Improved Magnetron Stability and Reduced Noise in Efficient Transmitters for Superconducting Accelerators

We evaluate the capacity and bandwidth efficiency of microcellular CDMA systems. Power control, multipath diversity system bandwidth, and path loss exponent are seen to have a major impact on the capacity. The CDMA system considered uses convolutional codes, orthogonal signalling, multipath/antenna diversity with noncoherent combining, and fast closed-loop power control on the uplink (portable-to-base) direction. On the downlink (base-to-portable), convolutional codes, BPSK modulation with pilot-signal-assisted coherent reception, and multipath diversity are employed. Both fast and slow power control are considered for the downlink. The capacity of the CDMA system is evaluated in a multicell environment taking into account shadow fading, path loss, fast fading, and closed-loop power control. Fast power control on the downlink increases the capacity significantly. Capacity is also significantly impacted by the path loss exponent. Narrowband CDMA (system bandwidth of 1.25 MHz) requires artificial multipath generation on the downlink to achieve adequate capacity. For smaller path loss exponents, which are more likely in microcellular environments, artificial multipath diversity of an order of as high as 4 may be needed. Wideband CDMA systems (10 MHz bandwidth) achieve greater efficiencies in terms of capacity per MHz. >

CEPC is a circular electron positron collider operating at 240 GeV center-of-mass energy as a Higgs factory, recently proposed by the Chinese high energy physics community. The CEPC study group, together with the FCC and ILC community, will contribute to the development of future high energy colliders and experiments which will ensure that the elementary particle physics remain a vibrant and exciting field of fundamental investigation for decades to come. Superconducting RF (SRF) system is one of the most important technical systems of CEPC and is a key to achieving its design energy and luminosity. It will dominate, with the associated RF power source and cryogenic system, the overall machine cost, efficiency and performance. The CEPC SRF system will be one of the largest and most powerful SRF accelerator installations in the world. The preliminary conceptual design of the CEPC SRF system is summarized in this paper, including the machine layout, key parameter choices and some critical issues such as HOM damping, emphasizing the new technology requirement and R&D focuses.

The International Linear Collider (ILC) main linac comprises a series of 12 m-long cryomodules. The cryomodule contains eight 9-cell superconducting (SC) RF (SRF) cavities and an SC quadrupole magnet combined with dipole correctors to focus and steer electron and positron beams. The magnets are installed between the SRF cavity string and at the longitudinal center of a common cryomodule/cryostat. These magnets are conductively cooled by pure aluminum channels thermally connected to a 2 K two-phase helium pipe for cooling the SRF cavities. A recent study shows that field-emitted electrons, so-called “dark current” initiated in the SRF cavities, are transmitted through the SRF cavity string and reach the SC magnet. The energy is inevitably absorbed in the SC coil due to the magnetic field, resulting in risks of a quench caused by the coil heating. We are investigating alternate magnet designs by using Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> Sn or <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{MgB}_{2}$</tex-math></inline-formula> SC and by adding the dark current absorber surrounding the beam-pipe to realize sustainable magnet operation under the dark current heating. We report the design study of the magnet system and interfaces to the cryomodule accommodating it with the SRF cavities.

In an LEO satellite CDMA system, power control can only be used to combat long-term signal loss due to long round trip delay, which calls for different considerations on power control algorithms. Based on the criteria of fast response, small overcompensation, oscillation-avoiding, and minimum power control error, the behavior of the transient response of fixed step power control, adaptive power control, and multistep power control are investigated and compared. A new multistep power control algorithm is proposed for LEO satellite communication systems suffering from high variation in the envelope of the received signal, i.e., high dynamics in slow fading.

We investigate whether adapting the transmit power optimally to the time-varying channel compares favorably, in terms of total energy consumption, to using a fixed link margin in wireless networks over short transmission distances. Over short distances, the circuit energy consumption dominates the transmission energy. For that reason, feeding back channel state information a requirement for power control - may not be a power efficient strategy. We investigate both slow and fast power control and conclude, somewhat surprisingly, that using a fixed margin is typically more power-efficient than using power control.

The European Spallation Source (ESS) is one of Europe’s largest planned research infrastructures. The project is funded by a collaboration of 17 European countries and is under design and construction in Lund, Sweden. Three families of Superconducting Radio-Frequency (SRF) cavities are being prototyped, counting the spoke resonators with a geometric beta of 0.5, medium-beta elliptical cavities (β=0.67) and high beta elliptical cavities (β=0.86). The ESS linac will produce 2.86 ms long proton pulses with a repetition rate of 14 Hz (4 % duty cycle), a beam current of 62.5 mA and an average beam power of 5 MW. A control system is being developed to operate the different accelerator systems. All operating modes of the superconducting linac shall ensure a safe operation of the accelerator. This paper presents the preliminary functional analysis and the operating modes of the 704 MHz SRF linac.

There is a significant interest in developing high-average power electron sources, particularly in the area of electron sources integrated with Superconducting Radio Frequency (SRF) systems. For these systems, the electron gun and cathode parts are critical components for stable intensity and high-average powers. In this initial design study, we will present the design of a 9-cell accelerator cavity having a frequency of 1.3 GHz and the corresponding field optimization studies.

CDMA is interference limited multiple access system. Power control is an effective way to reduce co-channel interference. Consequently, it can improve the system capacity. CDMA employs fast closed-loop power control in uplink in which signal-to-interference ratio (SIR) is estimated. Transmitting power is adjusted by comparing estimated SIR with desired SIR. In this paper a modified fixed step power control algorithm has been proposed, which can improve the performance of a CDMA system. The proposed algorithm produced a faster convergence and low outage probability compared to other power control algorithms discussed and analyzed to maintain the desired SIR target.

Performance of a reverse link wideband CDMA cellular system with fast closed-loop power control is studied in this paper. The channel is modelled as frequency selective fading. The probability of bit error, P c, is used as the performance criterion. A mathematical formulation is derived which takes into consideration the updating frequency of the power control mechanism, the fading rate of the channel, the number of resolvable multipaths, number of RAKE receiver branches, number of simultaneous users and the processing gain. The interaction between these parameters and the Pc performance is studied. The frequently assumed log-normal approximation of the received signal power has also been verified to be valid in the range of 10-3⩽Pc⩽0.5 provided the number of simultaneous users that is allowed in the system K⩾8. Finally, the analysis shows that the received signal power estimation error is the major cause of degradation in the Pc performance

Transmission power control (TPC) is essential in third generation systems for several reasons. Fast power control (or closed-loop power control) is intended to track fast channel fading. We investigate fast power control for FDD-UMTS with the help of Markov chain models in order to obtain analytical results of the performance. The effect of TPC errors and SIR estimation errors is explained. The Markov chain models are derived for small velocities, but can be considered as bounds for moderate speeds as well. We also explain how SIR estimation errors can be modelled as TPC errors.

eRHIC project requires construction of a FFAG ring to accelerate electrons and connect to the existing ion ring of Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. This new ring will have the same radius as the RHIC ring. Synchrotron radiation lost in the electron ring should be compensated by a CW superconducting radio frequency (SRF) cavity. Here we propose an 845 MHz single cell harmonic cavity. This cavity will experience a high average current (∼0.7 A) passing through it. With this consideration, this cavity design requires optimization to reduce higher order mode power. On the other hand, the cavity will operate at relatively high gradient up to 18 MV/m. Current design requires fundamental couplers to handle 400 kW forward RF power and HOM couplers to extract 2.5 kW HOM power.

The benefit of generalized RAKE reception to downlink system capacity is studied. System capacity is evaluated for certain cell and user outage probabilities. Fast fading, noise correlation, power control, and soft handoff are all included in our study. The site selection diversity transmission (SSDT) approach to power allocation between soft handoff links is shown to be optimal in the sense that it minimizes total transmit power to a given user. Numerical results show that G-RAKE receivers can improve system capacity by as much as 30-45% in a highly dispersive channel.

Microresonator frequency combs can be an enabling technology for optical frequency synthesis and timekeeping in low size, weight, and power architectures. Such systems require comb operation in low-noise, phase-coherent states such as solitons, with broad spectral bandwidths (e.g., octave-spanning) for self-referencing to detect the carrier-envelope offset frequency. However, accessing such states is complicated by thermo-optic dispersion. For example, in the Si3N4 platform, precisely dispersion-engineered structures can support broadband operation, but microsecond thermal time constants often require fast pump power or frequency control to stabilize the solitons. In contrast, here we consider how broadband soliton states can be accessed with simple pump laser frequency tuning, at a rate much slower than the thermal dynamics. We demonstrate octave-spanning soliton frequency combs in Si3N4 microresonators, including the generation of a multi-soliton state with a pump power near 40 mW and a single-soliton state with a pump power near 120 mW. We also develop a simplified two-step analysis to explain how these states are accessed without fast control of the pump laser, and outline the required thermal properties for such operation. Our model agrees with experimental results as well as numerical simulations based on a Lugiato-Lefever equation that incorporates thermo-optic dispersion. Moreover, it also explains an experimental observation that a member of an adjacent mode family on the red-detuned side of the pump mode can mitigate the thermal requirements for accessing soliton states.

Several future accelerator projects at Brookhaven for the Relativistic Heavy Ion Collider (RHIC) are based on energy recovery linacs (ERLs) with high-charge high-current electron beams. Their stable operation mandates effective higher-order-mode (HOM) damping. The development of HOM dampers for these projects is pursued actively at this laboratory. Strong HOM damping was experimentally demonstrated both at room and at superconducting (SC) temperatures in a prototype research and development (R) five-cell niobium superconducting rf (SRF) cavity with ferrite dampers. Two room-temperature mock-up five-cell copper cavities were used to study various damper configurations with emphasis on capacitive antenna dampers. An innovative type of ferrite damper over a ceramic break for an R SRF electron gun also was developed. For future SRF linacs longer cryomodules comprised of multiple superconducting cavities with reasonably short intercavity transitions are planned. In such a configuration, the dampers, located closer to the cavities, will be at cryogenic temperatures; this will impose additional constraints and complications. This paper presents the results of simulations and measurements of several damper configurations.

The European Spallation Source (ESS) is a pan-European project. It will be built by at least 17 European countries, with Sweden and Denmark as host nations. The Superconducting Radio-Frequency (SRF) linac is composed of one section of spoke cavity cryomodules (352.21 MHz) and two sections of elliptical cavity cryomodules (704.42 MHz). These cryomodules contain niobium SRF cavities operating at 2 K. Following a redesign of its accelerator, SRF linac design shall comply with a new set of requirement, like an increase of the beam current from 50 mA to 62.5 mA and an increase of the peak electrical surface field from 40 MV/m to 45 MV/m. Requirements and the main disciplines needed to construct this portion of the linac are presented.