For an analysis of the magnetic field configuration it is useful to represent the vacuum magnetic field by a set of harmonic functions. To exactly analyse a helical configuration, a harmonic function describing a pure helical mode of the configuration is desirable, especially in heliotrons, where good quasi-symmetry is regarded as an advantage for a built-in helical divertor. Owing to the specific mode numbers in the toroidal and the poloidal directions, toroidal harmonic functions are appropriate for a study of the helical field when a practical method for their numerical calculation is provided. In the paper, helical components are analysed by a numerical calculation with high accuracy. From a spectral analysis of the magnetic field generated by a continuous helical coil, the 'natural winding' law was found to be an excellent choice for the case where the components resonant with the helicity of the coil are dominant. A deviation from the natural winding law causes an enhancement of off-resonant components. The choice of unique toroidal co-ordinates and, hence, of a unique spectral representation follows naturally from the condition that the spectral series in which the magnetic field is expressed has the largest area for convergence. The high accuracy of the representation was used in a numerical investigation of the intricate structure of a helical divertor. A reticular structure of the scrape-off layer in a helical system was made visible by tracing an unclosed separatrix. The field line at the X-point is a nearly pure helix in the toroidal co-ordinates, and it was confirmed that the deviation due to perturbing fields is sufficiently small to allow a rigid divertor baffle to be installed.
磁場配位の解析には、真空磁場を一組の調和関数で表すことが有用である。螺旋配位を正確に解析するには、配位の螺旋モードを記述する調和関数が望ましく、特にヘリオトロンでは、良好な準対称性が螺旋ダイバータの利点とみなされている。トロイダル方向およびポロイダル方向の調和関数は、螺旋場を解析するのに適している。本論文では、連続螺旋コイルによって生成される磁場のスペクトル解析を高精度で行う。螺旋コイルの共鳴成分が支配的である場合、そのずれによる共鳴成分の影響は小さく、螺旋ダイバータのスクレイプオフ層の網目構造は、X点における磁力線がほぼ純粋な螺旋であることにより、ダイバータ用の固定バッフルを設置するのに十分な精度で追跡できることが確認された。