Continuous emissionsource covering the 50-300-A band
Stuart Bowyer
We have developed a continuous emission source for use in the soft-x-ray and extreme-UV spectralregions. The source and its characteristics are described.
Introduction
Because of the growing importance of extreme UVradiation, there is considerable interest in high-intensity laboratory sources for this spectral range.A variety of sources have been proposed for thesewavelengths, and many of them are described in theclassic book by Samson.' Most of these sources havesubstantial limitations, especially those that are in-tended for use below 1200 A. We2 and others havedeveloped continuous discharge sources that are stableand maintenance-free and that provide a large num-ber of intense lines at wavelengths down to 300 A.Soft-x-ray sources of the type developed and refinedby Henke and Tester3 are capable of producing sub-stantial amounts of soft-x-ray radiation. However,these sources have severe limitations for use atwavelengths longer than 50 A. For example, muchof the radiation produced is continuum radiation withrather low flux at any specific line. In addition theflux is peaked at shorter wavelengths. When dis-persed by a grazing-incidence monochromator, wave-lengths at first order and many higher orders emergefrom the exit slit, rather than a true monochromaticflux, which is usually desired. Essentially the onlysimple and relatively inexpensive sources availablethat can provide reasonable flux levels at wavelengthsbetween 50 and 300 A are capacitive discharge sourceswith attendant problems of severe electromagneticinterference and plasma discharge sources.
We have developed a plasma discharge source thatproduces a substantial number of lines with high in-tensity in the wavelength band from 50 to 300 A.This device is based on a design described by Finley et
The author is with the Center for EUV Astrophysics, Universityof California at Berkeley, Berkeley, California 94720.
al. 4 That source has undergone some six revisionssince its inception, and the current configuration isdescribed here. It is stable and easy to use andmaintain.
SourceThe source is shown in Fig. 1; a cutaway schematic isshown in Fig. 2. Electrons emitted from a cathodeare constrained to follow magnetic-field lines in tighthelices until they are scattered to an anode. Theseelectrons excite a variety of lines, including both linesintrinsic to the discharge gas and, equally important,lines produced by the excitation of highly ionizedmaterial sputtered from the cathode faces. Theaxial magnetic field is produced by a pair of rare-earthmagnets placed inside the cathode bodies, and themagnetic circuit is completed by six bolts that clampthe unit together. Considerable heat is generated inthe cathode bodies, and cooling water is circulatedthrough them. The anode is also water cooled.
In operation one controls the pressure in the gasdischarge region by pumping on the source whileadjusting the input gas-flow rate. The pressureinside the discharge region is difficult to measure.Relative readings, however, are sufficient for optimiz-ing the discharge. The discharge characteristics ofthis source are pressure dependent. In normal con-ditions the discharge is confined to the central anoderegion. At the high-pressure limit a glow dischargetakes place throughout the lamp. As the pressure islowered a critical pressure is reached below which adischarge cannot be sustained.
As the source is being operated, material is continu-ally sputtered off the cathode faces. However, thegeometry of the source is such that after more than100 h of operation, we observed no metal depositeddownstream of the source. Most of the sputteredmetal is deposited on ridges at the edges of the anode.When the cathode pieces are replaced, the anode
insert can be removed and a clean one can be in-stalled, if necessary.
The longevity of the cathode pieces depends on thematerial used and the operating characteristics of thedischarge. The source is designed to minimize the
MONOCHRCATrACHME
HIGH VOLTAGEGROUND CONNECTOR
ANODE ASSEMBLY
CATHODE INSERT
MAGNET/
HIGH VOLTAGECONNECTOR
CATHODE PLATE
SAFETY PLATE
SAFETY TUBE /
possibility of sputtering through to the water jacketby the addition of replaceable tantalum inserts be-hind the centers of the cathode pieces. When thecathode piece itself is sputtered through, the emissioncharacteristics of the source change dramatically,indicating that maintenance is required.
Output Characteristics
The spectrum shown in Fig. 3 is that of a neondischarge with magnesium cathodes. A magnesiumcathode gives very thorough coverage of the regionfrom 120 to 190 A with extremely bright Mg III linesat 232 A. The spectrum shown in Fig. 4 is that of aneon discharge with aluminum cathodes.
Finley et al.4 characterized some of the operatingcharacteristics of the source. They showed that thepressure dependency of individual lines varies greatlyfor different species. The short-wavelength, alumi-num-line intensities are strongly pressure dependent,dropping to half of their peak value when the pres-sure is increased by only 25%. The Ne iv lines dropto half of their original value, while the Ne iII linesdrop by a third, when the pressure is increased by afactor of 3. The aluminum 234-A line drops offmuch less sharply with pressure than the aluminum161-A line. Given that the gas column between thedischarge region and the entrance slit to the mono-chromator represents < 10-6 of an optical depth andthat the collision frequency of the ions is < 106 s51,self-absorption and collisional deexcitation are of
Wavelength (A)Fig. 3. Spectrum of magnesium and neon.
minimal importance. Thus the change in line inten-sities is probably accounted for by the lowered kineticenergies of the ions and electrons that result from theincreased frequency of collisions. The intensities ofthe metal lines decrease much more rapidly withincreasing pressure than the gas lines because thesputtering rate decreases, lowering the metal vapordensity.
800
.? 600
a)
* 400co
200
300
0
U
200
100
00 200 400 600
Voltage (V)
Fig. 5. Source current as a function of voltage. The gas is neon;
the pressure is 2 x 10-5 Torr.
The best combination for the 50-170-A region hasbeen found to be an argon discharge with aluminumcathodes. Lines between 170 and 200 A are providedonly by magnesium. Above 200 A a neon dischargewith aluminum cathodes is most satisfactory. Linesof argon and neon beyond 300 A are quite intensewith a discharge current of 300 mA or lower. Atthese discharge currents the aluminum sputteringrate is negligible, and the lamp can be used for dayswithout significant erosion of the cathode. Argonwas found to be effective in exciting metallic lines,while increasing the sputtering rate only slightly.
Typical Operating Parameters
The operational parameters of the source depend onthe source gas used, the pressure of the gas in thedischarge area, the applied voltage, the character ofthe high-voltage power supply, the value of the ballastresistor, and the condition of the cathodes. Conse-quently typical operating parameters are difficult tospecify; hence each user must characterize his or herown particular experimental setup. In particularthe character of the radiation is sensitive to thepressure in the discharge region in the source, andthis pressure is intrinsically unknown. As a practi-cal matter one measures the pressure at some pointnear the source and obtains a pressure reading that isrelated to the source discharge pressure by the inter-vening conductance.
Within the context of these limitations we showtypical operating parameters of the source in Figs.5-7. These measurements were made with a freshmagnesium cathode and with neon as a source gas.The pressure was measured in a monochromator at apoint immediately adjacent to the source attachmentflange. The voltages indicated reflect the total volt-
C:
a)
U
00 110 120 130 140
Wavelength (A)
Fig. 4. Spectrum of aluminum and neon.
300
200
100
00.001 Pressure (torr) 0. 0001
Fig. 6 Source current as a function of pressure. The voltagewas set to provide 750 V.
Fig. 7. Voltage as a function of pressure. The voltage was set toprovide 0.3 A.
age drop across the high-voltage power supply andthe ballast resistors.
Conclusions
With a limited number of cathode materials andexciting gases, this source provides nearly 40 usablelines between 50 and 300 A, many of which areavailable from a single cathode-gas combination.The source is quiet, continuous, and stable over mostof the cathode lifetime. It is well suited to calibrat-
ing photon-counting instruments. Although thecathodes eventually fail because of the erosion bysputtering, their longevity is sufficient for even longcalibration runs. When the cathodes do becomeexhausted, the refurbishment procedure is suffi-ciently simple that the source can be back on line in< 30 min.
The author thanks Pat Jelinsky, Dave Finley, andJim Gibson for useful discussions and acknowledgesthe support of a Guggenheim Fellowship and NASAgrant NGR05-003-450.
References
1. J. A. R. Samson, Techniques of Vacuum Ultraviolet Spectros-copy (Wiley, New York, 1967).
2. F. Paresce, S. Kumar, and S. Bowyer, "Continuous dischargeline source for the XUV," Appl. Opt. 10, 1904-1908 (1971).
3. B. L. Henke and M. L. Tester, "Techniques of low energy x-rayspectroscopy," Adv. X-Ray Anal. 18, 76-121 (1974).
4. D. S. Finley, S. Bowyer, F. Paresce, and R. F. Malina, "Continu-ous discharge Penning source with emission lines between 50 Aand 300 A," Appl. Opt. 18, 649-654 (1979).
