiar with its battery of techniques. A summary of the proposed technology and methodology, the initial scientific problems to be addressed, and a model management structure are discussed in the white paper.

[C. Helsley, S. Sharma, R. Burke and K. Patel (1996) PEARL: Pan-oceanic Environmental and Atmospheric Research Laboratory; A Consortium Proposal for a National Facility for Marine and Atmospheric Research, pp. 27 (unpublished). S.K. Sharma, C.E. Helsley, R.J. Burke, D.M. Tratt, R.L. Collins, and C.K.N. Patel (1997) Ship-Based Free Electron Laser (FEL) Lidar for Oceanic and Atmospheric Research, in N.K. Saxena (Ed.), Recent Advances in Marine Sciences and Technology, 96 , pp. 191-204, PACON International, Honolulu, HI.]

Characteristics of the PEARL FEL

Free-electron lasers use gain media that are not subject to the same restrictions as conventional lasers. In a FEL, perturbation of a relativistic electron beam by a periodic magnetic field generates tunable laser light. The optical wavelength is determined by the electron energy and the period and strength of the magnetic field, and tunability is becoming a normal operating feature of many FELs. The FEL wavelength is given by where is the magnetic field wiggler periodicity, is the relativistic energy of the electrons, and is a function that determines the dependence of the FEL wavelength on the magnetic field of the wiggler.

The pulse format of the laser beam mimics that of the electron beam produced by the accelerator. Using the existing 3 GHz RF accelerator, the PEARL's output will be a train of "micro pulses" of a few picoseconds duration within a "macro pulse" whose duration and repetition rate match the application of the RF power to the accelerator (Figure 2). The use of a high energy accelerator requires that the FEL be contained within a radiation shielded environment.

  
Depending on the research objectives that they support, operational FELs have been designed to produce laser output in selected spectral ranges from the ultraviolet to the far infrared. For the present program, an existing accelerator will provide the range of electron beam energy needed to tune the FEL from the near ultraviolet to the far infrared. This accelerator system will be configured for the PEARL to allow rapid (up to 1.5 GHz) wavelength agility over substantial wavelength ranges. This is accomplished by inserting a short section of the electron accelerator which follows the main accelerator (Figure 3). The short section provides of energy to the electron beam. This agile accelerator will be placed inside the optical resonator cavity, just upstream from the in-line wiggler magnet, to eliminate a need for complicated and slow adjustments of the electron beam transport.
If the radio frequency used in the short accelerator is the same as that of the main accelerator either in phase or out of phase, we can get the entire macropulse energy either shifted up or down from the undisturbed energy. Thus alternate macropulses can have differing FEL wavelength. One can also excite the auxiliary accelerator at half the frequency of the principal accelerator. This would give alternate micropulses whose wavelengths are shifted up and down from the undisturbed FEL wavelength. And in yet another manner, one can excite the auxiliary accelerator using a frequency which is, say, 10% higher than that used in the primary accelerator. This will produce a series of micropulses, each of which has slightly different FEL wavelength than the previous one. These cases are shown in Figures 4 (a), (b), and (c).

To cover the full spectral range, the optical resonator will have two sets of mirrors, and therefore gross spectral changes will be substantially slower (1 Hz).

With no conventional gain medium to overheat, a FEL can be designed for extremely high average power. Based on the existing system, the PEARL's FEL will provide average power of 2 kW in the near infrared, somewhat more in the mid-IR, and somewhat less for operation in the visible and ultraviolet parts of the spectrum. Figure 5 shows an estimated power output spectrum of the FEL as a function of its wavelength. Peak powers in the picosecond-long micro pulses will reach gigawatts, and these will average to megawatts over the microsecond-long macro pulses.

For information please contact any one of the following:

Shiv Sharma
Asociate Director
Hawaii Institute of Geophysics and Planetology, SOEST
University of Hawai'i
2525 Correa Road
Honolulu HI 96822
sksharma@soest.hawaii.edu

Charles E. Helsley
Director
Sea Grant College Program, SOEST
University of Hawai'i
1000 Pope Road
Honolulu HI 96822
sg-dir@soest.hawaii.edu

C. Kumar N. Patel
Vice Chancellor (Research)
University of California, Los Angeles
2138 Murphy Hall
Box 951405
Los Angeles CA 90095
patel@research.ucla.edu

Robert J. Burke
Arcata Systems
114 Limestone Lane
Santa Cruze, CA 95060
phone/fax: (831) 420-1772
rjburke@earthlink.net

Also, for more information about the FEL, please visit the UH Free-Electron Laser Group.

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