摘要:

Hydrogen‐ and deuterium‐fueled glow discharges are used for the initial conditioning of magnetic fusion device vacuum vessels following evacuation from atmospheric pressure. Hydrogenic glow discharge conditioning (GDC) significantly reduces the near‐surface concentration of simple adsorbates such as H2O, CO, and CH4, and lowers ion‐induced desorption coefficients by typically three orders of magnitude. The time evolution of the residual gas production observed during hydrogen‐glow discharge conditioning of the carbon first‐wall structure of the TFTR device is similar to the time evolution observed during hydrogen GDC of the initial first‐wall configuration in TFTR, which was primarily stainless steel. Recently, helium GDC has been investigated for several wall‐conditioning tasks on a number of tokamaks including TFTR. Helium GDC shows negligible impurity removal with stainless steel walls. For impurity conditioning with carbon walls, helium GDC shows significant desorption of H2O, CO, and CO2; however, the total desorption yield is limited to the monolayer range. In addition, helium GDC can be used to displace hydrogen isotopes from the near‐surface region of carbon first‐walls in order to lower hydrogenic retention and recycling.

ISSN:0094-243X    

DOI:10.1063/1.39072

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Techniques for cleaning and conditioning the vacuum vessel of the Advanced Toroidal Facility (ATF) and its internal components are described. The vacuum vessel cleaning technique combines baking to 150 °C and glow discharges with hydrogen gas. Chromium gettering is used to further condition the system. The major internal components are the anodized aluminum baffles in the Thomson scattering system, a graphite‐shielded ICRF antenna, two graphite limiters, and a diagnostic graphite plate. Three independent heating systems are used to bake some of the major components of the system. The major characteristics used for assessing cleanliness and conditioning progress are the maximum pressure attained during bakeout, the results of gas analysis, and revelant plasma parameters (e.g., time to radiative decay). Details of the various cleaning and conditioning procedures and results are presented.

ISSN:0094-243X    

DOI:10.1063/1.39063

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

It has been found necessary to perform a series of first‐wall conditioning steps prior to successful high power plasma operation in the Tokamak Fusion Test Reactor (TFTR). This series begins with glow discharge cleaning (GDC) and is followed by pulse discharge cleaning (PDC). During machine conditioning, the production of impurities is monitored by a Residual Gas Analyzer (RGA). PDC is made in two distinct modes: 1) Taylor discharge cleaning (TDC), where the plasma current is kept low (15–50 kA) and of short duration (50 ms) by means of a relatively high prefill pressure, and 2) aggressive PDC, where lower prefill pressure and higher toroidal field result in higher current (200–400 kA) limited by disruptions at q(a)≊3 at≊250 ms. At a constant repetition rate of 12 discharges/minute, the production rate of H2O, CO, or other impurities has been found to be an unreliable measure of progress in cleaning. However, the ability to produce aggressive PDC with substantial limiter heating, but without the production of X‐rays from runaway electrons, is an indication that TDC is no longer necessary after ≊105pulses. During aggressive PDC, the uncooled limiters are heated by the plasma from the bakeout temperature of 150 °C to about 250 °C over a period of three to eight hours. This limiter heating is important to enhance the rate at which H2O is removed from the graphite limiter.

ISSN:0094-243X    

DOI:10.1063/1.39064

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Helium glow wall conditioning (HeGWC) is the primary technique for conditioning the DIII‐D walls which are composed of Inconel and graphite. Presently, 40% (31 m2) of the DIII‐D plasma facing surfaces consist of graphite tiles covering the inside, top, and bottom DIII‐D walls which are the high heat flux surfaces for divertor and inside wall limiter discharges. HeGWC with Twall≤50 °C is effective both in removing lowZimpurities and desorbing hydrogenic particles from this large graphite ‘reservoir.  The effective desorption of hydrogen species from the graphite has led to lower recycling and improved particle control during tokamak discharges.We will first discuss the DIII‐D glow discharge apparatus, the characterization of the glow discharge, the dependence of particle removal rates on the glow parameters, particularly electrode voltage and the duration of the glow session, and the subsequent discharge particle balance. After installation of graphite tiles covering 40% of the first wall (prior overage was 9%, primarily in the lower divertor region), previously successful discharge cleaning and wall conditioning techniques in hydrogen failed to give reproducible high‐quality discharges. With HeGWC, H‐mode discharges were again reproducibly obtained. The main advantages of HeGWC during this phase of initial conditioning were: (1) disruption recovery during tokamak operations by removal of impurities such as CO, and (2) reduced fueling from the walls, allowing reproducible burn‐through and plasma density control.Finally, the experience of routinely applying HeGWC beforeeverytokamak discharge will be presented. Both impurity behavior and improvements in tokamak operation will be discussed. These improvements include: better lowqand high &bgr; operation, faster disruption recovery and more reliable high current operation, a technique for avoiding locked modes which lead to discharge termination, and lower density discharges. In addition, the parameter space in which DIII‐D operates has been significantly expanded with the achievement of inside wall limiter H‐mode and Ohmic H‐mode discharges.

ISSN:0094-243X    

DOI:10.1063/1.39065

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Conditioning of the plasma facing surfaces of a fusion device is a necessary prerequisite for the generaton of pure, hot and stable fusion plasmas. Thin layers of carbon or of boron containing carbon deposited plasmachemically on the entire inner surfaces of a tokamak have proven to be a very effective technique for wall prehandling.Radiofrequency assisted dc glow discharges (RG‐discharges) in a throughflow of appropriate gases are a flexible tool to produce the reactive species for plasma assisted deposition of thin films. The presence of a well defined cathode sheath yields sufficient homogeneity of the deposits even in the complex geometry of fusion devices. The low operation pressure of RG‐discharges (≲10−3mbar) leads to a high kinetic energy of the ionic species impinging on the surface, which is significant for film properties such as gas content, hardness, and adhesion to the substrate.The carbonization technique, i.e. the deposition of amorphous hydrogenated carbon (a‐C:H)‐layers and the boronization which leads to boron containing a‐C/B:H films as they have been developed for the J¨ulich tokamak TEXTOR are shortly described. The specific properties of the RG‐discharges for film deposition are outlined and some material properties for plasma surface interaction are discussed. The reductioon in the impurity concentration of tokamak discharges following carbonization and boronization will be addressed.

ISSN:0094-243X    

DOI:10.1063/1.39073

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The energy distribution of ions incident on the grounded surface (cathode) of a direct current (DC) glow discharge is measured. This incident energy determines the effectiveness of glow discharge cleaning in removing gases and impurities from the vessel surface. We have found that the incident ion energy falls significantly below the anode potential when the mean free path for charge exchange is less than the width of the cathode sheath which is approximately the Child‐Langmuir sheath width. The DC discharge provides a current density at the grounded surface of typically 17 micro‐Amps/cm2. A gas mix of deuterium and argon at pressures up to 40 mTorr is ignited to form the plasma. A mass and energy analyzer measures the energy distribution of the ion stream which passes through an aperture in the center of the electrically grounded surface.

ISSN:0094-243X    

DOI:10.1063/1.39074

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Protium‐rich carbon films were deposited by an RF‐assisted DC glow‐ discharge in methane gas. The desorption characteristics of the trapped protium by ion bombardment in a subsequent glow‐discharge were studied using partial pressure analysis of the desorbed species. Desorption by both helium and deuterium glow‐discharges was investigated. In a helium glow, protium was released primarily as H2and in a deuterium glow, as H2and HD. The desorption rate over time cannot be described by a single e‐folding rate. A desorption cross‐section was estimated by fitting an exponential to the data measured near the beginning of the desorption run sequence. At a bias voltage of 350 V, both D and He ions show a near identical desorption cross‐section of ∼7.5×10−17cm2. Deuterium glows were found to desorb ∼4 times more protium than those from helium glows at comparable operating conditions.

ISSN:0094-243X    

DOI:10.1063/1.39075

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The performance of a storage ring is measured in terms of beam lifetime, beam size, beam stability and recovery of normal operating pressure. In order to insure successful operation, one starts with the proper material selection, followed by the desired conditioning of all components before they are installed in the storage ring. Then the entire ring is leak‐checked and thoroughly baked‐out. During the bake‐out, all gauges, mass spectrometers and pumps are degassed and conditioned. After cool‐down the average pressure in the ring reaches the 10−10Torr range with hydrogen consituting more than 90% of residual gas. The final clean‐up, known as beam commissioning, is done by the particle beam itself. Throughout the above steps, partial pressures of main gases are monitored in order to assess the progress in vacuum conditioning.In this paper a review and discussion of the following will be presented:a. Chemical cleaning techniques for the most commmonly used metals (aluminum, stainless steel and copper); b. RF and DC glow discharge treatments and their effect on both outgassing and photon stimulated desorption; c. Reactive gas cleaning; d. RF cavity conditioning including multipactoring suppression; e. Final bake‐out; f. Beam commissioning.

ISSN:0094-243X    

DOI:10.1063/1.39076

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A technique was developed to clean and condition low‐ and medium‐energy accelerators by alternating between a glow discharge with hydrogen gas and a glow discharge with oxygen gas. The technique was initiated on a 2.5‐MV Van deGraaff accelerator and has been used on both lower‐energy (down to 10‐keV) and higher‐energy (up to 10‐MV) accelerators with equal effectiveness. The conditioning time for attaining the nominal maximum voltage on the 2.5‐MV accelerator was reduced from about ten days using pumping and voltage conditioning to one day using the glow discharge technique. After a glow discharge conditioning sequence, the 2.5‐MV accelerator could be operated effectively at energies of ≤3.4 MV. In addition, the accelerator tube life was found to be significantly longer than expected. The technique is described, and safety considerations are discussed.

ISSN:0094-243X    

DOI:10.1063/1.39066

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The optimization of a chemical cleaning method for Al alloy vacuum chambers using as criteria: surface analysis, thermal outgassing, electron, X‐ray and synchrotron radiation induced neutral gas desorption, is described.The results of the vaccuum conditioning of the 128 radio‐frequency (RF) accelerating cavities for the CERN LEP e+e−storage ring are presented. The gases desorbed with RF were H2, CH4, CO and CO2and their decrease with running time recorded.Argon glow discharge cleaning of stainless‐steel UHV chambers revealed that up to 77 monolayers of gas—mainly CO—could be desorbed from the surface. Measurements of the so‐called roughness factor—the real surface area seen by the adsorbed gas—gave numbers as high as 15 for stainless‐steel, thus only about 5 monolayers of gas are desorbed.In a dedicated beam line at the DCI storage ring at the LURE Laboratory, Orsay, France, synchrotron radiation induced neutral gas desorption from baked Al alloy and stainless‐steel chambers was measured under controlled conditions.The gases desorbed were H2, CH4, CO and CO2and with time the H2and CO desorption descreased as D−1/2where D is the radiation beam dose in mA hours. A model of gas diffusion from near surface layers and from the bulk to the surface which determined the desorption characteristics was proposed. This model well described the behavior of the desorption for H2.Measurements of the photoelectron currents produced at 11 mrad glancing angle of incidence and also at normal incidence, when compared with calculation, showed differences which could be explained if the photons with energies below about 1.5 keV were reflected at 11 mrad with a reflectivity approaching 1.In a special test chamber equipped with sets of photoelectron probes, it was established that, at glancing angles of incidence of 11, 20 and 40 mrad, up to 20% of the synchrotron radiation is scattered around the vacuum chamber from the point of impact.

ISSN:0094-243X    

DOI:10.1063/1.39067

出版商:AIP    

年代:1990

数据来源: AIP