Although the dominant approach to drug development is the design of compounds selective for a given target, compounds targeting more than one biological process may have superior efficacy, or alternatively a better safety profile than standard selective compounds. Here, this possibility has been explored with respect to the endocannabinoid system and pain. Compounds inhibiting the enzyme fatty acid amide hydrolase (FAAH), by increasing local endocannabinoid tone, produce potentially useful effects in models of inflammatory and possibly neuropathic pain. Local increases in levels of the endocannabinoid anandamide potentiate the actions of cyclooxygenase inhibitors, raising the possibility that compounds inhibiting both FAAH and cyclooxygenase can be as effective as non-steroidal anti-inflammatory drugs but with a reduced cyclooxygenase inhibitory 'load'. An ibuprofen analogue active in models of visceral pain and with FAAH and cyclooxygenase inhibitory properties has been identified. Another approach, built in to the experimental analgesic compound N-arachidonoylserotonin, is the combination of FAAH inhibitory and transient receptor potential vanilloid type 1 antagonist properties. Although finding the right balance of actions upon the two targets is a key to success, it is hoped that dual-action compounds of the types illustrated in this review will prove to be useful analgesic drugs.
This paper is concerned with a detailed analysis of the dominant modes of a slotted helix loaded in an empty waveguide, with a view to improving the efficiency of plasma production using such structures. To this end the dispersion relation for a slotted helix loaded inside a cylindrical waveguide is first derived. As in the case of a tape-helix-loaded waveguide, the different dominant modes of the system can be identified by suitable asymptotic expansions of the exact dispersion relation. For large diameter helices (kvrh>1, where kv is the free-space wavelength of the wave and rh is the helix radius), the system has one slow wave and one fast wave as its dominant waves. The field patterns of these two modes are studied (for both slotted and tape helices) and the dominant field components corresponding to each type of helix and mode, identified from the point of view of ease in the excitation of the slow wave for plasma production. Since the fast and slow waves have overlapping dominant field components, methods are also discussed for suppressing the fast wave which can be excited more easily due to the presence of the waveguide. Finally, the state of polarization (left/right handedness) of the slow-wave mode is discussed with relevance to the coupling of the microwave to the plasma electrons in an external magnetic field.
This paper presents a detailed study on the optimal use of helical slow-wave structures (both wire and slotted) for plasma production at microwave frequencies. For this purpose a feed optimization study was undertaken in which different feed structures located within the helical coils were used for exciting the helices. Each feed structure excites a preferred component of the wave electric/magnetic field inside the helices. It is seen that the efficacy of plasma production using the different feeds depends directly on the relative importance of the field component (excited by each feed) for the slow-wave mode of the helixloaded waveguide. The best feed for both wire and slotted helices is shown to be a dipole antenna, oriented so as to excite the radial component of the electric field within the antenna. For the wire helices, an axially oriented monopole antenna (which excites an axial electric field) is also shown to yield comparable results. In order to avoid a spontaneous excitation of the dominant fast-wave mode of the helix-loaded waveguide system preferentially, mode-selective structures (resonant or anti-resonant cavities) tuned to the slow-wave mode have been used. The experiments show that the performance of the resonant helical coils is uniformly superior to that of the nonresonant one. The plasma so produced was characterized with respect to the microwave power and magnetic field at the antenna region. A second set of experiments was also performed, where a second helical coil (in addition to the helical antenna being used for plasma production) was used. This helical coil is a long, large diameter, closely wound wire helix which fits snugly into the plasma chamber or the waveguide. Thus the plasma produced by the helical antenna now flows into this long helical transmission line which acts like an extended waveslowing structure. This plasma (surrounded by the long helical line) is characterized once again to study the changes in the plasma parameters induced by this line. It is found that using the long helical transmission line gives some improvement in the plasma density, the bulk electron temperature, and their radial profiles. However, the improvement in the hot and intermediate electron temperatures is seen to be quite significant, which indicates the efficacy of this structure for plasma-heating purposes.
The paper is concerned with identifying the dominant electromagnetic modes of a tape helix loaded in a conducting waveguide. Two distinct regions of operation of the helix are found. For knu rh<1(knu :free space wavenumber; rh:helix radius), the helix supports slow as well as fast waves, while for knu rh>1, it supports only fast waves. For each region separate asymptotic formulae have been derived. These have also been verified experimentally. Since the exact dispersion relation for a helix is extremely complex these formulae serve to eliminate the associated computational problems. Moreover, a helix in a waveguide supports a large number of fast wave modes. This makes the simplified formulae for fast waves extremely important since these isolate the dominant fast wave directly.
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