Summary: Determining the leakoff coefficient from a minifracture pressure decline has become a relatively common industrial procedure. A main assumption of the method, which is often referred to as the Nolte analysis, is a constant leakoff coefficient. Frequently there is no constant leakoff coefficient. The definition of the coefficient is based on a constant pressure differential and a prescribed mode of leakoff. In contrast, the analysis for fracture-pressure decline, introduced by Mayerhofer et al.,10 couples unsteady-state linear flow from the fracture with a varying skin effect at the fracture face, and superposes the leakoff history on the pressure decline. This guarantees a correct rate convolution to account for pressure-dependent fluid loss. The reservoir permeability, fracture-face resistance, and leakoff area can be determined. A comparison of the two methods and their relationship is presented in this paper, using real field cases. We show that the use of modern well-testing log-log diagnostic plots to determine fracture closure pressure is superior to drawing a straight line on a G-function plot or a square-root-of-time plot.

Radio frequency cavities are among the most challenging and costly components of an accelerator facility. They are usually manufactured in individual parts, which are then joined by complex processes, e.g., several brazing steps. 3D printing has become an alternative to these conventional manufacturing methods due to higher cost efficiency, freedom in design, and recent achievement of high print quality for pure copper. A fully functional 3 GHz drift tube linac (DTL) prototype was 3D printed in one piece, made from pure copper by selective laser melting (SLM). To achieve a higher surface quality, the DTL geometry was optimized for the SLM process. The DTL design is related to the design of the DTL part of the side-coupled DTL modules used in linac-based proton therapy facilities. The quality factor (8750) and the shunt impedance per unit length $(102\frac{\mathrm{m}\mathrm{\Omega }}{\mathrm{m}})$ of the printed prototype are already comparable to traditionally manufactured DTL structures and can be further enhanced by surface treatments.

Purpose Radiotherapy plays an important role for the treatment of tumor diseases in 2/3 of all cases but it is limited by side effects in the surrounding healthy tissue. Proton minibeam radiotherapy (pMBRT) is a promising option to widen the therapeutic window for tumor control at reduced side effects. An accelerator concept based on an existing tandem Van de Graaff accelerator and a linac enables the focusing of 70 MeV protons to form minibeams with a size of only 0.1 mm for a preclinical small animal irradiation facility, while avoiding the cost of an RFQ injector. Methods The tandem accelerator provides a 16 MeV proton beam with a beam brightness of as averaged from 5 µs long pulses with a flat top current of 17 µA at 200 Hz repetition rate. Subsequently, the protons are accelerated to 70 MeV by a 3 GHz linear post‐accelerator consisting of two Side Coupled Drift Tube Linac (SCDTL) structures and 4 Coupled Cavity Linac (CCL) structures (design: AVO‐ADAM S.A (Geneva, Switzerland)). A 3 GHz buncher and four magnetic quadrupole lenses are placed between the tandem and the post‐accelerator to maximize the transmission through the linac. A quadrupole triplet situated downstream of the linac structure focuses the protons into an area of (0.1 × 0.1) mm². The beam‐dynamics of the facility is optimized using the particle optics code TRACE 3‐D. Proton transmission through the facility is elaborated using the particle tracking code TRAVEL. Results A study about buncher amplitude and phase shift between buncher and linac is showing that 49% of all protons available from the tandem can be transported through the post‐accelerator. A mean beam current up to 19 nA is expected within an area of (0.1 × 0.1) mm² at the beam focus. Conclusion An extension of existing tandem accelerators by commercially available 3 GHz structures is able to deliver a proton minibeam that serves all requirements to obtain proton minibeams to perform preclinical minibeam irradiations as it would be the case for a complete commercial 3 GHz injector‐RFQ–linac combination. Due to the modularity of the linac structure, the irradiation facility can be extended to clinically relevant proton energies up to or above 200 MeV.

Linear particle accelerators (Linacs) are primarily composed of radio frequency cavities (cavities). Compared to traditional manufacturing, Laser Powder Bed Fusion (L-PBF) holds the potential to fabricate cavities in a single piece, enhancing Linac performance and significantly reducing investment costs. However, the question of whether red or green laser PBF yields superior results for pure copper remains a subject of ongoing debate. Eight 4.2 GHz single-cell cavities (SCs) were manufactured from pure copper using both red and green PBF (SCs R and SCs G). Subsequently, the surface roughness of the SCs was reduced through a chemical post-processing method (Hirtisation) and annealed at 460 °C to maximize their quality factor ($Q_0$). The geometric accuracy of the printed SCs was evaluated using optical methods and resonant frequency ($f_R$) measurements. Surface conductivity was determined by measuring the quality factor ($Q_0$) of the SCs. Laser scanning microscopy was utilized for surface roughness characterization. The impact of annealing was quantified using Energy-Dispersive X-ray Spectroscopy and Electron Backscatter Diffraction to evaluate chemical surface properties and grain size. Both the SCs R and SCs G achieved the necessary geometric accuracy and thus $f_R$ precision. The SCs R achieved a 95% $Q_0$ after a material removal of 40 µm. The SCs G achieved an approximately 80% $Q_0$ after maximum material removal of 160 µm. Annealing increased the $Q_0$ by an average of about 5%. The additive manufacturing process is at least equivalent to conventional manufacturing for producing cavities in the low-gradient range. The presented cavities justify the first high-gradient tests.

Compared to conventional manufacturing, additive manufacturing (AM) of radio frequency (RF) cavities has the potential to reduce manufacturing costs and complexity and to enable higher performance. This work evaluates whether normal conducting side-coupled linac structures (SCCL), used worldwide for a wide range of applications, can benefit from AM. A unit cell geometry (SC) optimized for 75 MeV protons was developed. Downskins with small downskin angles $\alpha$ were avoided to enable manufacturing by laser powder bed fusion without support structures. SCs with different $\alpha$ were printed and post-processed by Hirtisation (R) (an electrochemical process) to minimize surface roughness. The required accuracy for 3 GHz SCCL (medical linacs) is achieved only for $\alpha >{45}^{\circ }$. After a material removal of 140 µm due to Hirtisation (R), a quality factor $Q_0$ of 6650 was achieved. This corresponds to 75% of the $Q_0$ simulated by CST^®. A 3 GHz SCCL concept consisting of 31 SCs was designed. The effective shunt impedance $Z{T}^2$ simulated by CST corresponds to $60.13\frac{\mathrm{M}\mathrm{\Omega }}{\mathrm{m}}$ and is comparable to the $Z{T}^2$ of SCCL in use. The reduction in $Z{T}^2$ expected after Hirtisation (R) can be justified in practice by up to 70% lower manufacturing costs. However, future studies will be conducted to further increase $Q_0$.

The enormous potential of additive manufacturing (AM), particularly laser powder bed fusion (L-PBF), to produce radiofrequency cavities (cavities) has already been demonstrated. However, the required geometrical accuracy for GHz $T{M}_{010}$ cavities is currently only achieved by (a) avoiding downskin angles $<{40}^{\circ }$, which in turn leads to a cavity geometry with reduced performance, or (b) co-printed support structures, which are difficult to remove for small GHz cavities. We have developed an L-PBF-based manufacturing routine to overcome this limitation. To enable arbitrary geometries, co-printed support structures are used that are designed in such a way that they can be removed after printing by electrochemical post-processing, which simultaneously reduces the surface roughness and thus maximizes the quality factor $Q_0$. The manufacturing approach is evaluated on two $T{M}_{010}$ single cavities printed entirely from high-purity copper. Both cavities achieve the desired resonance frequency and a $Q_0$ of approximately 8300.