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In this paper, the authors examine the field emission performance of stainless steel polished to varying degrees, both before and after being coated with a nitrogen-implanted silicon oxynitride layer. The deposition procedure utilizes the simultaneous sputtering of silicon dioxide from a dielectric quartz window and ion implantation of nitrogen from an RF inductively coupled plasma. Here, the scanning field emission microscopy results indicate that prior to being coated, the number of emission sites increased drastically from 12 to more than 300 as the surface roughness increased from 4 to 64 nm, corresponding to polishing with 1-mum diamond paste to 15-mum (600 grit) silicon carbide paper. However, after being coated with nitrogen-implanted silicon oxynitride, all the samples displayed zero to five emission sites at electric field strengths at least three times higher than the uncoated stainless samples. Thus, neither the roughness of the underlying stainless steel nor that of the top surface of the coating had an effect on suppressing field emission. Depth profiling using Auger electron spectroscopy determined that the 0.24-mum-thick silicon oxynitride coating contained approximately 15% nitrogen. Fourier transform infrared spectroscopy of a coated silicon wafer confirmed this stoichiometry and bonding. The technical impact of this work is that coating the large contoured stainless steel surfaces with a nitrogen-implanted silicon oxynitride layer may eliminate the need for expensive labor-intensive polishing procedures currently used in high-voltage electrode structures, such as those found in electron injectors

A new THz/IR/UV photon source at Jefferson Lab is the first of a new generation of light sources based on an Energy-Recovered, (superconducting) Linac (ERL). The machine has a 160 MeV electron beam and an average current of 10 mA in 75 MHz repetition rate hundred femtosecond bunches. These electron bunches pass through a magnetic chicane and therefore emit synchrotron radiation. For wavelengths longer than the electron bunch the electrons radiate coherently a broadband THz ∼ half cycle pulse whose average brightness is >5 orders of magnitude higher than synchrotron IR sources. Previous measurements showed 20 W of average power extracted [Carr, et al., Nature 420 (2002) 153]. The new facility offers simultaneous synchrotron light from the visible through the FIR along with broadband THz production of 100 fs pulses with >200 W of average power. The FELs also provide record-breaking laser power [Neil, et al., Phys. Rev. Lett. 84 (2000) 662]: up to 10 kW of average power in the IR from 1 to 14 μm in 400 fs pulses at up to 74.85 MHz repetition rates and soon will produce similar pulses of 300–1000 nm light at up to 3 kW of average power from the UV FEL. These ultrashort pulses are ideal for maximizing the interaction with material surfaces. The optical beams are Gaussian with nearly perfect beam quality. See www.jlab.org/FEL for details of the operating characteristics; a wide variety of pulse train configurations are feasible from 10 ms long at high repetition rates to continuous operation. The THz and IR system has been commissioned. The UV system is to follow in 2005. The light is transported to user laboratories for basic and applied research. Additional lasers synchronized to the FEL are also available. Past activities have included production of carbon nanotubes, studies of vibrational relaxation of interstitial hydrogen in silicon, pulsed laser deposition and ablation, nitriding of metals, and energy flow in proteins. This paper will present the status of the system and discuss some of the discoveries we have made concerning the physics performance, design optimization, and operational limitations of such a first generation high power ERL light source.

2004 was the centenary of John Ambrose Fleming’s momentous patent on the thermionic diode that can be called the birth of electronics. The “Edison effect” was discovered in 1882; this was later shown to be the result of thermionic emission of electrons from a heated filament into a vacuum. Edison did not make any significant devices based on this discovery, and the effect was ignored for more than 8 years. In 1890 Fleming explained the effect and showed that the thermionic diode could be used as a rectifier. Fourteen years later Fleming filed his 1904 patent on the thermionic diode. It was the first public announcement of the electron tube; this revolutionized the development of radio and led to the invention of the thermionic triode by Lee de Forest in 1906. The background to these events will be described.

Over the last 50 years increasingly large and more sophisticated devices have been designed and put into operation for the study of particle and nuclear physics, magnetic confinement of high temperature plasmas for thermonuclear fusion research, and gravity wave observatories based on laser interferometers. The evolution of these devices has generated many developments in ultrahigh and extreme high vacuum technology that were required for these devices to meet their operational goals. The technologies that were developed included unique ultrahigh vacuum vessel structures, ultrahigh vacuum compatible materials, surface conditioning techniques, specialized vacuum pumps, and vacuum diagnostics. Associated with these technological developments are scientific advancements in the understanding of outgassing limits of UHV-compatible materials and particle-induced desorption effects.

Field-emission studies have been performed on a GaAs wafer and a sample of its stainless-steel (SS) support electrode that are part of a photocathode gun for the 10 kW Upgrade infrared free electron laser at Jefferson Lab. The objective of the studies presented here is to characterize the effect of both the cleanliness of the wafer and the plasma–source ion-implanted layer on the electrode to suppress field emission. Field emission is the limiting factor to achieve the required 6 MV/m at the surface of the wafer. Potential field emitters are first located on the surface of 1 in. diameter samples with a dc scanning field-emission microscope at 60 MV/m, then each emitter is characterized in a scanning electron microscope equipped with an energy dispersive spectrometer. The GaAs wafer was hydrogen cleaned before the study. The results show three emitters caused by indium contamination during wafer handling. The GaAs wafer thus shows good high-voltage characteristics and the need to maintain cleanliness during handling. The SS sample is hand polished with diamond paste to a 1 μm surface finish, then implanted with N2/SiO2 in a plasma–source ion-implantation chamber in preparation for the field-emission studies.Field-emission studies have been performed on a GaAs wafer and a sample of its stainless-steel (SS) support electrode that are part of a photocathode gun for the 10 kW Upgrade infrared free electron laser at Jefferson Lab. The objective of the studies presented here is to characterize the effect of both the cleanliness of the wafer and the plasma–source ion-implanted layer on the electrode to suppress field emission. Field emission is the limiting factor to achieve the required 6 MV/m at the surface of the wafer. Potential field emitters are first located on the surface of 1 in. diameter samples with a dc scanning field-emission microscope at 60 MV/m, then each emitter is characterized in a scanning electron microscope equipped with an energy dispersive spectrometer. The GaAs wafer was hydrogen cleaned before the study. The results show three emitters caused by indium contamination during wafer handling. The GaAs wafer thus shows good high-voltage characteristics and the need to maintain cleanliness durin...

Jefferson Lab is in the process of upgrading the Free-Electron Laser Facility to provide higher output power as well as broader wavelength and timing flexibility. As part of the upgrade, a new optical cavity is being constructed. Using a near-concentric configuration, it will provide high average power (∼10 kW) output using one of three sets of dielectrically coated mirrors. A fourth mirror set will provide broadband tuning throughout the mid-IR, but at a lower average power of ∼1 kW. The new optical cavity offers unique features such as in vacuo active stabilization of the mirror orientation and deformable high-reflector mirrors. The status of the construction of the optical cavity and a review of its capabilities will be presented.

A high-average power (kW) infrared (IR) free-electron laser (FEL) is currently being commissioned for the Jefferson Laboratory FEL User Facility. The IR FEL is driven by a unique superconducting rf linac which is recirculated to recover electron beam power that is not radiated in the FEL. The design and installation of the vacuum system for the FEL involved particular attention to minimizing particulate contamination which could cause problems with the superconducting acceleration cavities and the high power FEL optics. Particulate contamination levels of all vacuum components were monitored during the cleaning process using laser scattering. Cleaning, transport, and installation procedures were developed to minimize the contamination of the complete system. We will summarize a data base we compiled of particulate contamination levels of the various components installed in the FEL vacuum system.

As reported previously [1], Jefferson Lab is building a free-electron laser capable of generating a continuous wave kilowatt laser beam. The driver-accelerator consists of a superconducting, energy-recovery accelerator. The initial stage of the program was to produce over 100 W of average power with no recirculation. In order to provide maximum gain the initial wavelength was chosen to be 5 mu-m and the initial beam energy was chosen to be 38.5 MeV. On June 17, 1998, the laser produced 155 Watts cw power at the laser output with a 98% reflective output coupler. On July 28th, 311 Watts cw power was obtained using a 90% reflective output coupler. A summary of the commissioning activities to date as well as some novel lasing results will be summarized in this paper. Present work is concentrated on optimizing lasing at 5 mu-m, obtaining lasing at 3 mu-m, and commissioning the recirculation transport in preparation for kilowatt lasing this fall.

The International Workshop on “Wall Conditioning in Large UHV Devices” was held on 27 February–3 March at Chavannes de Bogis (Switzerland). The workshop was sponsored by the International Union of Vacuum Science and Technology Associations (IUVSTA) for the purpose of bringing together three communities that are concerned with the most effective means of treating (i.e. “conditioning”) materials for use in high performance ultra high vacuum systems. Practitioners from the field of accelerators, fusion devices, the new gravity wave observatories and semiconductor process equipment attended the workshop.

A model of water adsorption on metal oxide layers and water outgassing from metal surfaces has been developed. The oxide layer is assumed to have porous structure and a pore length (l) distribution of 1/l2. Numerical evaluation shows that the quantity of water adsorbed is logarithmic with time within a certain time range as experimentally observed. The outgassing rate from surfaces with adsorbed water distributed uniformly on the inner surfaces of individual pores is shown analytically to be inversely proportional to time. This result is consistent with frequently observed pumpdown curves.

The effectiveness of glow discharge cleaning (GDC) as a means of decreasing the pump‐down time of vacuum systems was studied. We measured the outgassing rate of an electropolished stainless steel surface after it was exposed to glow discharges fueled by various gases (He, He/1%–3%H2, He/1%–3%CH4, H2, Ar) following a standard 1 h of venting to ambient air. It is shown that He GDC is most effective in reduction of outgassing rate: a He GDC trial with a dose of 0.8 C/cm2 reduces the outgassing rate and subsequently the pump‐down time by a factor of 13. For given glow discharge currents, the outgassing reduction factor is directly proportional to the glow discharge duration. The reduction of post‐GDC outgassing is a result of enhanced desorption of H2, CO, and CO2 by He GDC as observed by residual gas analysis during the discharge. The GDC results are consistent with our previously proposed diffusion models for water outgassing from metal surfaces.

A design has been established for IR and UV FELs within the Laser Processing Consortium's (LPC) program for development and application of high-average-power FELs for materials processing. Hardware prototyping and testing for the IR portion of the system are underway. The driver portion has been designed based on the superconducting radio-frequency (SRF) technology now seeing large-scale application in the commissioning of CEBAF, the Continuous Electron Beam Accelerator Facility, where LPC activities are centered. As of July 1994, measurements of beam performance confirm SRF's benefits in beam quality and stability, which are applicable to high-average-power FELs.

The development of ultrahigh vacuum technology for high‐energy particle accelerators and magnetic fusion devices provided essential contributions to the progress in these two scientific endeavors over the last four decades. Storage rings were first proposed as an efficient means of producing intense high‐energy particle beams by G. K. O’Neill in 1956. The milestone demonstrations of the importance of this invention came with the operation of the ISR device at CERN in 1971, a 2 km long vacuum vessel which operated at 10−12 Torr and the SPEAR electron‐positron collider at Stanford in 1972. These achievements required development of new materials, materials processing techniques, vacuum components, and vacuum instrumentation. UHV development continues through the present design efforts for the 27 km LHC ring at CERN, and numerous high intensity synchrotron light sources worldwide. A similar progression in UHV development preceded the operation of the present generation of large tokamaks built to demonstrate the scientific feasibility of magnetic fusion energy: The JET device in Europe (U.K.), the JT‐60 in Japan, and TFTR at Princeton. Development efforts in UHV compatible first‐wall materials, efficient vessel conditioning procedures, and high capacity pumps are essential for the operation of these devices and continue to be active areas of research for the next generation of fusion demonstration devices.

The outgassing rate from an electropolished stainless steel (type 304) surface is measured after the surface is exposed to H2O vapor at pressures (p0) of 10−3–10 Torr for exposure periods (t0) of 3–600 min. The system is kept at 310, 350, or 390 K during H2O exposures and subsequent outgassing measurements. Our data show that the outgassing rate is proportional to pn0, where n=0.3, 0.26, 0.25 for T=310, 350, 390 K, respectively. The adsorption rate, defined as the quantity of H2O being adsorbed during exposures per unit time, is proportional to 1/t0. Continuing the previous work [Li and Dylla, J. Vac. Sci. Technol. A 11, 1702 (1993)] on correlating the outgassing with the source distribution of sorbed gas, we prove that the practical expression for the outgassing rate, Q(t)=Q10/tα, can be derived from a H2O concentration falling as (1/x)2α−1 with the spatial variable x defined as the distance away from the surface. The observed dependence of the outgassing rate on the exposure conditions is consistent wit...

Jefferson Laboratory currently operates a kilowatt average power, subpicosecond mid-infrared free electron laser as a user facility. We present plans to add an electron storage ring to this facility. Pulses of light from both sources will be synchronized at repetition rates up to 125 MHz to allow pump–probe dynamics experiments to be performed. We will present an outline of the new facility together with the operational parameters of the two sources. The new facility could be on line as a user facility in 2004.

A number of important applications of ultrahigh vacuum technology involve the use of vacuum systems which cannot be baked in situ, e.g., materials processing equipment and various designs of accelerators and fusion devices. A choice of vessel material and surface treatment which minimizes outgassing in unbaked systems has obvious advantages. We report on a systematic comparison of outgassing measurements from type 304 stainless steels and type 6061/63 aluminum alloys after various standard and advanced surface treatments. Correlations were made with high‐resolution secondary ion mass spectroscopy and x‐ray photoelectron spectroscopy profiles of the passivation oxide layer and profilometry measurements of the surface roughness. The observed outgassing rates Q obeyed a power law Q=Q 0 t −n where n varied from 0.94 to 1.27. Over the standard observation period (50 h), the outgassing was dominated (≳85%) by H2O. The absolute outgassing rates varied by only a factor of 2.8 for the stainless steels and by a factor of 1.4 for the aluminum alloys, even though the surface roughness differed by more than two orders of magnitude. The high‐value‐added surface treatments (i.e., electropolishing) did not have significantly lower outgassing in comparison to simple detergent washes.

Numerous measurements of outgassing from metal surfaces show that the outgassing obeys a power law of the form Q=Q10t−α, where α is typically near unity. For unbaked systems, outgassing is dominated by water. This work demonstrates that α is a function of the water vapor exposure during venting of the system, and the physical properties of the passivation oxide layer on the surface. An analytic expression for the outgassing rate is derived based on the assumption that the rate of water diffusing through the passivation oxide layer to the surface governs the rate of its release into the vacuum. The source distribution function for the desorbing water is assumed to be a combination of a Gaussian distribution centered at the interior surface driven by atmospheric exposure, and a uniform concentration throughout the bulk. We have measured the outgassing rate from a clean stainless‐steel (type 304) chamber as a function of water exposure to the chamber surface from

The applicability of the high quality, continuous electron beam from the CEBAF accelerator to free electron laser (FEL) research and photon production has been examined. A simple electromagnetic wiggler utilizing the output of the CEBAF injector (modified to include a new, continuous pulse train, high brightness photoemission gun) would produce output powers of ∼ 1 kW CW in the 45-20 μm wavelength range. With a similar 6 cm wavelength EM wiggler of twice the length with a 400 MeV beam from the North Linac comparable laser output could be achieved at wavelengths around 200 nm. Analysis to date includes conceptual design optimization and more detailed modeling of the optimized parameter space. Program plans as well as designs for the two FELs are presented.

To assure the cleanliness of gas flow components such as mass flow meters and controllers, there must be an initial concentrated effort on the design for cleanliness. The design effort must include all elements, such as materials, sensor, and control valve technologies. The design must then be implemented by an appropriate manufacturing philosophy, facility, and etiquette. We describe in detail the design considerations, cleaning procedures, and quality assurance measurement tools and methods used in the process. This methodology is compared with alternative techniques, and is shown to produce gas flow components that indeed verify the conceptual ideas. Experimental data show the results of surface analysis (profilometry, x‐ray photoelectron spectroscopy, scanning electron microscopy), particulate measurements using both laser particle and condensation nucleus, and moisture analysis (vibrating quartz crystal) instrumentation.

The design for the BPX power and particle handling system consists of a double-null (DN) poloidal divertor with an inner-wall bumper limiter. In addition, a substantial portion of the remainder of the inside of the torus is covered with graphite tiles to withstand high heat flux. In divertor operating modes, the diverted plasma is swept across the divertor plates to reduce the time-average heat loads on the plates. The poloidal divertor is designed both to provide access to H-mode operation and to ensure high recycling in the divertor region, which minimizes impurity production and consequent contamination of the main plasma. The device is capable of both single-null (SN) and DN divertor operation. In addition, an inner bumper limiter is designed to provide an alternative to divertor operation. Additional toroidally and/or poloidally localized limiters are provided on the outboard wall for plasma startup and for protection of the radio-frequency (RF) antennas. In this paper the distribution of plasma power on the first-wall components is discussed. Power handling and the choice of material for the divertor plates and limiters is discussed. A discussion is included of the surface temperature limits and erosion mechanisms. The effect of disruptions on the plasma-facing components follows.more » Finally, tritium retention and particle pumping issues related to the choice of graphite as the first-wall material are discussed.« less

Under some conditions, a very large influx of carbon into TFTR occurs during neutral beam injection into low recycling plasmas (the supershot regime). These carbon 'blooms' result in serious degradation of plasma parameters. The sources of this carbon have been identified as hot spots on the TFTR bumper limiter at or near the last closed flux surface. Two separate temperature thresholds have been identified. One threshold, at about 1650°C, is consistent with radiation enhanced sublimation (RES). The other, at about 2300°C, appears to be thermal sublimation of carbon from the limiter. The carbon influx can be quantitatively accounted for by taking laboratory values for RES rates, making reasonable assumptions about the extent of the blooming area and assuming unity carbon recycling at the limiter. Such high carbon recycling is expected, and it is shown that, in target plasmas at least, it is observed on TFTR. The sources of the carbon blooms are sites which have either loosely attached fragments of limiter material (caused by damage) or surfaces that are nearly perpendicular to the magnetic field lines. Such surfaces may have local power depositions two orders of magnitude higher than usual. The TFTR team modified the limiter during the opening of winter 1989–1990. The modifications greatly reduced the number and magnitude of the blooms, so that they are no longer a problem.

In the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Fusion 26, 11 (1984)], the highest neutron source strength Sn and D–D fusion power gain QDD are realized in the neutral‐beam‐fueled and heated ‘‘supershot’’ regime that occurs after extensive wall conditioning to minimize recycling. For the best supershots, Sn increases approximately as P1.8b. The highest‐Q shots are characterized by high Te (up to 12 keV), Ti (up to 34 keV), and stored energy (up to 4.7 MJ), highly peaked density profiles, broad Te profiles, and lower Zeff. Replacement of critical areas of the graphite limiter tiles with carbon‐fiber composite tiles and improved alignment with the plasma have mitigated the ‘‘carbon bloom.’’ Wall conditioning by lithium pellet injection prior to the beam pulse reduces carbon influx and particle recycling. Empirically, QDD increases with decreasing pre‐injection carbon radiation, and increases strongly with density peakedness [ne(0)/〈ne〉] during the beam pulse. To date, the best fusion resu...

Measurements by moveable Langmuir probes and edge spectroscopy diagnostics have documented the conditioning effect of low density helium-initiated discharge sequences on the Tokamak Fusion Test Reactor (TFTR) edge plasma. Langmuir probe measurements show in general that the edge electron density n{sub e} decreases by less than a factor of 2 while the edge electron temperature T{sub e} doubles. Radial profiles to the plasma boundary show that the density scrape-off length increases somewhat while the temperature scrape-off length decreases substantially. The particle flux density is unaffected. The spectral emission of C 2 decreases by a factor of 2, a much smaller change than that exhibited by the D{sub {alpha}} signal. These results complement previous accounts of the conditioning technique. Comparisons of these He conditioning measurements are made to edge measurements during a deuterium density scan experiment, showing many similarities, and to an existing edge model of the conditioning process, showing qualitative agreement. 20 refs., 5 figs.

This paper reviews three different methods that may be used for measurement of the electron densities and temperatures in low-temperature process plasmas: electrostatic probes, microwave interferometry, and Thomson scattering. Each has certain advantages: probes offer high spatial resolution, but are invasive and can be difficult to interpret; microwaves offer high time resolution and ease of interpretation, but offer only limited spatial information; Thomson scattering gives high spatial resolution and is non-invasive, but is practically restricted to densities above 10 11 cm −3 .

A plasma-enhanced CVD process was used to deposit thin (40–100 nm), amorphous carbon/boron films onto the first wall of TFTR. A series of plasma experiments was performed to test the effect of this first-wall modification on low-Z impurity (carbon, oxygen) behavior, suppression of metallic impurities, post-disruption recovery, hydrogenic recycling, and plasma density limits. The analysis of the plasma performance following the first few boronization attempts on TFTR shows: (1) suppression by a factor of 3-1.5 of carbon impurities immediately after boronization, although this suppression is short-lived (~10 shots); (2) long term suppression of oxygen and metallic impurities by a factor of two; (3) significant suppression of oxygen impurity emission following disruptions and during pulse discharge cleaning; and (4) no significant change in hydrogenic recycling properties in comparison to the pre-existing carbon first wall.

Measurements of the toroidal rotation speed vφ(r) driven by neutral beam injection in tokamak plasmas and, in particular, simultaneous profile measurements of vφ, Ti, Te, and ne, have provided new insights into the nature of anomalous transport in tokamaks. Low‐recycling plasmas heated with unidirectional neutral beam injection exhibit a strong correlation among the local diffusivities, χφ≊χi>χe. Recent measurements have confirmed similar behavior in broad‐density L‐mode plasmas. These results are consistent with the conjecture that electrostatic turbulence is the dominant transport mechanism in the tokamak fusion test reactor tokamak (TFTR) [Phys. Rev. Lett. 58, 1004 (1987)], and are inconsistent with predictions both from test‐particle models of strong magnetic turbulence and from ripple transport. Toroidal rotation speed measurements in peaked‐density TFTR ‘‘supershots’’ with partially unbalanced beam injection indicate that momentum transport decreases as the density profile becomes more peaked. In hi...

Magnetohydrodynamic (MHD) activity within three zones (core, half‐radius, and edge) of TFTR [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. 1, p. 51] tokamak plasmas are discussed. Near the core of the plasma column, sawteeth are often observed. Two types of sawteeth are studied in detail; one with complete, and the other with incomplete, magnetic reconnection. Their characteristics are determined by the shape of the q profile. Near the half‐radius the m/n=3/2 and 2/1 resistive ballooning modes are found to correlate with a beta collapse. The pressure and the pressure gradient at the mode rational surface are found to play an important role in stability. MHD activity is also studied at the plasma edge during limiter H modes. The edge localized modes (ELM’s) are found to have a precursor mode with a frequency between 50–200 kHz and a mode number m/n=1/0. The mode does not show a ballooning structure. While these instabilities have been studied on many other machines, ...

Helium pumping on argon frost has been investigated on Tokamak Fusion Test Reactor (TFTR) neutral beam injectors and shown to be viable for limited helium beam operation. Maximum pumping speeds are ∼25% less than those measured for pumping of deuterium. Helium pumping efficiency is low, >20 argon atoms are required to pump each helium atom. Adsorption isotherms are exponential and exhibit a twofold increase in adsorption capacity as the cryopanel temperature is reduced from 4.3 K to 3.7 K. Pumping speed was found to be independent of cryopanel temperature over the temperature range studied. After pumping a total of 2000 Torr l of helium, the beamline base pressure rose to 2×10−5 Torr from an initial value of 10−8 Torr. Accompanying this three order of magnitude increase in pressure was a modest 40% decrease in pumping speed. The introduction of 168 Torr l of deuterium prior to helium injection reduced the pumping speed by a factor of two with no decrease in adsorption capacity.

The Tokamak Fusion Test Reactor (TFTR) will generate a total of 3 × 10 21 neutrons during its planned D-T operational period. A maintenance manipulator has been designed and tested to minimize personnel radiation during in-vessel maintenance activities. Its functions include visual inspection, first-wall tile replacement, cleaning, diagnostics calibrations and leak detection. To meet these objectives, the TFTR maintenance manipulator is required to be operable in the TFTR high vacuum environment, typically −8 torr, ( −6 Pa). Geometrically, the manipulator must extend 180° either direction around the torus to assure complete coverage of the vessel first-wall. The manipulator consists of a movable carriage assembly, a non-articulated link section mounted on the carriage, and six movable articulated link sections which are driven by electrical actuators. The boom has vertical load capacity of 455 kg and lateral load capacity of 46 kg. The boom can either be outfitted with a general inspection arm or dexterous slave arms. The general inspection arm is designed to hold the leak detector and an inspection camera; it is capable of rotation along two axes and has a linkage system which permits motion normal to the vacuum vessel wall. All systems except the dexterous slave arms are operable in a vacuum.

Thomas Jefferson National Accelerator Facility, Newport News, VirginiaBackground and Objectives: In theory, infrared vibra-tionalbandscouldbeusedforselectivephotothermolysisoflipid-rich tissues such as fat, sebaceous glands, or athero-sclerotic plaques.Study Design/Materials and Methods: Absorptionspectraofhumanfatweremeasured,identifyingpromisingbandsnear1,210and1,720nm.Photothermalexcitationofporcine fat and dermis were measured with a 3.5–5 mmthermal camera during exposure to the free electron laser(FEL) at Jefferson National Laboratory. Thermal damageto full-thickness samples exposed at 1,210 nm through acoldcontactwindow,wasassessedbynitrobluetetrazoliumchloride staining in situ and by light microscopy.Results: Photothermal excitation of fat was twice that ofdermis, at lipid absorption bands (1,210, 1,720 nm). At1,210 nm, a subcutaneous fat layer several mm thick wasdamaged by FEL exposure, without apparent injury tooverlying skin.Conclusion: Selective photothermal targeting of fattytissues is feasible using infrared lipid absorption bands.Potential clinical applications are suggested by this FELstudy. Lasers Surg. Med. 38:913–919, 2006. 2006 Wiley-Liss, Inc.Key words: dermis; fat; tissue opticsINTRODUCTIONSelective photothermolysis (SP) is a widely used treat-ment approach that preferentially heats tissue ‘‘targets,’’whichabsorbapulseofopticalradiationmorestronglythansurrounding tissues [1]. Both source and tissue factorsaffect SP. In general, the source wavelength region andbeam optics are chosen to produce preferential absorptionin the desired targets; and pulse duration or exposure timeis chosen to be nearly equal to the thermal relaxation timeof the desired target structures [2]. Cooling at the tissuesurface can be employed before, during, and/or after lightdelivery to prevent thermal injury of a superficial tissuelayer [3]. Since its conception over 20 years ago, SP hasbeen developed clinically for treating microvascular skinmalformations [4], pigmented lesions [5], tattoos [6],pigmented hair [7], glaucoma [8], laryngeal lesions [9],and other conditions. In all of these applications, visible ornear-infrared lasers or intense pulsed light sources areusedtopumpelectronicabsorptiontransitionsofthetissue‘‘pigments’’ hemoglobin,oxyhemoglobin, melanin,ortattooinks.SPhasnot yet been developed using infrared vibrationalabsorptionbands.Thefar-infraredisknownasa‘molecularfingerprint’ spectral region because strong, narrow vibra-tionalmodesarecharacteristicofchemicalbondstructures.While in theory SP could be performed using far-infraredpulses, the utility of wavelengths greater than about2,500 nm is limited because strong absorption by waterprevents photons from penetrating more than about0.1 mm into tissue. In contrast, the near- and mid-infraredspectral region of about 900–2,800 nm includes the mosttissue-penetratingopticalwavelengths,specificallyaround1,200 nm.Inthisspectralregion,absorptionislowbecausethe photon quantum energy is less than electronic transi-tion energy in most organic molecules, and optical scatter-ing is weak [10]. In this region, there are multiple weakvibrationalabsorptionbands,consistingofovertonesofthefundamental modes responsible for strong far-infraredabsorption.In this study, we investigate the potential for SP of lipid-rich tissues, by pumping vibrational absorption bands.Lipids contain copious CH and CH

The Jefferson Lab (JLab) 10 kW IR Upgrade FEL DC GaAs photocathode gun is presently the highest average current electron source operational in the U.S., delivering a record 9.1 mA CW, 350 kV electron beam with 122 pC/bunch at 75 MHz rep rate. Pulsed operation has also been demonstrated with 8 mA per pulse (110 pC/bunch) in 16 ms-long pulses at 2 Hz rep rate. Routinely the gun delivers 5 mA CW and pulse current at 135 pC/bunch for FEL operations. The Upgrade DC photocathode gun is a direct evolution of the DC photocathode gun used in the previous JLab 1 kW IR Demo FEL. Improvements in the vacuum conditions, incorporation of two UHV motion mechanisms (a retractable cathode and a photocathode shield door) and a new way to add cesium to the GaAs photocathode surface have extended its lifetime to over 500 Coulombs delivered between re-cesiations (quantum efficiency replenishment). With each photocathode activation quantum efficiencies above 6% are routinely achieved. The photocathode activation and performance will be described in detail.

We describe a > 100 Watt broadband THz source that takes advantage of the relativistic enhancement of the radiation from accelerating electrons according to the formula assigned the name of Sir Joseph Larmor[1, 2]. This is in contrast to the typical 1 milliwatt sources available in a laboratory. Specifically, for relativistic electrons the emission is enhanced by the fourth power of the increase in mass. Thus for 100 MeV electrons, for which the mass increases by a factor of ∼ 200, the enhancement is > 109. The experiments use a new generation of light source called an energy recovery linac (ERL) [3], in which bunches of electrons circulate once, but in which their energy is recovered. In such a machine the electron bunches can be very much shorter than those, say, in storage rings or synchrotrons.The Jefferson Lab facility operates in new limits of emission from relativistic particles involving both multiparticle coherence and near-field emission in which the velocity (Coulomb) term in the classical electrodynamical theory becomes as important as the acceleration term (synchrotron radiation).The sub-picosecond pulses of light offer unique capabilities in 2 specific areas, namely time-resolved dynamics, and imaging. High resolution THz spectroscopy has recently revealed sharp vibrational modes for many materials including malignant tissue, proteins, DNA, pharmaceuticals and explosive materials. Energetically the THz range embraces superconducting bandgaps, and regions of intense interest in the understanding of systems in which correlated motions of electrons are important, such as colossal magneto-resistive and high-Tc materials. The very high power levels of the new source will allow non-linear effects to be observed as well as the creation of novel states of materials, including electric-field driven localization[4]. We will give examples of existing work in these areas and present opportunities afforded by the new source.

We report initial lasing results from the IR Upgrade FEL at Jefferson Lab (Proceedings: 2001 Particle Accelerator Conference, IEEE, Piscataway, NJ, 2001). The electron accelerator was operated with low average current beam at 80 MeV. The time structure of the beam was 120 pC bunches at 4.678MHz with up to 750 µs pulses at 2 Hz. Lasing was established over the entire wavelength range of the mirrors (5.5-6.6 µm). The detuning curve length, turn-on time, and power were in agreement with modeling results assuming a 1 ps FWHM micropulse. The same model predicts over 10kW of power output with 10mA of beam and 10% output coupling, which is the ultimate design goal of the IR Upgrade FEL. The behavior of the laser while the dispersion section strength was varied was found to qualitatively match predictions. Initial CW lasing results also will be presented.

Introducing tritium into the plasmas of the controlled fusion program will require stringent control and processing of effluents from the vacuum system. The reduction in the amount of gas required for discharge cleaning of the vacuum vessel during D-T operations is discussed. A steady-state glow discharge at pressures of 5*10/sup -3/ torr (D/sub 2/) for periods of up to 24 h are currently performed as part of the procedure to prepare the TFTR (Tokamak Fusion Test Reactor) vacuum system for high-power pulsing. The present vacuum system, with a speed of 5000 L/s, would generate about 5 mol. of tritium-contaminated effluent per hour. Purification and real-time recycling of the discharge cleaning gas using a Stirling cycle cryopump (10 K to 20 K) in series with a turbopump would reduce the discharge cleaning gas requirements to a single gas filling. The usual charcoal coating on the low-temperature stage of the cryopump will be removed to prevent hydrogen isotope sorption pumping. Impurities such as CO, H/sub 2/O, CH/sub 4/, etc., will be cryopumped on the 80 K and 10-20 K cryo-surfaces. At the termination of glow discharge, the cleaning of a single charge (250 torr-L) of tritium contaminated gas will be transferred to a tritium cleanup system, possibly for recycling. The cryopump is then regenerated and the tritium-contaminated impurities are transferred to the tritium clean up system. >

The original design of the Tokamak Fusion Test Reactor (TFTR) and tritium handling are described. Modifications to these designs inspired by operational experience gained from running TFTR since 1982 with deuterium-deuterium (D-D)-fueled experiments are also presented. The presently installed gas handling systems work well for the D-D phase of operation, but they do not have the level of sophistication needed for tritium injection, tritium-contaminated-gas exhaustion, and gas detritiation for deuterium-tritium (D-T) operations. The pumping and gas-handling systems were designed prior to the operation of TFTR to handle the tritiated process streams anticipated during the DT phase. Since then, it has been found that certain changes and refinements to this design would make tritium handling simpler and safer. Most (90%) of the process gas (deuterium fuel) is pumped by the cryopanels in the neutral beams during operations. This nearly pure elemental hydrogenic gas is radiologically 25000 times more safe to handle than the corresponding amount of tritium oxide. It has been decided to transfer the hydrogenic species from the neutral beam cryopanels into special containers for off-site shipment as elemental tritium rather than as oxide. This reduces the exposure of oil-lubricated pumps from large contact with tritium-laden gas and reduces the risk of exposure to tritium oxide. Conditioning of the torus will be processed by impurity cryopumps, designed to pump only the nonhydrogenic species (impurities) and reduce the total amount of tritium used during conditioning. >

The kW‐class infrared (IR) Free Electron Laser (FEL) at Jefferson Lab had the capability of producing intracavity Thomson scattering of the IR off the electron beam thus producing high average flux, sub‐picosecond x‐rays. We have measured these x‐rays and demonstrated the energy tuneability range from 3.5 keV to 18 keV. The corresponding flux and brightness has been estimated and will be discussed. This year, 2002, the FEL was disassembled and has been reconfigured to produce 10 kW average power IR. We present the estimated x‐ray capabilities for the new FEL and discuss potential applications.

The number of installed and operational /spl beta/=1 superconducting RF cavities has grown significantly over the last two years in accelerator laboratories in Europe, Japan and the U.S. The total installed acceleration capability as of mid-1993 is approximately 1 GeV at nominal gradients. Major installations at CERN, DESY, KEK and CEBAF have provided large increments to the installed base and valuable operational experience. A selection of test data and operational experience gathered to date is reviewed. >

High-performance superconducting radio frequency (SRF) cavities are placed under UHV and low particulate conditions when they go through the surface treatment process, and are kept under these conditions permanently thereafter, both before and during use in the accelerator. The outgassing of the gasket material in the gate valves and the outgassing of the solvents that come in contact with the gasket and the cavity surfaces determine the ultimate pressure that can be obtained in the cavity. A systematic study of gaskets and various solvents was undertaken in order to understand the conditions that lead to a low ultimate pressure in a short period of time. The results with Kalrez, Viton O-rings and solvents like methanol, acetone, isopropyl and freon will be presented. Also, we discuss the effect of a slow or fast initial pumpdown in order to prevent the settlement of particulates due to condensation. >

Conceptual studies of two free-electron lasers (FELs) located at the output of the front end and north linac of the CEBAF (Continuous Electron Beam Accelerator Facility) accelerator are conducted. The high average beam power and the superior electron beam quality produced by the linac yield projections of tunable output power that substantially exceed existing and most proposed sources. The tolerances for most FEL components are not severe but the high optical power requires careful consideration and, perhaps, special optical cavity arrangements and mirror designs. >

Operation of the Burning Plasma Experiment (BPX) with a deuterium-tritium plasma and graphite divertor plates will result in significant retention of tritium in layers of eroded and subsequently redeposited carbon. This trapped tritium is at risk of release during a vacuum accident. An administrative limit of 2 g of releasable tritium has been established for assessment of the environmental impact of such an event. Analysis of the erosion and redeposition process indicates that the 2-g limit will be reached in about 200 full-parameter discharges. Helium-oxygen glow discharge cleaning will be used to remove the redeposited carbon and the trapped tritium. It is calculated that about 90 h of cleaning are required to remove the trapped tritium from 200 plasmas. The cleaning will be done during scheduled maintenance periods every third week. Studies indicate that the tritium inventory can be effectively managed with helium-oxygen glow discharge cleaning. >

CEBAF has developed a comprehensive conceptual design of an industrial user facility based on a kilowatt UV (150-1000 nm) and IR (2-25 micron) FEL driven by a recirculating, energy-recovering 200 MeV superconducting radio-frequency (SRF) accelerator. FEL users-CEBAF's partners in the Laser Processing Consortium, including AT&T, DuPont, IBM, Northrop-Grumman, 3M, and Xerox-plan to develop applications such as polymer surface processing, metals and ceramics micromachining, and metal surface processing, with the overall effort leading to later scale-up to industrial systems at 50-100 kW. Representative applications are described. The proposed high-average-power FEL overcomes limitations of conventional laser sources in available power, cost-effectiveness, tunability and pulse structure.

We have developed a conceptual design for an industrial-use kilowatt UV and IR FEL driven by a recirculating, energy-recovering 200 MeV, 1-5 mA superconducting rf (SRF) electron accelerator. In this paper we describe the accelerator design of this FEL. The accelerator consists of a 10 MeV injector, a 96 MeV SRF linac with a two-pass transport which accelerates the beam to 200 MeV, followed by energy-recovery deceleration through two passes to the dump. Technical challenges include high-intensity injector development, multi-pass energy-recovery operation, SRF modifications and control for FEL operation, development of tuneable, nearly-isochronous, large-acceptance transports, and matching of the beam to the FEL wiggler. An overview of the accelerator design is presented.

Jefferson Lab is planning a facility for studying ultrafast dynamical processes using a 1 kW average power IR/UV FEL, and a mode-locked Ti-sapphire femtosecond laser in the visible, combined with a 1 nm critical wavelength electron storage ring. Light pulses from the three sources will be synchronized at 125 MHz for pump-probe studies (including pump-pump-probe and pump-probe-probe options) in chemistry, physics, materials science, medicine and biology. The FEL operates with pulses as short as 300 femtoseconds (/spl sigma/), which will provide the narrow bandwidth pump at high peak as well as average power. The FEL is currently operating and will soon be upgraded to operate at up to 10 kilowatt average power at an extended wavelength range from 300 nm to 10,000 nm. A compact superconducting storage ring, Helios-1, has recently been donated to Jefferson Lab that has a critical wavelength of 10 /spl Aring/ and is capable of storing currents up to 800 mA. In addition to providing spectroscopy capabilities, the storage ring will also support exploratory X-ray lithography R&D, including a precision stepper-aligner for training purposes. The facility, which is expected to become available in 2004, will be described and the capabilities detailed.

Recent work at Jefferson Lab has demonstrated the viability of same-cell energy recovery as a basis for a high average power free-electron laser (FEL). We are now extending this technique to lase at average powers in excess of 10 kW in the infrared. This upgrade will also produce over 1 kW in the UV and generate high brightness Thomson back-scattered X-rays. The power increase will be achieved by increasing the electron beam energy by a factor of four, and the beam current and the FEL design efficiency by a factor of two. Utilization of a near-concentric optical cavity is enabled by the use of very low loss state-of-the-art coatings. The FEL will be placed in the return leg of the electron beam transport, giving a machine footprint quite similar to that of the existing 1 kW IR device. Some features of the upgrade are straightforward extensions of those in the present 1 kW design; others break new ground and present new challenges. These will be described. The required electron beam parameters and the laser performance estimates will be summarized. Changes required in the electron beam transport will be outlined and the optical cavity design briefly reviewed.

Field emission is one of the principal phenomena limiting the operating voltage of practical electron guns. There is interest in developing photoemission cathode based DC electron guns employing cathode field strengths and cathode-anode voltages well above the present state-of-the-art. Such electron sources could provide high brightness, high average current beams for energy recovered superconducting linear accelerators, for applications in next generation light sources, electron cooling, and electron-ion colliders. We have studied the effect of plasma-source ion implantation on the field emission behavior of large area stainless steel electrodes. Our apparatus allows the operation of disc-shaped electrode pairs with 100 cm/sup 2/ uniform field area to 125 kV. The cathode electrode is biased at high voltage, and the anode is electrically isolated, allowing measurement of the field emission current. Electrodes were either mechanically polished, or implanted with nitrogen ions. Two separate ion implanted electrodes have shown negligible field emission at 20 MV/m, and emission between 0.5 and 1.8 pA/cm/sup 2/ during multi-hour runs at 30 MV/m. These electrodes show very little conditioning effect.

At the beginning of the 1990 Tokamak Fusion Test Reactor (TFTR) experimental run, after replacement of POCO‐AXF‐5Q graphite tiles on the midplane of the bumper limiter by carbon fiber composite (CFC) tiles and prior to any pulse discharge cleaning (PDC), boronization was performed. Boronization is the deposition of a layer of boron and carbon on the vacuum vessel inner surface by a glow discharge in a diborane, methane, and helium mixture. The amount of discharge cleaning required after boronization was substantially reduced compared to that which was needed after previous openings when boronization was not done. Previously, after a major shutdown, about 105 low current (∼20 kA) Taylor discharge cleaning (TDC) pulses were required before high current (∼400 kA) aggressive pulse discharge cleaning (PDC) pulses could be performed successfully. Aggressive PDC is used to heat the limiters from the vessel bakeout temperature of 150 to 250 °C for a period of several hours. Heating the limiters is important to in...

(1) For shots 18528 and 20345 the limiter is believed to have had a high recycling coefficient; shot 21790 occurred after modest limiter 'conditioning', which produced some reduction in recycling, though much less than for 'supershots'; shot 22871 is a 'supershot'. These plasma regimes have been described previously [1, 2]. The older work does not include the 1987 corrections to the TVTS [2] electron temperatures (and Te dependent quantities) and so those data cannot be directly compared with the corrected data in the present tables.