Search

The neutron total cross-sections of thulium (169Tm) were measured in the neutron energy region from 0.01 eV to 100 eV by using the time-of-flight method at the Pohang Neutron Facility, which consists of an electron linac, a water-cooled tantalum target with a water moderator, and a 12 m time of flight path. Two thulium plates with different thicknesses were used for the neutron transmission measurement. The background level was determined by using a notch-filter of Co, In, and Cd sheets. The present measurement was compared with the previous ones, and a new set of resonance parameters of 169Tm isotope was obtained from the transmission rate by using the SAMMY code, with a comparison with the recommended parameters by Mughabghab.

The present work has been developed within the frame of the EFDA task “HCD-08-03-01: LH4IT, EU contribution to the ITER LHCD Development Plan” The use of rectangular oversized waveguides in the Main Transmission Lines (MTLs) of the Lower Hybrid Current Drive (LHCD) system of ITER, requires to investigate the problem of bends. The high number of involved waveguides (from 24 to 48) must be also taken into account. Thus, it has to consider not only the best choice in terms of curved framework, but also the proper allocation of all the waveguides. In this context, the principal specifications that characterize the design of the bends are: a) to minimize the reflection of the fundamental TE10 mode; b) to maximize the transmission of the fundamental TE10 mode; c) to minimize the coupling between the TE10 mode and other spurious modes that propagate at 5 GHz. This paper presents an overview about the bend options, and it compares the performances of several frameworks analyzed by using the Finite Element Method (FEM) commercial software, HFSS®. First of all, simple circular trajectory curves with different angulations, are considered. Then, the so called Mitre Bends alternatives are deeply analyzed. These curves are studied by several authors in the mono-modal configuration, with different techniques but the propagation in an oversized environment is a topic not much attended in literature. The only design parameter of the simple circular trajectory bend is the bending radius, so that the design is not flexible; the Mitre Bend structure is at least more flexible than the previous one and it is of great interest to study this type of bend to check the possible advantages. Finally an innovative modified Mitre Bend solution based on a cascade of trapezoidal elements is proposed.

In order to assist the design of the future ITER Lower Hybrid launcher, coupling codes ALOHA, from CEA Cadarache, TOPLHA, from Politecnico di Torino, and GRILL3D (Dr.Mikhail Irzak, A.F.Ioffe Physico-Technical Institute, St. Petersburg, Russia), from ENEA Frascati, have been compared with the initial (3 modules with 8 active waveguides per module) and updated (6 modules with 4 active waveguides per module) Passive-Active Multijunction (PAM) Lower Hybrid antennas. Both ALOHA and GRILL3D formulate the problem in terms of rectangular waveguides modes, while TOPLHA is based on boundary-value problem with the adoption of a triangular cell-mesh to represent the relevant waveguides surfaces. Several plasma profiles, with varying edge density and density increase, have been adopted to provide a complete description of the simulated launcher in terms of reflection coefficient, computed at the beginning of each LH module, and of power spectra. Good agreement can be observed among codes for all the simulated profiles.

A 20 MW Lower Hybrid Current Drive system using an antenna based on the Passive Active Multijunction (PAM) concept is envisaged on ITER for steady state operation, including the current ramp-up phase. In the frame of an EFDA task, a conceptual design of such a system has been performed. This paper gives an insight of the mechanical analysis, modeling and design carried out on two elements of the antenna: the front face, and the RF windows which constitute the first tritium barrier. The front face of the antenna will have to withstand high heat and fast neutrons fluxes directly from the plasma. It will be actively cooled and present a beryllium coating upon ITER requirement. The main issues were to ensure a reasonable margin versus a maximum temperature fixed by safety concerns (650 °C for plasma facing beryllium), and to understand and assess resistance of the assembly while in operation. Analyses were conducted on some variants of this PAM. Manufacturing scenarios considering ITER requirements and involving various techniques (brazing, high isostatic pressure bonding, explosion bonding) were explored, and simulations over subsequent residual stresses were conducted. Analyses showed that surface temperatures do not exceed 500°C, and stresses should not affect the integrity of the antenna mouth. The RF window is a critical component since it also has a safety function. We planed to design a water cooled 5 GHz CW RF “pillbox” window capable of sustaining 500 kW of transmitted power. An RF optimization conducted on this concept allowed to combine good RF properties and low dielectric losses. It resulted in a maximum thermal stress in the ceramic of 31 MPa, well below the static fatigue limit of 50 MPa. The residual stresses resulting from the brazing methods involved in the manufacturing of this multi-material assembly (copper, copper alloys and ceramic) were also analyzed. Both studies allow us to move forward and focus on critical issues, such as manufacturing processes (beryllium, bonding techniques…) and R&D associated programs including test of mock-ups.

A very preliminary design of the Lower Hybrid Current Drive system for ITER was developed in the early years of the 2000's. Many parameters of the system were defined on the base of knowledge and technology of those times. Experimental results obtained in the last few years, like the successful tests of a PAM launcher on FTU and on Tore Supra have definitely indicated this concept as a possible candidate for the LHCD launcher for ITER. The successful development of a prototype klystron at 5GHz with a target RF power of 500kW CW has indicated this power as an upper limit for the present high vacuum electron tubes technology at this frequency. From the physics point of view the ITER operational scenarios have been well defined, while the last experiments have pointed out the effectiveness of the LHCD waves at high plasma density. All these results imposed a revision of the existing design of the LHCD system. The RF power limited to a maximum of 500kW per klystron has determined a new modularity of the whole system and of the PAM launcher. In addition a relatively shorter distance between klystrons and ITER machine suggested the possibility of using rectangular oversized waveguides, excited in the fundamental TE01 mode, for the realization of the Main Transmission Lines (MTLs). Therefore all the main microwave components of the system have been revised on these basis and different and more suitable solutions have been investigated. In particular this paper compares the performances of the two different MTLs, based respectively on circular and rectangular waveguides, describes the main components of the proposed rectangular MTL and analyzes their main microwave characteristics. Companion papers in this Conference cover all the other aspects of the updated design of the LHCD system for ITER. The present revision has been carried out in the frame of the EFDA task “HCD-08-03-01: EU Contribution to the ITER LHCD Development Plan (LH4IT)”.

The extensive design effort for KSTAR has been focused on two major aspects of the KSTAR project mission - steady-state-operation capability and advanced tokamak physics. The steady state aspect of the mission is reflected in the choice of superconducting magnets, provision of actively cooled in-vessel components, and long pulse current drive and heating systems. The advanced tokamak aspect of the mission is incorporated in the design features associated with flexible plasma shaping, double null divertor and passive stabilizers, internal control coils and a comprehensive set of diagnostics. Substantial progress in engineering has been made on superconducting magnets, the vacuum vessel, plasma facing components and power supplies. The new KSTAR experimental facility with cryogenic system and deionized water cooling and main power systems has been designed, and the construction work is under way for completion in 2004.

The higher order modes (HOMs) of RF cavities at the Pohang Light Source (PLS) storage ring cause longitudinal coupled bunch mode instabilities. A longitudinal feedback system (LFS) is introduced to cure these instabilities. As a key component of the LFS, there is a single-ridged waveguide-overloaded cavity for a longitudinal bunch-by-bunch LFS kicker. Since the RF frequency is 500 MHz, the bandwidth of this kicker should be wider than 250 MHz to damp any coupled bunch mode. Also, a higher shunt impedance of the kicker is selected to use a lower power amplifier. One aluminum kicker is fabricated, and the bandwidth, HOMs, and the shunt impedance of the kicker are measured with a network analyzer. This kicker has several different features from the LFS kicker for DA/spl Phi/NE. First of all, there are four input/output ports to obtain a wider bandwidth. Second, there is a nose cone to obtain a higher shunt impedance. Third, the symmetric frequency response of the shunt impedance around the central frequency is provided. Finally, low beam coupling power (4 W per port at 400 mA) is obtained. According to the simulation result by the high frequency structure simulator (HFSS) code, we obtained the high shunt impedance of 621.3 /spl Omega/ (transit time factor considered value) and the wide bandwidth of 255 MHz. These are compared with the measured result.

The Korea Superconducting Tokamak Advanced Research (KSTAR) project is the major effort of the national fusion programme of the Republic of Korea. Its aim is to develop a steady state capable advanced superconducting tokamak to establish a scientific and technological basis for an attractive fusion reactor. The major parameters of the tokamak are: major radius 1.8 m, minor radius 0.5 m, toroidal field 3.5 T and plasma current 2 MA, with a strongly shaped plasma cross-section and double null divertor. The initial pulse length provided by the poloidal magnet system is 20 s, but the pulse length can be increased to 300 s through non-inductive current drive. The plasma heating and current drive system consists of neutral beams, ion cyclotron waves, lower hybrid waves and electron cyclotron waves for flexible profile control in advanced tokamak operating modes. A comprehensive set of diagnostics is planned for plasma control, performance evaluation and physics understanding. The project has completed its conceptual design and moved to the engineering design and construction phase. The target date for the first plasma is 2002.

For a high-performance, advanced tokamak mode in KSTAR, we have been developing a real-time control system of MHD modes such as sawtooth and Neo-classical Tearing Mode (NTM) by ECH/ECCD. The active feedback control loop will be also added to the mirror position and the real-time detection of the mode position. In this year, for the stabilization of NTM that is crucial to plasma performance we have implemented open-loop ECH antenna control system in KSTAR Plasma Control System (PCS) for ECH mirror movement during a single plasma discharge. KSTAR 170 GHz ECH launcher which was designed and fabricated by collaboration with PPPL and POSTECH has a final mirror of a poloidally and toroidally steerable mirror. The poloidal steering motion is only controlled in the real-time NTM control system and its maximum steering speed is 10 degree/sec by DC motor. However, the latency of the mirror control system and the return period of ECH antenna mirror angle are not fast because the existing launcher mirror control syst...

A 170 GHz Electron Cyclotron Heating and Current Drive (ECH/CD) system on KSTAR is designed to launch total 2.4 MW of power for up to 300 sec into the plasma. At present the first 1 MW ECH/CD system is under installation and commissioning for 2011 KSTAR campaign. The 170 GHz, 1 MW, 300 sec gyrotron and the matching optics unit (MOU) will be provided from JAEA under collaboration between NFRI and JAEA. The transmission line consists of MOU and 70 m long 63.5 mm ID corrugated waveguides with the eight miter bends. The 1 MW, 10 sec launcher is developed based on the existing two‐mirror front‐end launcher in collaboration with Princeton Plasma Physics Laboratory and Pohang University of Science and Technology, and is installed on the low field side in the KSTAR equatorial plane. The mirror pivot is located at 30 cm below from the equatorial plane. 3.6 MVA power supply system is manufactured and now is under commissioning to meet the triode gun operation of JAEA gyrotron. The power supply consists of 66 kV/55 ...

Theoretical 5 GHz lower hybrid current drive (LHCD) efficiency using power spectrum given by 0‐D Brambilla code and Lower Hybrid Simulation Code (LSC) have been studied for KSTAR. In LSC simulation, RF‐driven current and current drive efficiency has been found to be deeply dependent on the profiles of the plasma density and temperature as well as on current profile in order to obtain hollow current profile favorable for advance tokamak operation mode and steady state operation. The peaked density and broad temperature profile control has been found to be efficient in current drive with maximum RF‐driven current larger than 400 kA/MW with very high efficiency when the peak plasma density is ranged from 0.2 to 2.0×1020 m−3, and the peak electron temperature range of 2–20 keV together with toroidal field 2–3.5 T and Ip = 0.5–2 MA. The on‐/off‐axis current profile controllability is also investigated through parametric scan, and small negative magnetic shear is seen at the narrow region of the off‐axis for ve...

A novel system to measure the ratio of transverse velocity to axial velocity (α) of a low voltage, axis‐rotating electron beam is described herein. With the system, more information about the beam can be obtained than from the usual slit/phosphor screen system. Since most of the beam is undisturbed before the phosphor screen, the beam radius and azimuthal symmetry can be quantified in combination with the beam α along various axial positions. The diagnostic is applied to a Cusptron microwave tube to measure these electron beam parameters which are important for high tube efficiency. Two magnetic field profiles are used to study various causes of low beam quality, while a third profile is used in attempt to maintain a high quality beam. By using the described system, it is shown that the third profile significantly increases beam quality. The system can easily be extended to higher energy beams with an appropriate scintillator.

A 20 MW/5 GHz Lower Hybrid Current Drive (LHCD) system was initially due to be commissioned and used for the second mission of ITER, i.e. the Q = 5 steady state target. Though not part of currently planned procurement phase, it is now under consideration for an earlier delivery. An LH program has been initiated under EFDA, following the ITER STAC recommendation, to provide a pre‐design document including the conceptual design, costing, possible procurement allocation, WBS and R&D needs.

We designed and fabricated a C-band standing-wave accelerating structure for a compact industrial electron accelerator. It is capable of producing 4-MeV electron beams with 50-mA pulsed beam current. As an RF source, we use a 5-GHz magnetron with 1.5-MW peak power and 0.08% duty factor. This structure is bi-periodic and operated with the pi/2-mode standing-waves. It is on-axis coupled with magnetic coupling slots on the side wall. Each cavity in the bunching and the normal cells is designed by using the MWS code. The coupler cell is designed by using the Omega3p code for determining the resonant frequency and the external Q2. With the measurement of the aluminum prototype cavities, we determined the final dimensions for actual OFHC-copper cavity. In this paper, we present the low-power measurement results for the actual column: the pi/2-mode resonant frequency, the coupling coefficient, and distribution of the field strength.

The relative significance of the active optical guiding and the gain guiding is identified at arbitrary pump strengths. The optical guiding is observed to be turned on gradually as the pump strength increases. Power laws are found for the eigenmode radii.

The PAL (Pohang Accelerator Laboratory) 2.5-GeV linac is planed to be converted to a SASE-XFEL facility (PAL XFEL) that supplies coherent X-rays down to 0.3-nm wavelength. The PAL XEL requires a new 1.2-GeV linac that will be combined to the existing linac to increase a beam energy upto 3.7-GeV. This XFEL linac should supply highly bright beams with emittance of 1.2 mm-mrad, a peak current of 3.5 kA, and a low energy spread of 0.03%. The RF stability of 0.02% is required for both RF phase and amplitude to get stable SASE output. This stability is mainly determined by a low level RF drive system and klystron-modulators. The stability level of the modulator has to be improved 10 times better to meet the pulse stability of 0.02%. The regulation methods such as traditional de-Q'ing and precision inverter charging technology are reviewed to find out suitable upgrade shceme of the modulators. Design considerations for the stability improvement of klystron-modulators for PAL XFEL are presented.

The PAL XFEL will supply coherent radiations from VUV to X-rays. X-ray FEL for 0.3 nm lasing requires a minimum 3-GeV driver linac and a 60-m long in-vacuum undulator with a narrow variable gap. The linac should supply highly bright beams with emittance of 1.2 mm-mrad, a peak current of 3.5 kA, and a lower energy spread less than 0.03%. The beam quality is degraded along the undulator trajectory due to the energy loss, the wake field, and the magnetic field errors, etc. Especially the wake field effect is most sensitive parameter due to the narrow gap of the undulator. The preliminary design details of undulators for PAL-XFEL are presented with parametric analysis. The SASE performance is analyzed using simulation tool SIMPLEX [1].

An infrared FEL using the PAL Test Linac is proposed as one of new PLS beamline plans for users. For an infrared FEL some parts of Test Linac should be improved such as klystron modulator system and RF-gun. To get a higher intensity of electron beam a magnetic bunch compression system using chicane magnet will be incorporated; which can yield a sub-pico-second beam. Two FEL beam lines with different energy will be provided: one is a low energy FEL (25-35 MeV) and the other one is a high energy FEL (50-70 MeV). The IR FEL beamline will be officially included into the PLS beamline lists. The design FEL wavelength ranges from 1.0 to 20 microns and the repetition rate is up to 30 Hz. The laser macropulse length is more than 5.0 micro-seconds. PACS codes: 41. 50.Cr

Pohang Accelerator Laboratory(PAL) is working on a X‐ray self‐amplified spontaneous emission (SASE) FEL which will be driven by the upgraded injector linac. The preliminary idea is to upgrade the existing 2.5GeV injector linac to 3.0 GeV electron energy and use it as a driver for X‐ray SASE FEL. It also features utilization of the recent in vacuum undulator technology for shorter radiation wavelength . And the 3rd harmonic enhancement is considered for a possible option. In this report, the preliminary conceptual designs of the X‐ray SASE FEL at PAL including above features will be described.

Summary form only given, as follows. Recently, the plasma catalytic hybrid processes has been extensively studied for controlling NO/sub x/ effectively. We investigated the effect of NO/NO2 concentration ratio on removal efficiency in the catalytic process with changing of the the ratio of NO to NO/sub 2/ and the experimental results were compared with those obtained from the plasma catalytic hybrid process. In the experiment, the dielectric barrier discharge (DBD) reactor was combined with an NH/sub 3/ SCR catalyst.

Pohang Neutron Facility (PNF) is a pulsed neutron facility based on the 100-MeV electron linear accelerator. It was constructed for nuclear data production in Korea, and it consists of an electron linear accelerator, a water- cooled Ta target with a water moderator and a time-of- flight path with an 11 m length. The 100-MeV electron linac uses a thermionic RF-gun, an alpha magnet, four quadrupole magnets, two SLAC-type accelerating sections, a quadrupole triplet, and a beam-analyzing magnet. In design of a water-cooled Ta target, we used the Monte Carlo simulation codes, EGS4 and MCNP version 4B. The neutron energy spectrum is measured for different water levels inside the moderator and compared with the calculation results by the Monte Carlo N-Particle (MCNP) transport code system.

The cusptron microwave tube is a compact high-power device using the harmonic frequency generating scheme via the negative mass instability with an axis-rotating electron beam. Both the high and low harmonic modes of operation in the oscillator configuration have been successfully demonstrated, and the beam dynamics have been studied to improve beam quality. This device was shown to have superior quality in terms of mode purity and frequency stability. A program is now in preparation for conversion of the cusptron into an amplifier configuration. It is concluded that off-centering due to the unbalanced cusp is very harmful and that the staged field profile featuring a balanced cusp and an adiabatic compression phase is desirable. >

The PLS linac has been injecting 2-GeV electron beams to the Pohang Light Source (PLS) storage ring since September 1994. PLS 2-GeV linac has 11 units of high-power pulsed klystron-modulator (K&M) system for the main RF sources for the beam acceleration. The klystron has rated output peak power of 80-MW at 4 /spl mu/sec pulse width with 120 pps, and the matching modulator has 200-MW peak output power. The total accumulated high voltage run time of the oldest unit has reached beyond 20000 hours and the sum of all the high voltage run time is approximately 180000 hours. In this paper, we review overall system performance of the high power K&M system together with the description of the K&M system. A special attention is paid on the analysis of all failures and troubles of the K&M system which affected the linac RF operations as well as beam injection operations for the period of 1994 to May 1996.

A C-band (5,712 MHz) RF system for an e+e- linear collider at 500 GeV center-of-mass energy requires total 4,080 units of 50 MW pulse klystrons and matching 111 MW peak-power modulators for two main linacs. An initial design value of the wall plug power for the entire RF system is 150 MW with 2.5 /spl mu/sec RF pulse width and 100 Hz pulse repetition rate. A detailed circuit analysis shows that it is possible to get the overall RF system efficiency of approximately 24%, or equivalent to 210 MW wall plug power. This includes 85% DC high voltage charging efficiency using compact inverter power supplies, and 70% pulse efficiency using 3.5 /spl mu/sec pulse length (ESW) with less than 1% flat top ripple, and auxiliary powers for thyratron heater, klystron heater, focusing magnet, pulse transformer reset, and etc.

An average 30-kW MPC (magnetic pulse compression) modulator has been constructed and tested with 32 /spl Omega/ resistive dummy load. This modulator can produce high power pulses (250 MW peak@100 kV) with 200 nsec pulse rise time, 500 nsec (FWHM) pulse width, and 300 Hz repetition rate. This system is used for the production of intense streamer corona discharge in a plasma reactor for the simultaneous removal of NO/sub x/ and SO/sub x/ in the flue gases from the stack. We present the operational characteristics of the MPC modulator connected to a prototype plasma reactor for the stack gas treatment system equipped with a 0.5 MWe oil burning boiler unit.

The graphic-based realtime control system is developed and used for the normal operation of the PLS 2-GeV linac. Control and monitoring signals from local devices, such as magnet power supplies and modulators, are connected to the VME CPUs located in the field via special signal conditioning units. Data collected by these field CPUs are summarized and stored in the supervising VME CPUs located in the control room. Operators can control such devices simply by pointing and clicking with a mouse on the control panels which are X-windows loaded on a SUN workstation environment. Status and data to be monitored are displayed in terms of digital values and graphical presentations on individual status windows which are categorized functionally. Those windows are generated with the commercially available development software named RTworks. Every data transaction is done through a specified ethernet. There are four separate ethernets for effective data transactions. Fast signals, such as RF signals and modulator beam voltages, are captured by digital oscilloscopes and displayed in the control panel through the GPIB port. This paper presents main features and the general performance of the computer control system for the PLS 2-GeV linac.

The PLS 2-GeV electron linac at the Pohang Accelerator Laboratory (PAL) has been constructed as a full energy injector to the storage ring of the Pohang Light Source (PLS) project. The 150-m long linac uses 11 klystrons of 80-MW maximum output power driven by 200-MW modulators. There are 42 constant gradient acceleration sections and 6 quadrupole triplets. By December 10, 1993, we completed the linac installation with the beam analyzing station and the beam transport line to the beam dump of 30-m long. We completed the machine commissioning by June 30, 1994. Since September 1, 1994, it provides 2-GeV beams to the PLS storage ring for its commissioning, which is also completed by December 24, 1994 with 300-mA stored current. In this paper, we present the operational experience of the PLS 2-GeV linac.

In the Pohang Light Source storage ring, we empty 68 buckets of total 468 buckets to cure the conventional ion trapping. However, the fast beam-ion instability is occasionally generated at higher beam current. Immediately after the strong vertical beam shaking due to the fast beam-ion instability, the longitudinal coupled bunch mode instabilities are substantially damped down without any active feedback system. We have described the fast beam-ion instability driven longitudinal damping and its mechanism in the Pohang Light Source storage ring.

During the normal operation period in the Pohang Light Source (PLS) storage ring, the spontaneous bunch length oscillation or beating is generated by the nonlinear effects of the RF noise whose frequency is much lower than the bifurcation frequency. Due to the bunch length beating, we can not supply the uniform intensity beams to users. We have described the observed bunch length beating and its mechanism in the PLS storage ring with a simulation.

From the recent data analysis of the longitudinal feedback system and the streak camera, we have found a longitudinal relaxation beam oscillation whose frequency is a multiple of 36 Hz. When the relaxation oscillation is generated, horizontally oscillating wide beams are observed at the diagnostic beamline, and the beam lifetime is fluctuated. The strength of the longitudinal coupled bunch mode instabilities is also modulated sinusoidally. We have described the observed longitudinal relaxation oscillation in the Pohang Light Source storage ring.

In the Pohang Light Source (PLS) storage ring, we have occasionally experienced the abnormal spontaneous beam. lifetime reduction and strong longitudinal damping. Due to the abnormal beam lifetime reduction, available beam time is reduced substantially. From the recent data analysis of the PLS longitudinal feedback system (LFS) and the streak camera, we have found that the abnormal phenomena can be generated by the RF noise via the dipole mode parametric resonance of the RF phase modulation. We describe the observation of the dipole mode parametric resonance driven by the RF noise and compare it with a simulation result in the PLS storage ring.

A compact standing-wave linac has been designed using S-band microwaves for medical and industrial applications. It uses a bi-periodic structure with constant impedance. It consists of a buncher section with three-cells and on-axis coupled cavities with nine-cells. The total length is 740.0 mm including the electron gun. Using SUPERFISH, HFSS and PARMELA codes, it is capable to deliver 4.5 MeV electron beams with pulsed microwaves of 4.8 MW at 2,856 MHz. In this paper, we present the detailed design studies on a compact S-band standing-wave linac.

We have designed and fabricated a photocathode test-stand in order to develop the polarized electron source in Korea. The test-stand consists of a photocathode gun, a light source, a cylindrical condenser and a mini-Mott chamber. We report the status of the test-stand and the future plan for the development of a polarized electron source.

The single bunch collective effects such as the bunch lengthening due to the potential well distortion and the microwave instability which also increases the energy spread, the transverse head-tail instability, and the transverse strong head-tail instability are investigated in the Pohang Light Source storage ring. The threshold current and the Chao-Gareyte scaling factor of the bunch lengthening, the longitudinal broadband impedance, and the transverse impedance have been measured and compared with the ZAP code results.

The Pohang Accelerator Laboratory uses a 100-MeV electron linac for the pulsed neutron source as one of the long-term nuclear RD one for short pulse mode with the various repetitions between 2 ns and 100 ns and the other for long pulse mode with 1 /spl mu/s. The major beam parameters are as follows; the nominal beam energy is 100 MeV, the maximum beam power is 10 kW, and the beam current is varied from 300 mA to 5 A and depends on the pulse repetition. We constructed and tested a test-linac based on the existing equipment such as a SLAC-5045 klystron, two constant gradient accelerating sections, and thermionic RF-gun. We describe the characteristics of the test-linac and report the status of the pulsed neutron source facilities including a target system and time-off-flight paths.

The generation of microwave radiation at high harmonics of the cyclotron frequency by a low‐energy annular rotating electron beam (27 kV, 1 A, 3 μs) via interaction with the modes of a magnetron‐type conducting boundary has been studied experimentally. Details of producing the axis‐encircling E layer by magnetic cusp injection are presented. Narrow‐band microwave radiation has been obtained from the device at power levels of 9.1 kW at 6.78 GHz (eighth cyclotron harmonic) and 2.2 kW at 8.85 GHz (tenth cyclotron harmonic). Measurements of the radiated power spectra and confining magnetic field have allowed identification of operating waveguide modes and instability mechanisms. An experimental comparison with respect to threshold beam power is presented between waveguide structures operating in the π mode and the 2π mode.

This report presents observations on propagation of a mildly relativistic electron beam (γ=2.4) in several different gases (air, He, N2, and Ar) at low pressure (10–320 mTorr). At such low pressures, and with beam currents of several kiloamperes, beam‐induced ionization may result in self‐focusing of an electron beam. Beams propagating under this condition are said to be in the ion‐focusing regime of propagation. The injected beam has an rms radius of 0.7 cm and a transverse temperature of 35 keV. Beam transport efficiency was measured for He, N2, and Ar. A simple calculation was performed to determine the space‐charge neutralization fraction as a function of time. Using the results of this calculation, the charge‐transport efficiency is found and compared with the experimentally measured quantity.

The effects of low power discharge cleaning (Te=9–18 eV, ne=1010–1011 cm−3) on wall conditions in the ISX‐A tokamak have been studied in detail by means of Auger electron spectroscopy and residual gas analysis. It was found that discharge cleaning caused very little transport of metals and reduced the surface oxygen content on stainless steel samples located between the limiter and the wall to substoichiometric levels. This oxygen was probably removed from the samples as H2O molecules. On the other hand carbon, in the form of a metallic carbide, increased during discharge cleaning. The cleanliness of tokamak plasmas (Zeff=1.6–2) obtained after about three weeks of discharge cleaning was clearly correlated with improvements in the wall conditions and with the reduced level of residual gas species.

The radiation produced by a hollow, non‐neutral, rotating, relativistic E layer propagating inside a coaxial cylindrical drift tube has been investigated theoretically and experimentally. The measured radiation spectrum is very broadband in agreement with theory, though a shift in the spectrum can be achieved by a preloaded azimuthal density structure.

The effects of space charge and lens aberrations on a 5‐keV, 80–200‐mA electron beam focused by a solenoid lens are investigated theoretically and experimentally. A third‐order radial trajectory equation is derived with space charge added in the form of discrete annular current elements. This equation is numerically solved for the trajectories of focused electrons in a short solenoid lens. Trajectories launched near the axis are ‘‘reflected’’ from the axis at the waist due to space charge, and trajectories launched farther from axis, cross the axis. The resulting discontinuity is best illustrated in a transverse phase‐space plot where it may be described as a ‘‘tearing’’ of the electron distribution. Due to the combined action of the nonlinear lens forces and the space charge, the initially uniform radial current density distribution becomes markedly nonuniform (hollow or peaked depending on distance) downstream from the lens. Detailed experimental measurements of the density profiles at different positions are found to be in very good agreement with computations.

With successful demonstration of the ‘proof-of-principle’ experiment of the cusptron, the configuration of the cusptron waveguide gains importance. Although the dispersion relation for the cusptron rf circuit has been derived and experimentally verified before, the detailed properties of its dispersion characteristics are not well understood. In this paper, we will examine various aspects of the dispersion properties in conjunction with the design process of the cusptron tubes. In particular, two unique properties of the cusptron rf circuit, the mixture and the specific grouping of the azimuthal mode numbers are closely examined and demonstrated with simple physics. Special emphases are on the identification and designation of the modes, the profiles of various field components, the spectral distribution with respect to azimuthal mode numbers, and their relation to the cusptron microwave tube design